BIODIVERSITY AND BIOSPHERE IN THE ARCHEOZOIC ERA THROUGH THE CAMBRIAN PERIOD

Contents

Introduction
1. Reading the past in the stone pages
2. Age of Earth
3. Unfamiliar planet by the dim star
4. Time of space attacks
5. Dynamics of early Earth
6. Alive water
7. Primary ocean
8. Atmosphere and climate on young Earth
9. Origin of life
10. The oldest evidence of life
11. Mounds and columns: life-made landscape of early Earth
12. Trap for the sunbeams
13. Fossil record of early eukaryotes
14. Geochemical trends in early history of biosphere
15. Rise of oxygen: first ecological catastrophe
16. Who made the weather on early Earth?
17. Snow-ball Earth
18. Amidst the glacial period
19. End of the glacial era: beginning new life
20. Vendian Period in Earth history
21. Oldest known animals
22. Rise of biomineralisation in animals
23. Metazoan impact onto global environment
Glossary
Useful WWW links

According to the historians of science about 2000 species of the fossil plants and 25 000 species of extinct animals have been described by 1850, i.g. nine years before the first edition of Darwin's «Origin of Species» has been published. Practically all described fossil species were from Europe.

Recently about a quarter of million of the fossil species have been described from all the continents. More than 99 per cent of the fossil taxa are described from the Phanerozoic, the latest 550 million of Earth history. Why is this disproportion? And what about the older periods of life development?

Paleontology dealing with the long period of time has exposed the phenomena which are impossible to observe directly. Human life and even the historical time are too short to notice and to recognise slow and steady trends in biosphere masked by numerous reversible fluctuations. That is why the ideas of cyclic, static and linear time of the world have been equally competing through the whole history of the human culture (Grunbaum, 1969). Actually we do not have an actualistic methodological instruments to see difference between the long term events and fluctuations if their duration exceeds the human historical experience.

Fossil record being the integrative result of the long term processes and the fluctuations decreases the noise of the latter and, otherwise, represents more prominently the long and irreversible trends and events. That is why it was paleontology which gave the fourth dimension, namely the Time Arrow, to the recent model of the Nature. Idea of the biological evolution was the most fundamental and valuable discovery of the classical paleontology. Evolutionary approach has spread later on over the most of other areas of the natural sciences.

Introduction of the radiometric dating of rocks has revealed the great age of our planet, e.g. 4.5-5.0 billion years against 4-5 thousand years what was generally believed according to the Christian tradition (interpretation of Bible) and even against the 100 million years, an estimation based on the comparison of the amount of salt in the world ocean and annual input of salt by rivers (Burchfield, 1990).

Radiometric dating first invented by Bertram Boltwood, a Yale physicist, early in 20th century has led to the discovery of the great age of Earth but also the fact that the Phanerozoic makes about 1/9 of the whole geologic history. This fact meant that almost two centuries of the classical paleontology were devoted to the study of a rather small portion of the history of life. And though the Phanerozoic is the best documented part of life history, it is still the tail of the whole story.

Generations of geologists and paleontologists considered the Pre-Phanerozoic rocks as being non-fossiliferous. For example, the whole sequence of the Riphean sedimentary rocks accumulated between 1.6 and 0.6 billion years ago and exposed in the Ural Mountains in Russia has been called for many years as «ancient dumb formations» because these rocks seemed to be «mute» in respect of life. Apparent absence of the fossils in the Precambrian rocks was the reason to give the name Cryptozoic (from Greek «cryptos» that means concealed, covert, unrevealed) for the greatest (about 85%) and the early portion of life history.

Precambrian fossil record became the subject of intensive study relatively late if compared with the age of the paleontology as the special scientific discipline, i.g. effectively after 1960 though the pioneer works can be traced far back to the beginning of the century. Before the middle of our century the term «paleontology of the Precambrian» sounded strange and even later this term was rejected by some paleontologists on the basis that many objects in the Precambrian fossil record have uncertain nature while others, for instance, stromatolites are in fact the remnants of the peculiar geobiocoenoses but not the fossil remains of the organisms. Nevertheless during a few last decades a series of the fundamental discoveries has been made in the Precambrian fossil record which changed dramatically our understanding of life history and life as a phenomenon of a longer time retrospective.

Second half of 20th century is marked by the breakthrough of the man to the space towards the other planets of the solar system. Realisation of this long-term dream costs much human, energetic and material resources. Outstanding discoveries has been made during the space projects though the number of the scientific problems is growing with every new step from the Earth.

This lecture will provide you with an opportunity to travel to another planet, quite unfamiliar and strange. That was our Earth some billions years ago.

1. Reading the past in the stone pages

Earth crust is stratified. Its rocks lie bed above bed, layer by layer keeping the memory of the events which took place on our planet and in the nearest Universe millions and even billions years ago. As a rule, the age of the rocks decreases upward from the beds below to the top beds of the sediments. All exceptions are connected with the distorted areas of the earth crust (like some mountains) due to the movements of the continental plates, folding, great faults and cracks in the crust and other deformation.

It was noticed pretty early, and William Smith was probably the first in 18th century who discovered that different beds of the rocks contain different kinds of the fossil organisms. But what is more important is that while we follow from the lower beds to the upper ones the complexity of the organisms is growing. This fact of a fundamental importance is one of the strongest arguments in favour of the historical change of life on Earth from the simple forms to the complex ones including man.

We shall travel to opposite direction dawn to the origin of life and from this point we shall trace the rise of living systems and the growing diversity of organisms. We shall see how during the enormous time Life has changed an ocean, atmosphere and climate and converted the Earth from the strange and unfamiliar planet down to the place where all we can live.

2. Age of Earth

Since the rise of the human culture the man was puzzled by the questions of the origin of the Universe. Cosmogonies and theories of the Earth formation are as old as Egypt and Summer. Until the last few centuries there were no developed concepts of the geological change. That is why people generally believed that our planet has been created more or less instantaneously, fully formed, and in nearly present condition.

The rise of Christianity has changed the idea of the origin and age of Earth. Genealogies described in the Book of Genesis were considered as a complete and accurate chronicle of the rime since the first day of Creation. A number of attempts has been made to calculate the date of the Creation. The age of Earth, according to the Christian tradition, falls within 4-6 thousand years. The most influentive of the Biblical chronologies was the one by Julius Africanus (200-250 years) who placed the Creation at about 5500 years before Christ. The medieval chronologies, like ones by Luther (1483-1546) and Ussher (1581-1656) gave the age of Earth as 4000 and 4004 years before Christ correspondingly.

Radiometric dating of rocks first invented by Bertram Boltwood, a Yale physicist, early in 20th century has led to the discovery of the great age of Earth. The age of our planet turned out to be about 4.5-5.0 billion years against 4-5 thousand years what was generally believed according to the Christian tradition (interpretation of Bible) and even against the 100 million years, a later estimation based on the comparison of the amount of salt in the world ocean and annual input of salt by rivers.

Discovery of the great age of our planet has revealed the Phanerozoic makes about 1/9 of the whole geologic history. This fact meant that almost two centuries of the classical paleontology were devoted to the study of a rather small portion of the history of life. And though the Phanerozoic is the best documented part of life history, it is still the tail of the whole story.

What is the reason for such a conclusion?

The numbers 4.6-4.9 billion years (though some scientists calculate it firmly bracketed between 4.65 and 4.43 b.y.) are based on the study of the oldest rocks lifted up in some areas of the Earth surface as well on the study of the meteorite materials and the specimens from the Moon. With few exception, the ages of the meteorites fall within the range of 4.5-4.7 billion years. Major method of the study is the isotope radiometric dating.

Atoms of some elements exist in a few forms (isotopes) which have different weight. Some isotopes are radioactive. Rate of decay of the radioactive elements is constant. Neither change in temperature nor gravitation, magnetism or other forces can affect the rate of the radioactive decay. This process is measured by the volumes known as the «half decay period» which is 4 500 000 years for the uranium (U238) loosing half of his mass and converting into the led (Pb206) and 13 900 000 years for the thorium (Th232) converting into the led (Pb208). Uranium, thorium and led use to occur in one the same rocks in minor quantities. Measuring the number of atoms of these elements the scientists are able to calculate the age of the rock.

Thus, our planet is about 4 500 000 000 years old. Within approximately 50 million years of the time the Earth was formed and differentiated into silicate mantle and iron core. What was it look like? When did life first appear on it?

Well, to get an idea on the Earth 4-5 billion years ago we can have a look at one of our neighbours in the Solar System, Venus which has very dense and poisonous atmosphere, high temperature, no liquid water. Life as we know it seems to be impossible there.

3. Unfamiliar planet by the dim star

According to the recent theory of Sun developed by astrophysicists our nearest star slowly increase the intensity of its light through time. The Sun was 25-30% less luminous at the early days of Earth than it is today. But the planet was still hot. Early atmosphere of our planet was consisting of a mixture of nitrogen, carbon dioxide, carbon monoxide, water steam and a few percent hydrogen, hydrogen sulphate and, probably, methane. Concentration of carbon dioxide in early atmosphere must be no less than 100 times more than its present atmospheric level. There was no free oxygen.

Such an atmosphere provided a sufficient greenhouse effect to keep the planet surface hot. The surface temperature on Early Earth could be 80-100°C slowly dropping down to 30-50°C at 3.5-3.2 billion years ago. The first glaciation on the planet is documented some 2.3 billion years ago, and then there was a long period of warm climate until the 800 million years ago. Since that time the glacial periods became the more or less regular events of geological history. We are still living in the Quaternary glacial period though it is hard to imagine living in Italy. But polar ice caps and mountain glaciers are still in place.

4. Time of space attacks

The study of other planets in the solar system may help to reveal an earliest history of Earth. Thus, the research of our Moon, Mars, Mercury and the large satellites of Jupiter and Saturn have provided the evidences that all these planets were intensively bombarded by the other space bodies of different sizes shortly after the planets were formed. The lunar record demonstrates that the intensity of the impacts increased down to its present level about 4 billion years ago.

Our Earth seemed to have the same «star war» history though the crustal motion and intensive erosion of its surface has destroyed the craters that must have formed during that period of time. Accumulation of thick sediments during the subsequent history of our planet (in some places the sedimentary cover may reach tens of kilometres) has masked the traces of these tremendous attacks. The bombardment of the planet by the burning bodies from the space has been the essential part of the environment that gave rise to life. What a noisy cradle of life! What a tremendous birthday firework!

5. Dynamics of early Earth

Earth crust though consisting of the hard rocks occurs in a constant movement. This motion may be very fast in some places and people can observe it as catastrophic phenomena. Earthquakes are the most familiar. But there are very slow and steady movements that can not be seen directly because our life and even the historical time of the human beings are too short to fix minor changes. These changes have been «imprinted» in the composition, distribution, thickness and alternation of the sediments accumulated in the past. Due to the study of the sequences of the rocks geologists are able to reconstruct the long-term processes which took billions and even billions years, duration of time hard to imagine. That is why the sequence of the rocks from the oldest ones that are about 3.8 billion years old to the recent sediments is called as the «geological record».

What does the geological record tell us on the movements of the earth crust? It turns out that in the dimension of the geological time the lithosphere moves as if it is plastic or even liquid substance. Prior 3.8 billion years ago the lithosphere seemed to recycle totally. It was rather thin and the processes of the vertical motion like convection and submergence have reworked the crust many times. Young Earth was dominated by the oceanic lithosphere composed by the dark iron-enriched rocks resembling basalt. Vast and shallow-water ocean has covered the planet. We do not know how great was the primary ocean because very few rocks of that age are preserved.

Major land areas made of the lighter kinds of rocks were small and comprised less than 5% of the recent continental crust. Unstable volcanic islands were formed along the spreading zones (zones of the horizontal extension of the crust that use to be accompanied by active volcanism, hot vents etc). Continental growth by the accretion of the smaller fragments into the large land masses passed through the three major episodes. About 5% of the continental crust were formed 3.3-3.1 billion years ago. 58% of the continental crust were formed later between 2.7 and 2.3 billion years ago, and 33% of the continental crust were formed 2.1-1.6 billion years ago.

Rise of the continents had essentially changed the face of the planet. Being composed of the lighter rocks the continental plates moved (and still move) over the heavier oceanic crust like the ice slabs over the water. Put together east coast of South America and west coast of Africa and you will see how far these continents moved apart since the great supercontinent Gondwana has broken up 150 million years ago.

In case of collision of the continental plates the convergent plate margins are the sites of the formation of major mountain belts (the Alps, Andes, Himalayas, in the distant past, the Urals and Appalachians). Breaks in the plates can produce deep depressions or basins like the Red Sea and the Gulf of California). All these features are typical for the recent face of Earth. But at her early days our planet did not have such sharp contrasts in its relief. It was rather smooth until the rise of the large continental mass. The rise of the «plate tectonics» changed the geographic face of the planet permanently. Half a billion years ago the continents were isolated blocks scattered around the poles. During next 250 million years these isolated continents came into contact to form the «supercontinent» of Pangea. Pangea persisted for about 100 million years and then began to fragment. Modern geography, in particular three north-south oriented continental masses (both Americas, Europe-Africa, and Asia-Australia) with three semi-isolated large ocean basins has developed only within the past 160 million years.

Effect of the continental movements was resulted in the change of the climate, level of the ocean, continuity of the habitats of the organisms both in the sea and, especially, on the land. «Plate tectonic» is the term indicating an important role of the horizontal movements of the Earth crust fragments. Generations of geologists did not believed in the possibility of such a motion leaving the role of the dominant for the vertical movements of the crust. These group of geologists are called «fixists» in the contrary to the «mobilists» who are easy to move the continental plates in their theoretical models. Some geologists are developing a hypothesis of the expanding Earth which help to find the compromise between the two approaches. But the plate tectonic became the paradigm of the modern geology.

Late Archean and Early Proterozoic (2.6-2.2 Ga ago) was passive period of Earth history. Low tectonic activity, weak magmatism and slow accumulation of the pelagic and chemogenic sediments is typical for this time interval. However, between 2.5 and 1.7 there were episodes of intensive rifting accompanied by intensive sedimentation along the passive edges of the continents. This time interval is marked by maximum accumulation of the banded iron formations on the globe. Relative rare occurrences of the volcanic rocks in the time interval 1.8-1.2 billion years ago indicate a quiet regime of the Earth dynamics during that period.

Most of early continents were situated in the equatorial zone. Rather complex motion of the continents was resulted in the formation of great supercontinent Rodinia that has broken up at the end of the Proterozoic. At about 900-800 and 600 million years ago vast marine basins were formed at the places of the earth crust extension between the continent fragments.

6. Alive water

Oldest sediments accumulated in and transported by the water are at least 3.8 billion years old. Geologists discovered the rocks with particles reworked by water like the sand on some recent beach. Some oldest sediments have the desiccation cracks indicating the subaerial environment after the presence of water. This fact means that life was possible at that period because water is the most important condition of life and the component of all living beings (from 60 to 97% of individual mass!!!). Water is the medium of life, and as we know it from the fossil record, ninety percent of the evolutionary time had passed before the higher forms of life could emerge from water to populate the land.

Why water is so important?

Water supports the structure of the cells because of its high content in protoplasm. Water serves as a solvent and as a mean of diffusion, it participates in the most of the biochemical reactions (for example, photosynthesis), it is an environment of reproduction and transportation of the seeds, gametes, larvae of aquatic organisms. Water is the major mean of transportation of the substances important for life activity both inside and outside of the organisms. Water makes possible such important biological processes as the osmosis, turgor and initiation of the seeds in plants, water plays the main role in the processes of osmoregulation and termoregulation in animals and many, many other examples. All it is possible because of unusual qualities of water connected with the small size of its molecules and with their polarity, with the ability of the water molecules to join to each other by means of the hydrogen ties. These ties are not as strong as the ion ties, nevertheless they create various internal structures making water unusual in chemical and physical aspects.

Liquid water is a fleeting substance. It can persist only within a limited range of temperature at reasonably high pressure. Such conditions are extremely rare in the outer space and even in the Solar System. For instance, the temperature of the cold gases of space is about 10-20 times below, and the temperature of the star surface is about 10-20 times above the limits favourable for existence of the liquid water. High pressure favourable for liquid water use to be compatible with the surface gravity of the modest, cool, planet-sized bodies. All these conditions coincided on the surface of Earth due its size and position in the Solar System. This was the greatest lucky circumstance! Liquid water meant that the planet was prepared for life.

One of the macroscopic visible phenomena connected with the unusual qualities of water is the frost on the window glass usually seen during cold Russian winters. Fantastic diversity of forms that resemble flowers, trees, leaves and grass... Early Earth did not know winters, it was hot.

7. Primary ocean

Degassing of the planet interior in Early Archean had a catastrophic character. The mixture of the hot gases had the water as a steam that was the only form of water at the very early days of our Earth. Slow cooling of the planet surface made it possible the liquid water to exist. Later the condensation of the liquid water from the steam became possible in the deeper parts of Earth crust as far as the crust interior decreased its temperature. Russian scientists S.A.Ushakov and O.G.Sorokhtin have calculated recently that maximum input of the water into the ocean took place at the very beginning of the Proterozoic about 2.5 billion years ago.

Though the ocean has appeared pretty early it was fundamentally different from what we see today. The bathymetric contrast was very weak that means there were no great deeps. It was water layer of the moderate depth over the whole globe. Only small and temporary pieces of land, for instance, microcontinents and volcanic islands were scattered over the primary ocean.

Ancient ocean was strongly and permanently stratified. The ocean body consisted of two layers. Lower and thick stratum of water was anoxic and enriched by the dissolved gases and minerals injected by the numerous hydrothermal vents on the bottom. Surface and thin layer was enriched by the gases of the atmosphere, it was slightly oxidised because of the dissociation of the water by the UV light and because of the activity of the first primitive algae producing oxygen by the photosynthesis (in fact they might be the cyanobacteria) Archean ocean did not have the mechanisms of the ventilation of the lower portions of the water like it is today.

Recent ocean does have the mechanisms of the ventilation of the bottom strata of the water. One of the mechanisms is upwelling, the upward movement of the subsurface oceanic water mass toward the surface by the edge of the continents. Upwelling brings diverse nutrients to the surface water layer, for instance, phosphorus, which is necessary to the phytoplankton. Another important mechanism of the water mixing is the penetration of the cold, heavy and oxygen enriched waters from the polar regions of the planet down to the oceanic depth. Both mechanisms maintain the permanent ventilation of the ocean body and help to recycle the vitally important nutrients in the biosphere.

Archean ocean did not have such mechanism to mix the water body. The temperature of the ocean water in Archean was no less than 30-50°C. Even in the polar regions the water was warm enough to prevent the formation of the marine shelf ice. Surface temperature was declining through the early history of Earth but it was still rather high for a long period of time. It was recently calculated by D.R. Lowe that the surface temperature may have decline from 90-100°C at 4.1-4.0 to 30-50°C at 3.5-3.2 billion years ago. Small land areas (volcanic islands and microcontinens) could not cause the upwelling at the scale comparable with the recent one. That is why the surface water layer of Archean ocean was a nutrient-starved desert. Micro-organisms that use to reproduce rapidly in favourable conditions should exhaust the available nutrient very fast. Life oasises could exist in the vicinity of the shalow water hydrothermal vents and volcanic springs that supplied the bacterial communities by the necessary nutrients. We know many examples of such oasises in the recent ocean where bacterial communities produce enormous biomass by the bottom hot springs consuming the chemicals dissolved in the water of the hydrothermal vents. Many other higher and larger organisms consume the bacterial biomass as the major food but some animals live with these bacteria in the simbiosis having them inside by the gills or the blood vessels. Good example is the «black smokers» and fantastically abundant life around them on the bottom of the recent ocean.

Permanently stratified ocean existed untill the Early Proterozoic. Between 2.7 and 2.5 billion years ago large continents were formed and upwelling could become the factor of the global importance promoting the recycling of the biophyle elements in the biosphere. But no less important for the development of life on the planet was that the rise of the large continents has created the vast shallow water habitats along the continental edges or in the marine basins flooding the continens.

It was calculated that in modern ocean the shallow water environments (0-200 meter deep) occupy less than 8% of the ocean floor but the total mass of the organisms living here makes 82.6% of the total biomass of the benthic (bottom dwelling) organisms in the ocean. It is connected with the input of the nutrients from the continent and from the depth of the ocean by the upwelling as well as with possibility of the intensive recycling of the organic matter and other nutrients in the shallow water environment.

Thus we can expect the abundant life in the shallow water habitats in the Precambrian as well. And the fossil record tells it is true. Thick sequences of the carbonate deposits containing the traces of the bacterial activity (stromatolites) are wide spread on the many continents.

8. Atmosphere and climate on young Earth

If we were able to visit our Earth 3-4 billion years ago we would see unfamiliar planet with hazy air and dim Sun. We would not stay there because the life as we know it today was impossible.

Atmosphere of early Earth in Archean was composed mainly of the carbon dioxide, nitrogen, carbon monoxide, water vapour, with little or no hydrogen, hydrogen sulphate, ammonia and, probably, methane. It was a thick, dense atmosphere with pressures 4-5 times higher than at present and with a carbon dioxide concentration probably being 100 to 1000 times its present level (Holland, 1984; Walker, 1985). There was no free oxygen in the air.

According to the recent theory of Sun developed by astrophysicists our nearest star slowly increase the intensity of its light through time. The Sun was 25-30% less luminous at the early days of Earth than it is today. But the planet was still hot.

Atmosphere enriched by the carbon dioxide and water vapour provided a sufficient greenhouse effect to keep the planet surface hot. The surface temperature on Early Earth could be 80-100°C slowly dropping down to 30-50°C at 3.5-3.2 billion years ago. The first extensive glaciation on the planet is documented some 2.3 billion years ago, and then there was a long period of warm climate until some 800 million years ago. Since that time the glacial periods became the more or less regular events of geological history. We are still living in the Quaternary glacial period though it is hard to imagine living in Italy. But polar ice caps and mountain glaciers are still in place.

9. Origin of life

Great Italian naturalist and physician Francesco Redi (1626-1698) has developed a concept that is know as The Redi Principle «all alive is from alive». Up to now the scientists did not documented a case of the origin of the organism from the non-living substance. Biological reproduction is the only way of the appearance of the living beings in the biosphere. It is empirical conclusion.

But our historical experience is very short if compared with the age of Earth.

The idea of life arising from non-life, the idea of spontaneous generation, has been rather popular in medieval ages. In a sense, it was obvious that the worms are from the mud, maggots from decaying meat, and mice from old linen. The roots of the doctrine are coming back to Aristotle, Virgile and Lucretius. Similar ideas on life originated from non-living matter were developing on the East. Rig Veda, for instance, postulated the beginning of life from the primary elements while the Atharva Veda considered the ocean as the cradle of all living beings.

The possibility of sudden emergence of highly ordered systems (biomolecules, membranes etc.) from the chaotic, completely unorganised precursors is being intensively discussed recently by the mathematicians, theoretical physicists, chemists and other scientists. Some efforts are being made in the scientific laboratories.

The first historically documented experimental test of the concept of the spontaneous generation was carried out by Francesco Redi of Florence in 1668. His experiment was as simple as it was decisive. As far as the decaying meat in a jar was covered with a veil of muslin, no flies could lay eggs there, and therefore meat bred no maggots. All life is from the egg! Life is from life!

Well, but the theory of the spontaneous generation of life kept alive for a century or so, partially due to the invention of the microscope by Robert Hooke and Antony Leeuwenhoek (ca. 1660-1700). Many scientists who saw many moving micro-organisms amidst decaying vegetable mass were unable to explain their origin.

Central dogma of chemistry in 18-19 centuries was one established by famous Berzelius (1806): «The generation of the organic compounds from inorganic compounds, outside a living organism, is impossible». Synthesis of urea by Wohler (1828) and subsequent inventions of organic chemistry have destroyed this dogma.

In 1860, the French Academy of Sciences offered a prize to anyone who would provide a decisive experimental result to solve the old controversy. It was Louis Pasteur, who showed that life did not arise spontaneously. The intact swan-neck flask remained sterile, while the one with the broken neck did not. Pasteur announced the result of his experiment to the French public in the following words: «Life is a germ, and a germ is life. Never will the doctrine of spontaneous generation recover from this moral blow». Actually Pasteur has established the central dogma of biology: «The generation of a whole living organism from chemical compounds, outside a living organism, is impossible». Well, great Pasteur may be write in respect of all known forms of life. But what about the very first forms of life?

Since that time the problem of the origin of life was investigated by different scientific disciplines. In the laboratories scientists synthesise numerous organic molecules from the very simple chemicals in the artificial environment which model the conditions that could exist on the early Earth. A.I.Oparin (1926), and J.B.S. Haldane (1929), S.L. Miller (1953) and J.D. Bernal (1967) are making the list (which is far to be completed) of scientists who contributed much into the study of the origin of life both theoretically and experimentally. One of the main conclusions was made (and it may be another central dogma to be abolished sometimes) is that the generation of the first living organisms from inorganic compounds is impossible. There are however many experimental studies showing possibility of abiotic synthesis of complex organic molecules (see for instance, Huber, C., and G. Wдchtershдuser, 1998).

Recently the search for the answer to the major questions on the origin of life has expanded into the space. In the space the scientists discovered numerous organic compounds though no life has been discovered yet. It is interesting that 75% of molecules detected in the interstellar space are organic! This may be the reason to derive life from that chemical diversity. Some scientists, like Jian Oro (University of Huston, USA) suggests the following scenario consisting of the series of the events which are as low-probable as paradoxical: 1) the formation of circumstellar and interstellar organic molecules; 2) The accumulation of the terrestrial volatiles, primary from the comets, lately by accretion processes; 3) non-biological synthesis of biochemical monomers on the primitive Earth; 4) Prebiotic condensation of the monomers and synthesis of the biopolymers; 5) the self-assembly of the prebiotic membranes; 6) the encapsulation of coding and catalytic biomolecules within the structure of a protocell, which is triggered into action, or into life, by the high-energy bond of pyrophosphate or ATF, presumably generating Darwin's ancestral cell.

As Dr. Oro writes in his paper presented for the Nobel Symposium «Early Life on Earth» in Sweden: 'Whether this is an optimist's or a romantic's point of view, it is nonetheless certain that the bottom line is: we are made of stardust!»

One thing that is difficult to agree concerns the first ancestral cell. Charles Darwin wrote in his «Origin of Species»: «All the organic beings which have ever lived on this Earth may be descended from some one primordial form». But if this was true then the first organism should rapidly multiply the number of the individuals by the non-regulated reproduction (no predators), then during very short period of time all the nutrients were exhausted and the habitat was oversaturated by the non-digestible by-products of its life activity. So life should be inhibited.

Good examples of this process can be seen in the lab experiments with the bacteria. Reproductive rate of the bacteria is very high. In the laboratory conditions the bacterial cultures grow on the special nutrient substrates. Every 20 minutes bacterial cell divides giving two daughter cells. During 10 hours one bacterial cell can give 1 000 000 000 descendants. If this process could be unlimited then during 24 hours the number of the descendants could reach 1021 cells with total mass about 4 000 ton. Could you imagine what could happen during next 24 hours?

Reproductive rate of the bacteria in natural environments is essentially lower. But even in the lab experiment the rate of the reproduction is decreasing because of the exhaustion of the food resources and accumulation in the environment of the bacterial metabolite products which have a negative affect onto the bacteria. Natural limitations of the reproductive rates is connected with the predation and the competition with the other groups for the nutrients and energy.

Recent life is actually maintained by the tremendous diversity of the organisms which arrange the recycling of the major nutrients and energy inside the biosphere due to the different biochemical abilities. Thus, life from the very beginning should be diverse enough to maintain this recycling.

We do not know how life originated, whether or not there were multiple origins of life. It is not known what the first cell looked like. The recent discovery of the catalytic ribonucleic acids (RNA) has changed the opinion that the proteins were the original components of the living systems. Scientists change their mind in favour of the primary «RNA World» where RNA stored information and also built biological structures. These two functions are now generally divided between the deoxyribonucleic acid (DNA, the famous «double helix» molecule» that carries genetic information) and proteins, respectively.

What can geology and paleontology tell us on the origin of Life? Not as much as we would like to know. The matter is that we recognise life when it is already exists. Paleontologists are collecting such evidences of life as the oldest fossil organisms, the isotope composition of the oldest sedimentary carbon (carbon cycle in nature is strongly controlled by the organisms), they reconstruct the primary conditions of the environments where early life arose and how life expanded over the globe, they determine the role of life in the change of the environments and climate, the sequence and age of major biological events.

Some hints on the conditions for the origin of life can be obtained from the ecology and physiology of the present day microbes. For instance, many species of archaebacteria live in hot acidic conditions, growing best at temperatures approaching 100°C. These organisms along with some other hyperthermophylic prokaryotes occupy the lowest position on the universal tree of life, a phylogenetic tree of life based reconstructed from the comparative study of the ribosomal RNA (rRNA) of recent organisms (Woese, 1987). It has been suggested this lineage is more ancient than eubacteria, arising during primordial conditions on Earth.

10. The Oldest Evidence of Life

Precambrian palaeontology deals with a great diversity of microfossils: thalli of megascopic algae, animal body fossils and trace fossils, such as bioturbations produced in sediments, stromatolites and other biosedimentary structures, biogenic minerals, kerogens, organic films, biomarkers, and other organic chemofossils. Along with purely palaeobiological methods, during last two decades the geobiological approach develops actively, combining data from a wide range of geological subdisciplines in order to reconstruct the living conditions of the remote past.

The geobiological approach considers the biosphere as an integral system of interacting biotic and abiotic components in which life acts as the most active one. Recent models of Precambrian climates, of the atmosphere and the chemistry of the oceans, of sedimentary processes, etc. include the biota as an active, controlling factor (Fedonkin, 1996). Yet, their study, although often beyond palaeontology as such – for instance, isotope records of carbon and sulphur – sheds additional light on the Precambrian biota and environment.

Precambrian fossils have been collected worldwide from more than 3 000 localities and from rocks accumulated during a tremendously long time covering more than 3 billion years. Yet, so far, only just over 1 300 fossil taxa have been described at the genus level (Schopf and Klein, 1992). Some critically inclined palaeontologists even estimate the number of «real» Precambrian taxa at 500-900. A comparison of these numbers with the millions of recent species suggests that Precambrian life was indeed quite different.

Contrary to classical Phanerozoic palaeontology, the study of the Precambrian history of life concentrates on that of micro-organisms, particularly on that of prokaryotes which dominated most environments during the largest part of the history of the biosphere. Living in the present world, one can hardly imagine that this largest part of the history of life is, in fact, the history of microbes.

Chronologically and stratigraphically, the distribution of the Precambrian fossil localities is very uneven. Ca. 1-5 localities per 100 million years are common for the Early Proterozoic, and ca. 10-25 localities per 100 million years history for the Late Proterozoic. The Archean fossil record appears far poorer. For example, no more than 20 localities with stromatolites are known for all of the Archean. Thus, the number of sites with fossils decreases with the age of the rocks. Moreover, macroscopic organisms appeared relatively late in geological history, and even among these prokaryotic microfossils palaeontologists can only identify largest bacterial cells, particularly cyanobacteria. None of the other prokaryotes, such as the whole Kingdom of Archaebacteria have yet their fossil record because of their small cell size. However, some groups of micro-organisms were present in the Precambrian as judged on products of their activity.

Precambrian microfossils are studied in two kinds of preservation: mummified or organic walled microfossils and mineral pseudomorphs (ones replaced by silica or phosphates are the most usual). Being heterogeneous by their nature (bacteria, lower eukaryotic algae, lower fungii, protozoans, cysts, eggs and egg cases) and by their ecological specialisation the microfossils demonstrate two important trends through the Precambrian fossil record, namely, the growth of diversity and the increasing individual cell size.

Analysis of more than 200 genera of the silicified microfossils shows that they first appear as early as Early Archean (3.56 Ga b.p.) but they become really diverse and abundant after 2.2 Ga b.p., especially in the Late Proterozoic (Riphean and Vendian).

Conservative communities of cyanobacteria inhabiting the extremely shallow water environments do not demonstrate any visible morphological evolution during almost 2.5 Ga. These communities seem to have their morphological and ecological analogues in the recent cyanobacterial communities of sabkha, marshes and lagoons of the arid climatic zones. Extremely rare events in the communities are biological innovations such as the appearance of the spirally-cylindrical Obruchevella in the Late Riphean.

Microorganisms of the open marine environments show more dynamic change through the time. For example, an assemblage of morphologically complex prokaryotes which are common for the Gunflint Iron Formation of Southern Ontario, Canada (Kakabekia, Eoastrion) are known as early as 2.2 Ga b.p. but do not spread later than 1.8 Ga b.p. (Schopf & Klein 1992). Some problematic taxa of fossil microorganisms such as Eosphaera tyleri and Leptoteichos golubicii could be either eukaryotes and eubacteria (Knoll, 1992).

11. Mounds and columns: life-made landscape of early Earth

The most notable evidence of the microbial life activity in the Precambrian are the stromatolites. In the field they looks like the mounds, cones, columns or strange branching tree-like forms made of the thin laminated carbonate rock (limestone or dolomite). Size of the build-ups from a few millimetres to a few meters.

These biogenic sedimentary structures first appear as early as 3.5 billion years ago and became wide spread in Early Proterozoic as the dominating shallow water landscape. Abundance and morphological diversity of the stromatolites increased rapidly during Proterozoic and reached the maximum at about 1.0 billion years ago though frankly we still do not know the nature of this diversity. We can suspect that the general shape of the stromatolites (from flat laminated forms to the columnar branching ones), their laminations and peculiar microstructures may reflect somehow the composition of the bacterial communities and their biogeochemical activity. On the other hand, the builders of the stromatolites could not be indifferent to the factors of the environment (temperature and chemical composition of the water, hydrodynamic regime, depth and light, water turbidity etc.).

The key to the nature of these strange build-ups can be found in the study of the recent bacterial mats which precipitate some amount of the carbonates. Recent bacterial communities of sabkhas, marshes and lagoons of the arid climatic zones might be considered as analogues for the Precambrian stromatolite communities. Very few higher organisms can live here on the crust covered by the salt crystals or below the surface of the mat where almost no oxygen because of the decay processes on the deeper layers of the mat. Diversity of the faunas grazing in microbial mats is very low.

In the contrary to the recent bacterial mats which exist in the limited areas with abnormally high salinity the Precambrian stromatolites have covered the whole continents. Thus the globally dominated landscape through out the most part of the Precambrian life history seemed to be not as attractive as the recent shallow water marine environments full of the diverse and mobile life. It was really different world in the sea: no waving see weeds, no crawling molluscs, no fish... just strange immovable mounds, cones, branches made of carbonate and covered by the green slime.

Diversity of the Precambrian stromatolites might be in part connected with the different abilities of the bacterial communities to compete for the light and the nutrients dissolved in the sea water. Peculiar shape of the stromatolites in facts increased the surface of the contact (and metabolism) between the bacterial communities and the water environment that would be impossible in case of the flat bacterial mat.

Though the stromatolites are not the fossil organisms but are in fact the remnants of the former biogeocoenoses (living organisms and their environment as one system) or even as the by-products of the bacterial communities they are called by the Latin names to identify the formal morphological «genera» and «species» and are arranged into the formal systems according to their general shape, mode of branching, microstructure etc. This is done for the operational needs only though some microbiologists consider the microbial community as an organism with the dispersed cells.

The nature of the stromatolite morphological characters and of the stromatolite taxa is not well understood. However the taxonomy of the stromatolites became an important instrument to search the Precambrian life history. Distinct change in the stromatolite assemblages throughout the Precambrian fossil record discovered empirically by the paleontologists of the Russian biostratigraphic school may reflect both the intrinsic factors (such as change in the structure and composition of the microbial communities, biological and biochemical innovations of the stromatolite-building prokaryotes) and the extrinsic factors (such as the change in the chemistry of the ocean and the atmosphere, global climate and paleogeography, appearance of the eucaryotic groups competing with or feeding on the stromatolite bacterial communities).

12. Trap for the sunbeams

Green substance of the plants, chlorophyll molecules, are able to catch the light energy from the Sun and transform this kind of energy into the chemical energy which helps to create the primary organic compounds. Recent calculations has led to the conclusion that about 75% of the primary organic compounds are created by the plants of the continents and the rest is produced by the algae of the ocean. It is interesting that the surface of the Earth is less than 0.0001% of the surface of the Sun but the surface of the plants exposed to the sun light makes from 0.86 to 4.2% of the size of the Sun surface. This calculation was made first by Vladimir I. Vernadsky more than 60 years ago and the recent recalculations made similar results. Due to the activity of the plants the Earth has accumulated enormous quantities of the Sun energy conserved in the natural oil and gas, coal and other carbonaceous deposits. Making morning coffee or driving car we burn the sun energy which arrived onto the Earth some millions years ago and was trapped by the terrestrial plants or by the aquatic algae.

Oldest cells which might have had the chlorophyll are found in the rocks

about 3,5 billion years old. These fossils are the individual microbial cells which looks like the filamentous blue-green algae (cyanobacteria). Recent cyanobacteria are quite common in many recent environments. But they are especially abundant in those habitats where other, higher organisms can not live. Hypersaline marshes and hot springs are good examples of such habitats. The oldest known organisms seem to live under similar conditions. So both morphologically and ecologically the oldest fossils resemble the cyanobacteria.

The discovery of the oldest cells made in the Warrawoona Group in Western Australia has lead us to a few important conclusions. First, is that life has appeared on Earth prior the 3.5 billion years ago. Second, is that the way from the simple organic molecules which were synthesised abiotically to the first cell took the time no more than 500 million years between the final stage of the heavy bombardment of the inner solar system and the time of the oldest cell (4.0 to 3.5 billion years ago). And the third is that initial diversity was really higher than that preserved in the fossil record. The reason for the last conclusion is based on the understanding that one species of the organism can not live alone for a long period of time. Non restricted reproduction will be resulted in an enormous growth of the population which will consume all the nutrients very soon and pollute the environment by the products of the life activity. Life on Earth is maintained by means of the biological diversity. Heterothophs and decomposers are as necessary as the primary producer of the organic matter.

So they had to live at the same time with the earliest cyanobacteria. The reason why we see nothing but the cyanobacteria may be connected with low preservational potential of the other groups (for instance because of the smaller cell size) and with different environments they lived in. And the last conclusion is that the oxygen as the by-product of the cyanobacteria might have appear in biosphere pretty early. Most of the oxygen was spent immediately in the oxidation of the metal ions and some gases. So the concentration of the oxygen in the atmosphere could not grow much until the major mass of these metals and gases were oxidised and buried in the sediment.

13. Fossil record of early eukaryotes

The origin of the eukaryotic cell seemed to be not just a single, though very important, event but rather a series of events that took place both on the cytological and molecular level in the cell itself, and in the external environment as well. This story seemed to be very long and its early episodes still remain mysterious. The keys to this enigma seem to shift more and more back to the geological past (Knoll, 1992). Quite possible we shall never recognise this moment because the first eukaryotes could appear in very narrow habitats (see below).

Three independent lines of evidences indicate that eukaryotes have appeared not later than 1.7-1.8 Ga before present: 1) palaeontological data; 2) phylogenetic interpretation of the molecular genetics, and 3) taxonomic interpretation of the biomarkers. The study of the fossil record shows slightly varying time of origin of the eukaryotic cell. The chronological distribution of the sizes of organic-walled microfossils dates this moment not later than 1.75 Ga ago (Schopf, 1992). Time distribution of carbonaceous megafossils may extend the record of eukaryotes back to 2 Ga or slightly later (Hofmann, 1994). Eukaryotic biomarker compounds are common in the rocks of about 1.7 Ga old (Summons and Walter, 1990). More recently biomarkers characteristic for cyanobacteria and eukaryotes were discovered recently in Archean rocks from Western Australia (Brocks et al. 1997). These data show that a key attribute of eukaryotic physiology had already evolved 2700 million years ago.

Protein clocks date the origin of eukaryotes by 2 Ga ago (Doolittle et al., 1996). Below we consider the sets of data that were the base for an estimating the age of Eukaryotes.

Large spheroidal microfossils including Chuaria, the one with smooth wall surface, and Trachyhystrichosphaera, the form with processes, appear in the Late Riphean, and the Vendian period saw the rise of true acanthomorph acritarchs (Micrhystridium, Baltisphaeridium, Skiagia), reflecting the general process of eukaryotization of the planktic ecosystems.

Thus, the oldest organic walled microfossils are known from the Late Archean (2.9 Ga b.p.) but they become really diverse and abundant since the beginning of the Riphean (1.6 Ga b.p.) and in the Vendian. General trends in the history of this group of microfossils are the increase of the upper size limit of the sphaeromorphic and filamentous forms, the growing morphologic complexity, morphological innovations (appearance of acanthomorph and marginate acritarchs, etc.) and the growth of taxonomic diversity. On the background of the general trends mentioned above one can see at least three global events reflected in the paleontological record of microorganisms.

The first event is connected with the disappearance of the Gunflint-type microbiota in the Early Proterozoic (about 1.8 Ga b.p.) which was replaced by the biota of morphologically complex organic walled sphaeromorph acritarchs in the Lower Riphean (1.6 Ga b.p.). The second event is relaled to the rise of acanthomorph acritarchs (microorganisms with the the spines, tubes and other outgrowing structures like in Trachyhystrichosphaera), polygonal forms (Octoedryxium), ball-like (Tortunema) and other morphologically complex microfossils in the late Middle Riphean. And the third event is connected with the Vendian radiation of microorganisms marked by the appearance of Bavlinella, Micrhystridium, Leiomarginata and others with complex morphology.

Along with the biological innovations there were some trends and events in the aspect of the upper size limit of the cell as well as the change in diversity and ecology of the microorganisms. Forms having 60-100 µm in diameter have appeared no later than 1850 Ma ago, the forms with diameter of 200-600 µm – not later than 1400 Ma ago, the forms with diameter more than 1000 µm including morphologically complex acritarchs have appeared not later than 1050 Ma ago, and large sphaeromorph microfossils of 1-7 mm in diameter have become widespread about 850 Ma b.p.

Taking into account that the upper size limit of the prokaryotic cell is near 60 µm, the size trend described above may reflect the evolution of eukaryotes in the Precambrian. On the other hand, we know that the lower size limit of eukaryotic cell is about 20µm (and occasionally, may be as small as 1 µm), and thus their real history might begin far prior to 1850 Ma before present. But as far as we consider the palaeontologcal and palaeobiochemical evodence, the eukaryotes were the part of ecosystems not later than 1700-1800 Ma ago, with reservation that eukaryotic cell has already formed by 2 Ga ago.

Size trend in the realm of the eukaryotic microfossils may reflect the growing size of their primary biotopes dependent on the degree and stability of oxidation. Eukaryotes, and in particular microscopic algae seem to evolve rather slowly from the moment of their appearance up to 1200 Ma b.p. However, the later diversity of eukaryotes has been growing very rapidly, being probably connected with the development of the sexual reproduction in this group (Schopf 1992).

A maximum in abundance and taxonomic diversity of microfossils occured at the time interval between 950 and 850 Ma b.p. During this period of time the megasphaeromorphs (up to a few millimeter in diameter) were dominating among the phytoplankton but these gigantic microfossils became extinct prior to the end of the Proterozoic. After this 950-850 Ma maximum one can observe a sharp decrease in abundance and diversity of microfossils (Vidal & Knoll 1982, 1983). Interpreted as the earliest recorded extinction events in the Neoproterozoic protists, this collapse of the phytoplankton can be explained by the negative effect of CO₂ concentration decrease and of the growing O₂ concentration in the atmosphere upon the enzymes which controlled the photosynthesis. This hypothesis has an experimental confirmation by the study of recent populations of microalgae (Schopf 1992).

The change in CO₂/O₂ balance might be connected with the glaciations which took place in the interval between 850 and 650 Ma b.p. Unsuitable conditions of preservation offered by widespread glacial deposit may have influenced the fossil record of the microorganisms as well (Vidal 1992).

Between 850 Ma b.p. and the beginning of the Cambrian most of the planktic taxa with cell diameter more than 600 µm became extinct as well as the numerous groups of the morphologically complex acritarchs. After the great Varanger glacial period (650-630 Ma b.p.), a new radiation of the eukaryotic organisms took place but did not reach the former diversity which existed prior to the Vendian.

During the most of the Proterozoic the morphological diversity of acritarchs remained limited by simple sphaeroidal morphology. Biological interpretation of such forms is always doubtful. First moderately ornamented forms, including acantomorph (spiny) acritarchs appeared about 1 Ga ago or slightly earlier. Some of these later microorganisms might be interpreted as prasinophyte green algae (Knoll, 1994). The oldest multicellular protists that was assigned with confidence to extant phylum was a bangiophyte red algae preserved in Proterozoic silicified carbonates of Arctic Canada (Butterfield et al., 1990). The age of the host rocks is supposed to be between 950 and 1260 Ma. Multicellular green and probably chromophyte alagae are found in the deposits 950-750 Ma old Siberia (Hermann, 1990), Spitsbergen (Butterfield et al., 1988) and other regions of the world.

There were some ecological trends and events in the history of microorganism, such as an apparent increase in diversity of the planktic eukaryotic forms and a parallel decrease of diversity and the space distribution of benthic prokaryotes in the interval between 1500 and 1000 Ma b.p., followed by some increase in diversity of both procaryots and eukaryotes between 1000 and 850 Ma b.p. (Schopf 1992).

Large acanthomorph acritarchs, a few hundred microns in diameter and widespread between 950 and 800 Ma b.p., disappeared prior to the end of the Proterozoic. Diverse acanthomorph acritarchs which appeared during the Early Cambrian radiation of the phytoplankton did not exceed 75 µm in diameter.

Size increase in the acritarchs during Late Proterozoic may be interpreted as strategy of defense from planktotrophic organism, comparable by their body size to protozoans or even metazoan larvae (Burzin 1987). However, an increase of the cell size in the benthic microorganisms during the same period of time may indicate different causes.

An opposite trend can be observed during the time after 850 Ma b.p., i.e. the disappearance of most planktic taxa with a cell size more than 600 µm, and an explosive radiation of small acanthomorph acritarchs at the Atdabanian of the Early Cambrian may reflect a natural selection in favour of the forms with high surface/volume ratio. There are two ways to increase this ratio, namely to decrease the cell size and to develop special devices increasing the cell surface (spines, processes, ornamentations, sculptured elements etc.). Lower Cambrian acritarchs, demonstrating both morphological strategies, obviously have had increased abilities to extract the biophile matters per a volume unit of the cell.

14.Geochemical trends in early history of biosphere

The best way to feel the difference between the early Archean and present day atmosphere is to compare the composition of volcanic gas and normal air. These two extremes show us the major trend in the evolution of atmosphere during over 4 billion years. Living organisms are the major factor in this dramatic change being responsible, for instance, for the low concentration of carbon dioxide and high concentration of free oxygen in atmosphere. Concentration of these two gases influence the rate of weathering of the rocks and availability of some chemical elements in the biosphere. Rate of weathering essential for the cycling of the biophyle elements in the biosphere has been also influenced by the growing erosion and sedimentation rate due to the increasing gypsometric contrast of the planetary relief and because of shift in the composition of the major feeding provinces for the sedimentary basins – from the dark basic rocks to the less resistant acid and sedimentary rocks.

There are many other examples of geochemical trends and events related directly or indirectly to the life activity of biota.

Thus in the Precambrian ocean (600 million years ago and earlier) silica simply precipitated from the seawater and formed layers or nodules in the sediments. Rise of organisms capable to suck the dissolved silica from the water and use it in the production of their skeletons (sponges, radiolarians, and more recently, diatoms) has converted Recent ocean into almost a silica vacuum, seawater is strongly undersaturated with this element (Webstroek, 2000).

Many authors indicate decrease of Mg and increase of Ca component in the composition of the carbonate sediments through time and growing biological control over the carbonate precipitation in the ocean. There are so called «extinct sediments» typical for early geological history only, for example, uranium and gold bearing conglomerates, banded iron formations, laminated copper deposits in the clastic rocks, Pb-Zn ore in shales and carbonates, sedimentary Mn ore and phosphorites. These ore deposits reflect the geochemical peculiarity of biosphere of the remote past and biochemical activity of biota we have yet to decode.

15. Rise of oxygen: First ecological catastrophe

Tolerance of the bacteria to the wide range of the temperature, pressure, radiation, salinity and other environmental factors exceeds the tolerance limits of the higher organisms.

That is why one can wonder on a remarkable reduction of the stromatolites at the end of the Proterozoic. Stromatolites as the globally dominating biogenic landscape of the shallow water environments demonstrate marked decline in their abundance and diversity after 1.0 billion years ago and especially dramatic after 700 million years ago.

The possible cause for the stromatolite decline might be: a) negative effect of the new evolved grazing and burrowing metazoans (Awramik, 1971; Walter and Heys, 1985); b) appearance of the eucaryotic algae competing with the cyanobacteria for the nutrients, habitats and light (Monty, 1974); c) the major low-stands in the sea level (Gebelein, 1976); negative affect of the growing concentration of the biogenic oxygen upon the bacterial stromatolite-building communities (Krylov, 1988); d) decreasing carbonate saturation of the sea water during the Late Proterozoic (Grotzinger, 1990); e) climate change, in particular African Glacial Era and its paleogeographic and geochemical consequences (Semikhatov and Raaben, 1993). In fact, all these hypotheses may be complementary.

Decline of the stromatolites is in fact the very top of the iceberg. Rise of free oxygen in atmosphere and hydrosphere has essentially ceased the chemical diversity of the environments on the globe that has affected negatively the prokaryotic, and in particular, anaerobic biota.

Through the most of the geological history cyanobacteria and eukaryotic algae were the major photosynthesising organisms. Living inside the photic zone of the sea water and in the fresh water basins on the continents they consumed CO2 and released O2 to the atmosphere. But the oxygen, that is so important for the animal respiration, would never accumulated till such a high concentration it is today if the carbon would stay in the ecosystem in the same amount. Major source of carbon for the photosynthesisers was and still is carbon dioxide. Enormous volumes of carbon was removed from fast recycling and oxidation because it was buried for ever on the continents as the carbonate rocks (limestone, dolomite, siderite etc.) and as a dead organic matter converted later into the hydrocarbons such as oil and carbonaceous rocks.

Most of the specialists are in agreement that atmospheric oxygen concentrations prior to about 2.4 Ga B.P. were 10-14% PAL (present atmospheric level) or below. A few lines of evidence make this hypothesis correct. Nature of the uranium ore, presence of detrital unoxidised pyrite, paucity of oxidised redbeds and the relatively narrow range of isotopic composition of Archean sulphides and sulphates are used as evidence of an extremely low oxygen level prior to 2.4 Ga. Analysis of data from the study of the oxidation state of paleosols (fossil soils), the trace metal content of black shales, the age distribution of the banded iron formations, and the evolution of eukaryotes indicate that oxygen content of the atmosphere increased dramatically between 2.2 and 1.9 Ga B.P., from less than 1% PAL to about 15-20% PAL.

During the late part of Proterozoic after 1.6 Ga ago the rate of erosional weathering and the rate of accumulation of organic carbon were extremely low. However, strong variations of the isotopic composition of carbon and strontium in the Neoproterozoic rocks 850-545 Ma old seem to reflect the cardinal environmental change. This was time of greatest cold ever affected our planet.

After the Varanger glaciation which took place about 620-650 Ma B.P.. the rate of organic carbon burial increased rapidly to the volumes 2-4 times exceeding the average rate of organic carbon burial in the recent marine ecosystems. Conservation of the carbon in the sediment has been resulted in the rapid growth of an oxygen input into the atmosphere. This event took place immediately before the appearance of the megascopic forms of Ediacara fauna – oldest known fossil animals – in the Vendian Period about 600 million years ago. One may not exclude that the Vendian explosion of animal life was connected directly with the rapid growth of the free oxygen content in the biosphere. However, for many archaic forms of life adapted to the low concentration of oxygen or even to anoxic environment, this change was fatal.

Probably, the most negative consequence of the oxygenation of the biosphere was decreasing biogeochemical diversity of the environments. Lesser number of chemical elements became available for life due to oxidation. The fact that many biochemical reactions inside the the eukaryotic cell require strictly anoxia may show us that the rise of the cell complexity could be related to the neccesity to protect the internal environment from the oxygen.

The growing concentration of the atmospheric oxygen was accompanied by a rise in atmospheric ozone and the development of an efficient screen against solar ultraviolet radiation. some calculations predict that the an ozone UV screen was fully in place by 1.7 Ga that was especially important for the organisms colonising the land environments rather than for the marine forms.

16. Who made the weather on Early Earth?

Stromatolites, these stony columns and mounds built by cyanobacteria and other microbes might have had a tremendous impact on the Earth climate. As you know from above, stromatolite bacterial communities can make or trap sediment particles and cement then together. Thus, particle by particle, layer by layer these bacteria have accumulated thick strata of the carbonate rocks which cover extensive areas of the continents. Stromatolite bearing deposits can reach many kilometres in some regions like Urals or Siberia.

Fossil stromatolites date back some 3.5 billion years ago. About half a billion years ago they have almost disappear but during 3 billion years the carbonate sediment has been accumulating due to the bacterial activity. To make the carbonate rock like in stromatolites those bacteria were able to get ions of Ca and Mg from the see water and carbon dioxide from the atmosphere. Thus, these thick strata of the stromatolite rocks has conserved huge volumes of the carbon dioxide.Carbon dioxide is one of the green-house gases which can trap the infrared light reflected from the Earth surface and, thus, keep the atmosphere warm.

Due to activity of the cyanobacterial communities (and of the eukaryotic phytoplankton and algae later on) the concentration of the carbon dioxide dropped down to the level of the very sensitive balance about 850 million years ago. Since that time the climate became much closer to what we see now and the glacial periods became the normal and more or less regular episodes in the history of the biosphere. Role of the eukaryotic algae in the regulation of the atmospheric chemistry was growing during the Proterozoic and later on.

Ice caps and climatic zonation turned out to be extremely useful for the life as a whole. The major value of the glacial periods was an active circulation of the ocean water, ventilation of the deep ocean floor and recycling of the nutrients in the oceanic water.

17. Snowball Earth

Conservation of carbon in the sediments decreased the greenhouse effect of the primarily thick atmosphere that was resulted in cooling of biosphere. Though the first extensive glaciation is documented around 2.2 Ga, the ice periods became quite common during the very late part of Earth history. Our biosphere went through an extremely cold period late in its Neoproterozoic history. At least four (or perhaps five) severe glaciations developed on the planet between 750 Ma and 550 Ma. Neoproterozoic series of glacial periods caused the phenomena that Joe Kirschvink called Snowball Earth. Ice covered the whole planet from the poles to the equator with the tremendous consequences for life. But how convincing is this hypothesis? Every glacial episode has left its prominent signal in the geological record. The most remarkable evidence of the past glaciations is that from sedimentology: tillite or diamiktite, varve, drop stone and other (Table 3).

Geologists are able to read these evidence and reconstruct the magnitude of the glaciation. In 1964 W.B.Harland suggested the hypothesis on the severe glacial age of a global scale be the end of the Proterozoic. In late 70th N.M. Chumakov identified four major glacial episodes between 850 and 550 Ma ago. The most terrifying was Varanger glaciation some 650-620 Ma ago, that immediately preceded the first mass appearance of the animals in the history of biosphere. Enormous magnitude of this glacial was inferred from a wide geographic distribution of the tillites of this age all over the globe.

Climatic changes seemed to be very fast. Glacial deposits very often contain the fragments of the carbonate rocks, including the stromatolites, or tillites are often directly overlain by the carbonate deposits which are commonly precipitate in warm-water basins. Remarkably, that iron formations associated with the glacial rocks reappear again after a 1-billion-year of absence in the geological record. J. Kirschvink supposes that, sealed by ice, the ocean became anoxic and rich in dissolved ferrous iron.

In addition to the special kinds of the sediments, the glacial periods left some isotope evidence in the geological record. The most commonly used is the carbon isotope record: unusual C¹³ enrichment in carbonates and organic matter, punctuated by excursion to unusually low delta C¹³ values. These strong negative excursions are associated with glacial periods. These shifts in the isotopic ratio of carbon look enormous compared with the any analogous excursions in the preceding 1.2 billion years of the Proterozoic and in the whole Phanerozoic eon. Isotope shifts of such magnitude show really dramatic changes in the global ecosystem, and first of all, collapse of biological productivity in the surface ocean for millions of years.

According to the Snow Ball Earth model, supported by some prominent geologists, almost total surface of the globe was covered by ice shield over a mile thick. This picture looks especially shocking because most of the Precambrian continents covered by ice occupied very low latitudes at that time. Global shield of ice could stay in place for a very long time, from 4 to 35 million years according to various estimation.

18. Amidst the glacial period

Glacial periods are normally accompanied by the strong environmental shifts including radical climatic and geographic changes. Many, but not all, of the Vendian continents were covered by an ice shield over a mile thick. Permafrost was wide spread over a great areas. Those continents by equator that were free of ice were dry deserts. Water erosion of land, and input of the nutrients to the ocean, were close to zero. Life on the frozen continents reduced to the limited habitats like hydrotherms and subsurface realm where liquid ground water could be present. Marine environments have been affected by the vast shelf glacials and by millions of cubic kilometres of the floating ice. Sea level dropped 250-300 meter and fluctuated strongly during a few millions of the glacial period. Most of the shelf was exposed to the air. Area of the shallow water habitats on the shelf reduced down to the small strip at the edge of the platforms. Along with the cold climate, vast temperature gradients, change in atmosphere circulation, and increasing frequency of the storm events took place. These glaciations were accompanied by the glacioeustatic drop of the see level, regressive tendencies of the sea, decreasing of the shelf areas, growth of the land surface, radical shrinkage of the areas of benthic environments and the shift of the life to the pelagic realm. Geographic isolation of the species might increase because of growing land surface and climatic differentiation. Low content of the biophile elements (nutrients) in the open ocean might have been partially compensated by the input of the metabolites from the deeper ocean: heavy cold water was sinking down and pushed up the bottom water rich with the dissolved nutrients. The same mechanism cause the «ventilation» of the ocean: sinking water brought oxygen down the depth.

All these phenomena must have affected biota. The most negative affect of glaciations was the destruction of the benthic shallow water habitats of the shelf. Just to realise how important are the shallow water habitats we remind the fact that 83% of the total benthic biomass in the recent ocean is concentrated on less than 8% of the bottom surface, i.e. on the shelf. So, during the Varanger glaciation the marine life was concentrated in the pelagic waters which are nutrient poor and ecological homogeneous. Beside, large part of the phytoplankton was subjected to elimination being isolated from the sunlight. The only kind of communities that might not notice the global cold were those organisms existing around the deep ocean hot vents. But we do not know whether metazoans were there or not.

That was a prelude to the Vendian radiation of those metazoans who survived under the severe conditions.

19. End of the glacial era: beginning new life

Glaciation is a peculiar process. There are good explanations why it starts, but it is hard to understand why it is over. Glacier is a mass of ice, formed by recrystallization of snow that flows forward or has flowed at some past time under influence of gravity. While the area of ice is growing, albedo of the planet increases, climate is becoming cooler that cause further growth of ice mass. Why then all the glacial periods ended with the melting of ice shields, rise of the sea level and warming climate?

At some point the feed back processes turn on. Thus, while the glacial shield is growing over the globe the water evaporation decreased due to cold temperature and shrinking area of open water. So, the snow precipitation decreases and the ice shield grows very slowly. Meanwhile, carbon dioxide produced by the volcanic activity accumulates in the atmosphere because the major consumers, phytoplankton, algae and terrestrial flora are oppressed by ice. So, accumulation of CO₂ can be very fast that cause the greenhouse effect, rise of global temperature, and rapid melting of ice. It looks like that the growth and shrinking of the ice caps became the part of the global homeostasis, self-regulation of climate due to the feed-back processes.

P. Hoffman and A. Kaufman have analysed negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia. They came up with a conclusion that biological productivity in the surface ocean collapsed for millions of years. This collapse was related to a global glaciation. Remarkably, that this severe glaciation ended abruptly due to volcanic outgassing that raised atmospheric carbon dioxide to about 350 times the modern level! The change of the global environment from the icehouse to greenhouse was very fast. This hypothesis explains the poorly understood nature of the cap dolomites observed globally above the glacial deposits. It seemed likely that huge volume of carbon dioxide was transferred from the atmosphere into the ocean. Hypersaturation of the sea water with CO2 was resulted in the rapid precipitation of carbonate sediments in warm surface waters.

Many aspects of the Snow Ball model remain uncertain. Specialists debate the number of the glacial episodes and their timing on every continent. One of the recent hypotheses suggests that the axis of Earth rotation was inclined to the ecliptic so that the poles got more solar energy than the equator where the most of the continents were situated during the Neoproterozoic. All these hypotheses stimulate new research and promote the better understanding of the environments that gave birth to the organisms of the highest biological complexity.

20.Vendian Period in Earth history

Vendian Period as the terminal period of the Proterozoic was discovered by Russian geologist B.S.Sokolov (1952). Vendian fossil record is marked by a mass appearance of the megascopic multicellular animals and algae. Two global events make the time frame for the Vendian Period: Varanger Glaciation that began about 620 Ma ago the Cambrian evolutionary explosion 543 Ma ago. Varanger Glacial Period was the most severe one in the series of glaciations that took place during Neoproterozoic, and as N.M. Chumakov thinks, it was the greatest cold ever affect the biosphere. Varanger Ice Age is marked by the wide spread glacial deposits all over the world even at low paleolatitudes. These deposits begin the geological record of the Vendian Geological System, the whole amount of the rocks formed since the glacial period till the Cambrian. What do they tell us about the global environment which harboured the first known animals?

Huge ice shield melted fast and, correspondingly, sea level elevated. Various estimation of the sea level rise are in between 120 up to 500 m. Post-Varanger glacio-eustatic transgression on the vast continent eroded by the glacial buldosering saw the rapid radiation of megascopic soft-bodied invertebrates, algae and all sorts of micro-organisms in the huge new habitats. New continental basins provided enormous ecological opportunities for the new colonisers, and those took their chance. Geologically, it was short but dramatic episode in the history of life. We cannot yet detect the details of this colonisation and but at least two aspects of it are pretty clear: 1) post-Varanger metazoan radiation was almost instantaneous in the geological sense, and 2) there were no large geographic barriers for the global expansion and biological connection due to high sea level. Vendian was the first period when animals really manifested their important role in the global ecosystem. All of them were the direct descendants of those who survived the great cold of the Varanger Ice Age. This hard and cryptic period of their earlier history is yet to be recovered.

The end of Proterozoic eon was marked by extensive tectonic activity. Break-up of the supercontinent Paleopangea (or Rodinia), rapidly subsiding extensional basins were culminated in the opening of Iapetus paleoocean and other ocean basins during the Vendian Period. Collision and amalgamation of smaller continental plates into what became Gondwana continent in Paleozoic was also a part of the tectonic evolution in the Vendian. J. Kirschvink and D. Evans think that at that time continental plates moved at speeds exceeding several feet per year! This is much above the «plate-tectonic speed limits» known at present day that is a few inches per year. These tremendous processes changed paleogeographic situation on the globe due to the continental dispersal and aggregation, opening and close up of the vast basins, transgression of the sea on the continent. Ocean currents, places of the highest bioproduction, geographic isolation and connection, ways of the distribution of larvae etc, everything was subjected to constant change. Active tectonics was accompanied by intensive volcanism documented in numerous lava flows and ash beds in the Vendian deposits in many places over the globe. Hydrothermal activity associated with the tectonics injected huge mass of iron, manganese and other metals into ocean body that could increase the primary productivity of bacteria and phytoplankton but could decrease the oxygen content, especially in the deeper parts of the ocean. Rise of the mountains and increased erosion of the continents documented by a major shift in the isotopic record of the seawater strontium to unusually high ratio (Sr⁸⁷/Sr⁸⁶ ca. 0.7085) may indicate essential input of metabolites from the land.

Vendian Period was marked by further growth of oxygen concentration in atmosphere due to very high primary production of phytoplankton in the oceans and vast marine basins that cover all the continents. Important aspect of the growing oxygenation was massive burial of organic carbon in the sediments protected from the erosion, oxidation and recycling. This hypothesis is supported by the carbon and sulphur isotopic record. There are persistent indications to the change of the seawater chemistry during the Vendian Period, in particular, decline Mg/Ca ratio and increase in phosphorus, the «major biophile element» close to the end of the period.

Mass appearance of the Vendian fauna was predated by the Big Bang of protistan evolution. Major radiation of planktonic micro-organisms in late Riphean reached its peak about 700 Ma ago. However soon after the major decline is documented in the history of the phytoplankton that, according to G. Vidal, might be related to the Varanger Glacial Age. Vendian sow new diversification of the protistans though they did not get the former diversity until the Cambrian.

21. Oldest known animals

Since the down of paleontology the specialists failed to find something like animals fossil below the Cambrian deposits that are full of trilobites and other extinct creatures. One of the most common explanations was presumed small size of ancestors First animals must have been tiny beasts that could not be preserved. Many authors claim that metazoans could not attain large size before the atmospheric oxygen concentration crossed the some minimum threshold that is 5-10% of PAL. It sounds logical but reality is quite opposite. What really amazes everybody who first time see the oldest known animals is their large body size. Some are really gigantic for the invertebrates, especially for the earliest ones. That is something we have yet to understand. Forms like medusiform Ediacaria up to a meter in diameter, leaf-like segmented Dickinsonia up to a meter long, and frond-like Charnia and Charniodiscus of even greater size make a shocking contrast to the small shelly fossils of a millimetre scale that appear in abundance in the Lower Cambrian fossil record.

Second revelation is unexpectedly high diversity of the body forms. Geometry of the oldest animal fossils resemble dicks and cones, combs and bushes, feathers and leaves, tubes and rosettes, eggs and bags, cups and pots. In a sense there is nothing special in this list of the forms because all of them are common among the recent marine invertebrates.

Third unusual aspect is the soft-bodied nature of the majority of the Vendian animals. Soft-bodied forms like sea anemone and medusa were common while the animals with the hard parts are extremely rare. There is some evidence of the chitin-like parts (Redkinia), organic tubes (Paleolina and Saarina), or organic shell (Kimberella). We suspect the presence of a thin carapace in some organisms but there is virtually very few data on the mineralised skeleton. Tubular calcareous Cloudina from the Late Vendian rocks in Namibia and recently discovered sponges with the skeleton consisting of the siliceous monaxonial spicules from the Early Vendian (about 580 Ma old) rocks of China are pretty much an exception. Soft-bodied nature of the oldest animals poses very serious question on how preserved in the fossil record.

Another impression when one looks at the Vendian fossil is that most of the organisms were flat like pancake or blanket. Many speculations and far-going conclusions were made on this matter. But this is false impression related to the preservation of the animals. In most cases the soft tissues decayed and left nothing but imprint or cast of the body that was voluminous and fleshy. Recent discoveries of the fossils preserved as a core (three-dimensionally) Flat shape of the fossils is explained by the body volume decrease during the process of the decomposition under the weight of the sediment above the buried body as well as by the process of the subsequent compaction of the sediment.

Many questions related to the oldest animals depend on our capacity to read the fossil record, to decode the complex processes that took place between the time when the animal was alive and the time when it became a fossil. Sudden appearance of diverse Ediacara-type metazoan fossils in the Vendian rocks indicates the probability of even older history of animal life. This suggestion is consistent with the «molecular clock» models that predict an earlier origin of multicellular animals (from 1700 to 700 million years ago according to different authors). Proterozoic fossil record still keeps the secret of the origin of animals. Recent discoveries of the tubular or worm-like metazoan fossils in the rocks over 700 miliion years old (Sun et al., 1986; Gniliovskaya, 1998; Gniliovskaya et al., 2000), though rare, show the potential of the late Proterozoic fossil record to reveal the roots of the metazoan phylogenetic tree.

22. Rise of biomineralisation in animals

Structural definition of the skeleton in general is impossible and useless: its nature is so different when one compare the complex perforated shell of tiny radiolarians and massive bones of a dinosaur. At the beginning of the Cambrian Period we see explosive skeletonization of the multicellular animals. But in the history of biomineralisation the Cambrian was not the greatest events. Middle Ordovician saw the rise of the far more large forms with the massive skeleton.

Far more interesting is the functional and genetic aspects of the skeleton. Biomineral skeleton is not so a product but rather a process, very strictly balanced due to the feed-back loops in the organismal biochemistry. However, I would like to remind you that majority of the phyla remains unmineralized. Why?

By the beginning of the Cambrian Period some 544 Ma ago phosphatic, carbonate and siliceous biomineralization have risen almost simultaneously in a variety of the invertebrates. This coincidence make us to seek for an external, ecological causes of this event. Among the major of those we would indicate: colonisation of the warm carbonate basins, acquisition of the photosynthesising algae, growing length of the trophic chains, expansion of the photic zone due to biofiltering and fecal pellet transport, predator-grazer growing pressure as a factor of selection (Table 4).

23. Metazoan impact onto global environment

An explosive radiation of invertebrates was accompanied by an appearance of new physiologies that could affect strongly the environments in the Late Proterozoic and Early Cambrian. This affect could be especially strong during the periods of the growing abundance of the metazoans. The most important consequences for the biosphere as a whole could have been resulted from the following phenomena connected with the life activity of the metazoans:

1. Bioturbation of the sediment has been resulted in its better aeration which, in its turn, has allowed the progressive colonisation of the sediment down below its surface by a wider variety of the aerobic organisms. Both aeration and increasing life activity inside the sediment has promoted the recycling of the metabolites in the marine ecosystems. On the other hand, the bioturbation disrupted the substrate stability necessary for the formation of such biogenic structures as the stromatolites.

2. Biomineralization has been resulted in the formation of the bioclastic deposits and in the creation (with other non-metazoan groups of organisms) of the reefs as mechanically stable biotope and a special ecosystem with great diversity of habitats.

3. Filtration of the ocean water by the actively filtering organisms has a great impact on the global ocean ecosystem. The rise of active suspension feeding or the filtering in metazoans at the beginning of the Cambrian has radically changed the properties of the sediment and water habitats. The study of the filtering in the marine planktic crustaceans (Vinberg, 1967) has revealed that during 24 hours 1 milligram of living weight of the organism is capable to filter 360 milliliters of water. Calculations made by Bogorov (1974) demonstrated that the volume of water equal to the volume of the world ocean is filtering during the half of a year. The most inhabited portion of the ocean water (0-500 meter depth) is filtering by the organisms during 20 days. One fresh example: it has been estimated recently by Dankers (1993) that on average the western Datch Wadden Sea contains 294 x 106 kg of mussels (fresh weight). This population would pump 920 x 106 m³ of water every day. The western Wadden Sea contains 4,500 x 106 m³ of water at low tide, the volume, that would be cleaned biologically during about 5 days by the mussels only. But there are other filter feeders as well. Environmentally important aspect of the biofiltering is that non-digested fine particles packed into the fecal pellets precipitate down to the bottom far more rapidly than separate fine particles suspended in the sea water.

The rise of active filter-feeding organisms such as sponges, brachiopods, mollusks, some arthropods and echinoderms in the Early Cambrian should make the ocean water clear and the photic zone deeper thus providing additional opportunities for the photosynthesizing organisms to occupy lower levels of the water column and deeper benthic environments. The expansion of the photic zone thus could have resulted in a better oxygenation of the pelagic and bottom habitats via the activity of the chlorophyll-bearing organisms. Removal of the fine particles from the sea water and packing them into the pellets should have increased the permeability of the sediment that could lead to a better aeration and colonisation of the subsurface bottom environments and to more rapid oxidation of the buried organic carbon.

4. Growing length of the trophic chains during the Vendian and in particular in the Cambrian has decreased the loss of the major biophile elements and of energy from the ecosystems because of more efficient biological recycling. That could lead to the global oligotrophication of the

ocean waters. This hypothesis is consistent with the general decrease in the buried organic carbon during the Early Cambrian time as well as by the radiation of the Early Cambrian phytoplankton having the external processes, spines, ornamentation and very small cell size. All these morphological peculiarities of the Early Cambrian phytoplankton can be interpreted as the mean to develop the very large surface-volume ratio that could give some advantage in the oligotrophic environment. One could remind that 70% of the biomass and 80% of the chlorophyll belong to the picoplankton in the oligotrophic waters of the Recent ocean. Oligotrophication of the Early Cambrian ocean made inefficient some of the feeding habits so common in the Vendian metazoans (for instance, passive sedimentation of many sedentary suspension feeders). This factor may has been the cause for an elimination of some Ediacara species from the shallow marine habitats in the Early Cambrian or even slightly earlier.

Acritarchs. Mummified or organic-walled microfossils of uncertain nature and unclear relationship to existing and fossil organisms (perhaps the unicell algae but some of them may be lower fungii, protozoans, cysts, eggs or egg cases). Especially common in the Precambrian time.

Annelids, Annelida (phylum). Worms whose body plans include division of the body into similar segments and other characteristic-shared features. Recent representatives are earthworms, leeches, many marine polychaete worms. The root of this phylum goes down to the Precambrian.

Archaebacteria. The archaebacteria (also called the Archaea) are a subkingdom of bacteria considered to be ancient compared to other bacterial kingdoms, and possibly the most ancient life forms and the ancestors of all eukaryotes. They typically exist in extreme environments, and include the methane-producing bacteria (methanogens), the «salt-loving» bacteria (halophilic bacteria), and the sulfur-acid tolerant thermoacidophilic bacteria.

Archaeocyatha (phylum). Vase-shape immobile calcareous organisms that evolved, then became extinct, during the Cambrian Period. Traditionally considered to be the sponge-like animals.

Archean eon. The 1 billion years interval preceeding the Proterozoic eon. Spans the time from 3.8 to 2.5 billion years ago, diring which life originated and simple procaryotic organisms prolifirated.

Arthropods, Arthropods (phylum). Animals with joint legs, external skeletons, and segmented bodies. A huge group of animals, dominant on the modern Earth (crustaceans, spiders, insects, centipedes and others). Trilobites were the most numerous artropods in the Paleozoic along with the other forms which became extinct.

Banded-Iron Formarion (BIF). Alternating layers of reduced and oxidised iron minerals. Major kind of the iron ore formed largely in the Precambrian between 2.7 and 1.7 billion years ago, that give evidence of the initial stage of introduction of oxygen to the Earth's atmosphere. These deposits might be accumulated due to direct or indirect activity of the iron-precipitating bacteria.

Benthic. Refers to the bottom of the sea or fresh water basins. Benthic organisms are those who lived on or in the bottom sediment.

Brachiopods, Brachiopoda (phylum). Bivalved shellfish, superficially similar to clams, but fundamentally different in their anatomies. Extract small food particles from the water fitering it through the special spirally coiled arms calles «lophophore». Known from the beginning of the Cambrian Period these animals were much more prominent members of the marine communities in ancient time than they are today.

Calvin cycle. Ribulose bis-phosphate-utilizing biosynthetic cycle of carbon fixation exhibited by C3 autotrophs.

Cambrian Period. First period of the Paleozoic Era. Main biological events include the first widespread appearance of organisms with the mineralised skeleton. Like during the Vendian, this period was the time of active «experiments»in the body plan of the multicellular animals. Alongwith the sponge-like archeocyathids, molluscs, brachiopods, echinoderms and arthropods there existed some forms of uncertain nature (so called «problematics»). Most of these taxa were short living groups which did not have descendants. Some of the problematics may have been the «hopeful monsters» which gave rise to the later widespread groups of animals.

Coelenterates, Coelenterata (superphylum). Jellyfish, corals, sea anemons and related living or extinct animals. Coelenterates are usually subdivided into two phyla: Cnidaria, the most numerous group of the organisms having stinging cells, and Acnidaria or Ctenophora without such cells. Coelenterate-like animals seem to be the most diverse and abundant group of organisms at the beginning of the animal evolutionary history in the Precambrian.

Coelomates. Animals which have the a hollow, nondigestive, nonrespiratory space (a «coelom») in the interior of their body. Most of the multicellular animals of the tissuie grade of organisation (metazoans) are the coelomates with exeption for coelenterates which are acoelomates. Origin of the coelomates took place in the Late Precambrian.

Cyanobacteria. Bacteria (sometimes called «blue-green algae») whose photosynthesis liberate oxygen. Cyanobacteria first appeared about 3.5 billion years ago or even earlier. Their activity added oxygen to the atmosphere and created reef-like carbonate buildups (stromatolites) which covered the bottom of the wast shallow basins during the most of the history of life. Cyanobacterial mats could be one of the major sources of the organic matter preserved as the carbon-rich rocks and oil known in the Precambrian.

Detritivores. Animals that live by eating loose edible muck (»detritus») on the sea bottom like recent sea cucumbers, certain clams and worms, etc.

Echinoderms, Echinodermata (phylum). Marine animals with more or less plated exterior surface, peculiar internal hydralic system and other shared anatomical characters. Starfish, sea urchins (echinoids), brittle stars, sea cucumbers, crinoids, and many related forms that are now extinct. The phylum may have the Precambrian ancestors, and some forms of the Ediacara soft-bodied fauna (see Ediacara fauna) are interpreted as echinoderms.

Ediacara Fauna. An assemblage of the fossil soft-bodied invertebrates discovered by R. S. Sprigg in the Pound Qurtzites at Ediacara Hills, South Australia in 1947. This assemblage of the large and exeptionally well preserved animals was studied later by M.F.Glaessner, M. Wade and other australian paleontologists and became the classical fossil collection which is used for the comparison with the similar fossils found in the Vendian (see below) deposits elsewere in the world. These oldest animal fossils are poorly understood in terms of their anatomy, taxonomic relationship and their place in the early evolution of the animals.

Epifauna. Those animals that live on the sea bottom (as opposed to those that burrow into the sediment, the «infauna».

Eucaryots, Eucaryota (kingdom). Organisms whose cells consist of the organells (e.g. nucleus, mitochondria etc.), chromosomes and other structures. All higher organisms are built of the eukaryotic cells but there are many single-celled eucaryots (protists). Complex life was impossible untill the rize of the eukaryotic cells which evolved from the prokaryotes (see below). This event took place no later than 1.7 billion years ago.

Extant. Refers to organisms of taxa that are living today, as opposed to «extinct» organisms of the groups that have no living representatives.

Extinction. Dissapearance of a single species, a group of species, or higher taxa consisting of few or many species. There were five great mass extinctions and 10 or more lesser mass extinctions during the late 550 million years of life history (Phanerozoic eon, see below). Marine fossil record has documented five mass extictions in Cambrian Period, three in the Mezozoic Era, and two in the Cenozoic Era. The mass extinction at the end of the Cretaceous Period is famous by elimination of the dinosaurs and many other organisms. Another extinction that occured during the final ten million years of the Permian Perion has eliminated as many as 96% of the species of marine animals. Precambrian fossil record demonstrates great extinction of the phytoplancton (see below) at about 850 million years ago, elimination of the stromatolite (see below) bacterial communities at about the same time, and extinction of some groups of invertebrates at the very end of the Precambrian.

Greenhouse effect. A warming of the earth's surface and lower atmosphere resulting from a process involving selective transmission of short wave solar radiation by the earthh's surface, and its reradiation as infrared which is absorbed and partly reradiate back to the surface by atmospheric «greenhouse gases» such as carbon dioxide, methane and water vapor. This effect was maintained during the most part of the Precambrian time slowly decreasing to the end of the Proterozoic.

Hyolithida, hyolithids. Small tapering subcylindrical shells with opercula (lids) that belinges to the extinct group of organisms first appeared in the Cambrian Period and existed to the Permian Period. The relationship to other phyla is uncertain.

Infauna. Those animals that burrow into the sediment of the sea bottom or live inside the sediment.

Isotope. One of two or more, radioactively stable ot unstable atomic species of chemical element. Isotopes have the same number of protons in the nucleus but a different number of the neutrons. Actively used for the dating of the rocks and for the reconstruction of the geochemical and biogeochemical processes of the geological past.

Metaphytes. Multicellular plants composed of cells that are differentiated into several distinct types (tissues), and whose activities are coordinated.

Metazoans, Metazoa. Multicellular animals composed of cell that are differentiated into several types (tissues), and whose activities are tightly coordinated, that is complex animals. This term opposes the metazoans to protozoans (or «protoctists»), organisms that are either unicellular or composed of many similar cells. Among important distinguishing characteristics of metazoa are cell differentiation and intercellular communication. For certain multicellular colonial entities such as sponges, some biologists prefer the term «parazoa».

Multicellular. Refer to any organism whose body is composed of a large number of cells whose metabolic activities are coordinated.

Pelagic. Refers to the water that is not close contact with the bottom of the sea. Pelagic animals are those found up and off the see bottom. Pelagic realm is the mid-water environments far from the shore.

Phanerozoic eon. The last 550 million years of Earth history, spanning the entire time for which the fossils are abundant due to their large size, durable skeletons and mineralised parts (shells, bones etc.) that constitute the bulk of the fossil record. The Phanerozoic eon is subdivided into three long eras and eleven relatively short periods. The subdivision is based on the study of the fossils. Phanerozoic fossil record was the major object of the study during almost two centuries of the paleontology as a science.

Phylogeny. The evolutionary history of a species or group of species in terms of their derivation and relationships. A «phylogenetic tree» is a schematic diagram that represents that evolution.

Phylum. A group or category of those animals that share a similar basic body plan which differs essentially from the body plans of all other animals. Example: the animals of the phylum Cnidaria all possess stinging cells, radial symmetry, bodies composed of two layers of cells, and no anus, a combination not seen in other animals. Each phylum is subdivided into smaller categories (classes) which are furter subdivided.

Precambrian. The time-frame of the Precambrian period is 4.5 billion years to approximately 545 million years ago (when the Cambrian period begins). The Precambrian is divided into 3 eras, characterized as follows: Hadean (4.5 – 3.9 billion years ago): From the formation of the Earth until the first appearance of sedimentary rocks, with no record of fossil organisms. Archaen (3.9 – 2.5 billion years ago): Appearance of sedimentary rocks, stromatalites, and benthic prokaryotes. Proterozoic (2.5 -0.545 billion years ago): Appearance of planktonic prokaryotes, followed by appearance of eukaryotic cells, followed by appearance of multicellular organisms.

Priapulids, Priapulida (phylum). Worms with a distinctive body plan. A few obscure species exist today as the relicts of the former more diverse group. Rather diverse priapulids have already existed in the Cambrian Periods.

Problematic taxa (problematics). Extinct organisms of unusual anatomy that make it difficult (or impossible) to relate these forms to any known fossil or recent taxa even at the high taxonomic level. The number of the problematic taxa was especially high at the very early stage of animal evolution.

Prokaryotic. A term that describes cells whose internal construction and activities are relatively simple. Such cells lack organells, chromosomes, and other complex internal units that are common in the eucaryouts (see above).

Prokaryote. An organism built of one or more procaryotic cells; mainly bacteria and cyanobacteria. Most procaryotes are single-celled organisms, or consist of simple filaments or sheeth of cells. The oldest fossils represent prokaryotes. Eukaryotic cells evolved from procaryotes.

Proterozoic eon. The 2 billion years interval preceeding the Phanerozoic eon. Spans the time from 2.5 to 0.55 billion years ago, during which single-celled organisms acquired bisexuality, diplooidy, and multicellularity, the biological starting points from which complex life evolved.

Rubisco or ribulose 1,5-bis-phosphate (RuBP) carboxylase/oxygenase. Enzyme of the Calvin cycle catalyzing the carboxilation (with CO₂) and clivage of ribulose 1,5-diphosphate to yield two molecules of 3-phosphoglycerate.

Species. All of the organisms that are capable of interbreeding with each other under natural conditions to produce fertile offsprings (bisexual organisms) or all of the morphologically and genetically similar descendants of some inferred ancestral individual (asexual organisms).

Stromatolite. A layered (usually mineralised) flat, dome-shapes or columnar formations, sometimes reaching the sise of a reef, created by the activities of bacterial communities (mostly cyanobacteria). Common in the Precambrian shallow water basins. Much rare today (salt marshes, sabkhas etc.).

Taxonomic group. Categories into which organisms are classified. This makes it easier to understand the life processe and to evaluate the evolutionary relationships. Taxon: one such category, whether large and inclusive or small and exclusive. Plural, taxa.

Trace fossils (or ichnofossils, or bioturbations). Trails, tracks and burrows produced by extinct animals on the surface or inside the soft sediments which then became the rock. This category includes borings in hard substrates (stone, wood, shells) and some other evidences of life activity. Trace fossils are the «behavior written in stone» because they reflect the mode of locomotion, strategy of feeding, reactions of the animal to the external factors of environment etc.

Upwelling. The upward movement of the subsurface oceanic water mass toward the surface by the edge of the continents. Upwelling bring to the surface water layer the diverse nutrients, for instance, phosphorus, which is necessary to the algae and profitable to the organisms consuming algae etc. further along the trophic chane towards fish, birds and mammals (including man).

Vendian Period. Terminal period of the Proterozoic. Spans from 650 million years ago, the time of great glaciation, untill the beginning of the Cambrian Period about 550 million years ago The Vendian Period saw the rize and global expansion of the large soft-bodied animals (Ediacara fauna, see above). Many groups of the invertebrates which appeared and flowrished later had their ancestors among the Vendian fauna.

• http://geosciences.org
• http://www.ucmp.berkeley.edu/Paleonet/
• http://animaldiversity.ummz.umich.edu
• http://www.geology.com
• http://www.uni-wuerzburg.de/mineralogie/palbot1.html
• http://biodiversity.bio.uno.edu
• http://www.gla.ac.uk/Project/originoflife
• http://www.paleo.ru/museum/index.html
• http://www.geocities.com/CapeCanaveral/Lab/8147/index.html
• http://www.earthwise.org
• http://gaim.unh.edu

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