Precambrian Period
6,000 to 570 million years ago

6,000 MILLION YEARS AGO

The solar system was born from a fraction of interstellar gas and dust that appeared when an ancient star underwent a massive supernova explosion.

4,500 MILLION YEARS AGO

Like a raindrop forming around a dust particle, a round hot ball of red, steamy molten rock grew out of the interstellar gas and dust to form the Earth.

An artist impression of the surface of a young Earth nearly 4,500 million years ago on a very rare clear night.
The origins of the Moon is also interesting. After measuring the abundance of the isotope 182-tungsten in materials collected by Apollo astronauts in one of the solidified "magma" oceans on the Moon and checking to see whether there is a difference in its abundance compared to the Earth's own magma, scientists have discovered the Moon was created approximately 30 million years after the Earth even after taking into account errors in the measurement. Because the Moon does not have an iron core as the Earth does and yet both bodies have similar compositions, scientists are lending their scientific weight in support of the theory that the Moon was probably created when a Mars-sized body collided with the Earth at this time, shearing off a reasonable outer chunk of the Earth. Fortunately the speed of ejection of the material was not strong enough for it to escape Earth's gravity. As a result, the material quickly solidified and, together with various other much smaller collisions, eventually pushed it into a circular orbit around the Earth. Finally, the material's own gravity was strong enough to shape itself into a sphere. When the Moon was cool enough, various meteorites would hit the Moon and mould its surface to create the patterns we see of this body today. (1)
Looking closer to our planet, we can see how as the surface cooled to form the Earth's crust over the next 150 million years, gases trapped in the planet's interior and from considerable quantities of icy comets (2) crashing to the Earth were released and held gravitationally in place to conceive the Earth's atmosphere consisting mainly of nitrogen, methane, ammonia, hydrogen sulphide and hot water vapour.
The process of outgassing from Earth's interior has persisted to this day but in much lesser quantities, known as volcanism.

3,800 - 3,700 MILLION YEARS AGO

Oldest known rocks on Earth were found from Greenland. The technique for dating rocks has been done by taking a sample of rock from Greenland and other places and measuring the proportion of radioactive rubidium-87/potassium-40 and non-radioactive strontium-87/argon-40 elements (ie. the final products in the long radioactive decay process), respectively.

3,800 to 3,600 MILLION YEARS AGO

Water vapour in the atmosphere condenses to the liquid state to fall as rain for at least 60,000 years to form the great oceans of the world.
As the rains poured from the heavens, the highly penetrating and disruptive ultraviolet rays from the early Sun and the great electrical storms in the early hazy atmosphere of Earth assisted in the dissemination of methane, ammonia and water into smaller and highly reactive molecular fragments called free radicals. Then, as the energy briefly dissipated, these energised fragments would reassemble near the surface of pools or inside tiny water droplets in the primitive atmosphere to form a variety of new molecules.
A similar event would also be occuring not far from the hot hydrothermal vents peppering the floor of the great early oceans and the numerous shallow seas and lakes scattered generously throughout the ancient muddy, hot and humid desert-like continents of the world. Under these extreme conditions of hot and cool and dry and wet, high water temperatures would break molecules apart and the cooler waters nearby would help to reassemble energised fragments to form new and interesting molecules, some of which would be crucial to the development of life.
Then as water in some pools began to evaporate between major rains, the concentration of these molecules would increase in a smaller volume of water, and so increasing the probability of new molecules being formed.
Add to this the fourth complicating factor of icy comets surviving the fall to Earth and releasing their frozen cargo and it is possible another means of transferring vital chemicals or possibly primitive "alien" life by way of bacteria to Earth could also have started the process of life on Earth. Scientists are not certain how much contribution this "extraterrestrial influence" could have. It assumes that extraterrestrial life exists in the universe, that life can survive the long journey to Earth, and that certain catastrophic events will cause some bacteria from other planets to be suddenly trapped in ice and shot into space until through incredible luck the comet manages to reach and survive the plunge to the surface of an Earth-like planet. NASA scientists are currently investigating this possibility by understanding how long bacteria can potentially survive in extreme conditions such as ice and radiation.
Could very primitive extraterrestrial life exist in a state of suspended animation inside icy comets?
The opposing argument is that life could also begin on Earth. Among the important scientific work being conducted in this area is one from Professor David Deamer of the University of California, Santa Cruz. He is currently working on the idea that bubbles (or vesicles as he calls them) could play an important role in the formation of the first single-celled organism. If he is right, bubbles could have formed on the surface of certain types of clays containing the right metals to attract amino acids and help them form long chains of macromolecules called proteins. Because if enough of these proteins can be produced, the protective bubble could be reinforced by a protective protein-like membrane and so maintain the structure for longer. Then it is just a question of how long before the first DNA-like molecule is created to help replicate this first living single-celled organism. Already evidence has found that these clays can also create chains of nucleotides (the basis for building a DNA molecule).
For example, in 1977, while working at the National Aeronautics and Space Administration (NASA) Ames Research Center in California, USA, Dr James Lawless and a visiting scientist from Israel, Dr Nissim Levi, found evidence to support the 1947 claim made by British physicist Dr John Desmond Bernal (1901-1971) that ordinary clay can concentrate small chemical molecules in a hot "organic soup" and then act as a kind of prebiotic scaffolding for producing larger, more complex molecules at the very surface of the clay. Although most clays destroy a number of important amino acid structures, Lawless and Levi found that clays containing metals can attract certain amino acids and nucleotides without damage.
In particular, Lawless and Levi discovered two types of clays, one containing nickel and the other zinc, which not only attracts all the twenty amino acids found in living things on Earth and all the various nucleotides comprising DNA respectively, but which has been observed to form macromolecules of as many as eight amino acids and several chains of nucleotides. Longer, protein-like chains and DNA-like structures seem likely if given sufficient time for their synthesis.
If we assume life was created on Earth, an unimaginable amount of time must have passed during which the process of dissemination and reassembling of molecules continued unabated. Eventually, more and more complex molecules consisting mostly of fatty acids, sugars, tannins, amino acids and nucleic acid bases slowly accumulated near the surface of ancient clays existent beneath the hot springs (3).
Why was the water hot? The key to answering this question may lie not only in the Earth's hot magma heating up the water, but also in how asteroids achieve the same heating up effect. As some British scientists studying the Houghton crater on Devon Island, Canada, have found, there is evidence to suggest that asteroids crashing into the Earth can heat up the rocks, causing some rocks to vaporise while others develop tiny cracks as the rocks remain warm to hot for anywhere between 1,000 years and 1 million years. It is these cracks together with the heat of the rocks where some scientists believe life on Earth could have began.
As Dr Charles Cockell of the British Antarctic Survey in Cambridge said:
'What we've discovered is that rocks inside the crater are more heavily colonised by microbes than the rocks outside the crater. So what we have here is an example of how impact events can create a habitat for life and not merely act as agents of destruction.' (4)

Then suddenly, without warning (perhaps with the assistance of hydrothermal vents and new and varied catalysts in the clays and in the water; or as suggested by some scientists life may have already been controversially produced somewhere in the universe and was somehow frozen inside a comet until it hit one of the Earth's early warm ponds to release the microscopic life just like a human embryo being fertilised by a sperm (5)), a massive macromolecule made of proteins and nucleic acids was brought to life which was able to make crude copies of itself from the rest of the molecules in the organic soup. This self-replicating macromolecule is the ancestor of Deoxyribonucleic Acid (DNA).

A computer-generated image of a tiny segment of DNA. Each ball shown here represents an atom.
From the great turmoil of the early Earth came the first single cell organisms. The organisms, looking like tiny, perhaps semi-opaque bubbles floating around in the warm waters, consisted of a spherical-shaped protein membrane to protect the DNA molecule as it went about its incessant and rather important activity of replicating itself and crudely 'learning' from its environment on a regular basis.

3,500 MILLION YEARS AGO

The oldest direct evidence of life on Earth were simple bacteria fossilized inside rocks from Western Australia. The discovery was announced in 1993 by J. William Schopf of the University of California at Los Angeles, USA. (6)

3,500 to 3,400 MILLION YEARS AGO

The first single-cell organisms multiplied rapidly. Eventually, a time came when the food supply consisting of the freely-available organic molecules in the pools dwindled and the need for a more dependable source of food became increasingly more important for these tiny organisms.
At first, perhaps being situated close to an ocean or large inland sea, a number of these single cells found their way from the pools into larger and deeper waters. This may have helped the cells to find more food and plenty of reliable water to drink.

Layers of single-cell organisms grew on clay surfaces in shallow seas. As one generation of organisms grew on top of another over many millions of years, these rocks called stromatolites were formed (in Shark Bay, Western Australia). Source: Morrissey 1995, p.4. Photographed by Reg Morrison.
But after many more millions of years had passed, one of the single cells just happened to stumble across an important molecule called chlorophyll (7) which could trap the free and reliable energy from the sun and use it as a means of breaking down the highly abundant carbon dioxide in the Earth's atmosphere for food rather than depending on the availability of basic chemical building blocks in the oceans.
These chlorophyll-loving single-cell organisms became the first plants to appear on Earth. The highly primitive plant life probably looked very much like blue-green algae which still exists today, but they were not blue-green algae at all. Rather, the first plant cells to appear on Earth were actually photosynthesizing bacteria.

Chains of photosynthesising bacteria. These fossils were found in central Australia inside rocks estimated to be about 1,000 million years old.
When enough of these photosynthesizing bacteria filled the oceans of the world, another simple living cell evolved and, possibly after experiencing a moment of food shortage, discovered one day how it could consume or break-down this highly abundant 'plant-like' bacteria as a more stable source of food. This new living cell became the first animal to appear on Earth. An example of this is Protozoa, the oldest known animal fossil. Protozoa would swim or float around and consume smaller bacteria.

An example of a modern freshwater protozoa known as Stylonychia mytilus. Photograph by Hermilo Novelo. Image available from http://www.ucr.edu/pril/peten/images/el_eden/protozoa.jpg.
The concentration of carbon dioxide in Earth's atmosphere was at least 5 times higher than today with a density only about half that of present-day conditions.

3,200 MILLION YEARS AGO

Some of the earliest known fossilised evidence of ancient life on Earth has recently been discovered by scientists as tiny microbes living around hot, deep sea hydrothermal vents.

2,750 to 2,000 MILLION YEARS AGO

Photosynthesising bacteria permeated every corner of the world where water was abundant.
This was an interesting time when many of the pools on land and some shallow seas had turned a distinctly greenish colour (a striking contrast to the bluish oceans and brown desert-like expanses of the great continents), showing the presence of countless millions of photosynthesising bacteria.
The bacteria also provided one other benefit for the single-celled animals emerging throughout the world. As a natural by-product of their photosynthesising work, the water became well-oxygenated. In fact, so much oxygen was being produced by the bacteria that it slowly accumulated in the air, and so benefiting the development of more sophisticated animal life over the next 2,500 million years. Also the minerals and rocks of Earth containing iron, manganese, uranium and other elements progressively oxidized as levels of oxygen rose.
While the primitive plants continued its ferocious appetite for carbon dioxide, the atmosphere of the Earth nearly 2,750 million years ago had increasingly fewer greenhouse gases by way of carbon dioxide molecules to effectively trap the heat from the sun. This meant that the Earth was getting cooler. But fortunately the orbit of Earth (in addition to the possibly high volcanic activity along regions where the thin, cool crust cracked) prevented the planet from heading towards an irreversible ice age.
It is believed that clouds covering Earth finally broke-up sometime during this epoch, and so benefiting the development of more sophisticated plant life the world has ever seen.

The clouds of Earth finally break up, allowing the great oceans and continents of the world to be bathed in visible light from the early Sun. During this time, the volcanic ash still remaining in the air would have created spectacular sunsets like this one.

635 MILLION YEARS AGO

Life in the great oceans of the world suddenly diversified and multiplied in great numbers as the first multi-cellular organisms appeared on the Earth.

One of the earliest multicellular organisms of the Precambrian period. This flat worm is called Dickinsonia and was found in sandstones nearly 600 million years old near Ediacara in the Flinders Ranges, South Australia. Source: Long 1995, p.15.
The success of these new creatures was made possible only because a new cell had arrived on the scene with the great idea of working together (or "socialising") with other similar cells to achieve more easily the basic tasks of surviving the harsh environment aand therefore ensure a better chance of self-replicating the primitive DNA. It is also a way for a large number of cells to physically survive for a longer period of time without having to change significantly than if they were on their own.
But what would drive these "social" cells to change or combine in such a way as to make the organism look like it is adapting (eg. getting bigger, developing survival-based appendages etc)? It was probably the presence of other living organisms competing for the same sorts of food, not to mention the impact predators (8) may have had on some living organisms to change and adapt.
However, the highly independent single-cell organisms would not suddenly disappear overnight because of these new 'social' cells. In fact, single-cell organisms would continue to exist side-by-side with the "social" cells to the present day because of their ability to evolve more quickly into something unique and advantageous to help handle the diverse and changing environmental conditions much better than the social cells. We can see evidence of the power of these single-cell organisms to adapt quickly to something in those bacteria which become resistant to human-made antibiotics.
At any rate, these new 'social' cells were more complex than their predecessors. Apparently, the cells grew in complexity with the advancement of a nucleus (the control centre of the cell) which may contain additional structures needed to help master the art of socialising with its own kind while still maintaining some aspect of its individualism necessary to keep itself alive.
Soon millions of these highly successful 'social' cells came together to produce simple 'animals without backbones', such as worms and jellyfish. As for the plant-like cells, they combined to produce seaweeds and sponges to name a few.

Soft-bodied creatures like jellyfish and worms were the earliest multicellular organisms. Source: Reader 1986, p.45.
The eventual evolutionary progression from single-celled microbes to many-celled organisms took a very long time. Indeed, the time gap of at least two billion years to accomplish this next biological feat in the evolution of life has not been fully explained. But once certain cells learned how to come together without eating each other, the progression to multi-celled organisms took off at a very rapid pace. (9)

650 - 600 MILLION YEARS AGO

The success of multicellular plant life, covering almost all parts of the world's oceans and inland seas as it feverishly consumed the carbon dioxide in the atmosphere, had probably caused massive glaciation to occur on a worldwide basis at this time. These great ice sheets may have extended near the tropical latitudes.

Earth in the Late Proterozoic era around 650 million years ago. Image available from http://www.geologie.uni-stuttgart.de/down/maps2/pl1.jpg.
NOTE: There are some scientists claiming the entire Earth may have been frozen several metres thick at the equator and thicker in temperate and polar regions. Scientists are still searching for evidence to support this "snowball Earth" theory.
While worldwide temperatures dropped throughout much of the planet nearly 630 million years ago, many of the multicellular animal life that existed nearly 600 million years ago developed a primitive nervous system of the level at least of present-day jellyfish. The reason for developing a nervous system is not absolutely clear. Perhaps it is in response to the colder conditions in the oceans and/or the need for the animals to find new ways of coping with a more complicated environment where other similar creatures have now adapted to consuming animals for food.
Nevertheless, the development of a nervous system would have become necessary to coordinate and handle the increasing amount of diverse and sophisticated functions that had to be performed by the multicellular organism as it adapted to new environmental conditions in the oceans.

600 MILLION YEARS AGO

Two large supercontinents drifted across the globe. They would later join-up in another 400 million years as one great land mass.

NOTES

Analysis of the isotope 182-Tungsten was conducted by a team under the head research scientist Thorsten Kleine from the Mineralogy Institute in Munster, Germany, in October-November 2005. The Canberra Times: Collision with Earth made Moon: scientists. 26 November 2005, p.24.

Scientists suspect icy comets could have contributed significantly to the formation of the world's oceans and possibly to life on Earth more so than from the interior of the Earth. So, on 4 July 2005, scientists have crashed a 372-kilogram spacecraft, dubbed the Impactor, travelling at 10 kilometres per second into a comet known as Tempel 1 measuring 14 by 4.6 kilometres in size. The debris emitted from the impact will hopefully reveal a detailed chemical composition for the Deep Impact spacecraft nearby to analyse.
Results of the impact have yet to be fully analysed and understood. Assuming the comet doesn't explode and break up with parts flying in all directions, scientists will hopefully expect the results to prove comets are made of mostly water and enough carbon and nitrogen to indicate the possibility that amino acids could be locked away in the ice.
As of 23 March 2006, a University of Hawaii graduate student named Henry Hsieh and his mentor Professor David Jewitt believe the origin of the world's oceans could lie in another class of icy comets known as main-belt comets. The composition is similar to normal icy comets except they act more like asteroids and are located in the asteroid belt of our solar system.
This is a new discovery as scientists before this time generally thought asteroids and comets were distinct objects.
The findings came after observations of asteroid-like comets following along similar paths to asteroids in the main asteroid belt were found to be emitting dust (eg. Asteroid 118401) behind them like a comet because the paths were closer to the Sun than other asteroids. Observations were made on 26 November 2005 using the Gemini North Telescope on Mauna Kea, Hawaii.
As Hsieh commented:
'The main-belt comets are unique in that they have flat, circular, asteroid-like orbits, and not the elongated, often tilted orbits characteristic of all other comets, At the same time, their cometary appearance makes them unlike all other previously observed asteroids. They do not fit neatly in either category.'

Further observations are taking place to confirm the composition of the dust (believed to be mostly ice).

Scientists from the University of Calgary will be visiting a rather large yellow stain on a patch of snow near a glacier in Ellesmore Island on 21 June 2006. Realising no Eskimo with a serious binge drinking problem could create such a massive stain, the scientists decided to investigate. On landing, the scientists couldn't help noticing the smell of rotten gas, the tell-tale signs of sulphur compounds.

The yellow sulfurous stain on the snow is visible in the foreground of this picture.
A brief examination of the contaminated snow has shown not only sulfur springs existent below the surface, but has revealed a unique and potentially new bacteria. So unique is this bacteria that scientists are thinking this could be how life first appeared on Earth. It was a chance discovery during a routine helicopter flight!
If this is true, NASA intends to send a probe to Jupiter's moon known as Europa. With its sulphurus stains on the icy surface, the likelihood of finding the world's first evidence for extraterrestrial life seems high.
Benoit Beauchamp, executive director of Arctic Institute of North America at the University of Calgary will join a team to investigate more closely the interesting discovery.

Quote from Dr Charles Cockwell. Connor 2004, p.15.

A number of US government scientists are exploring this possibility by studying how extreme and for how long the conditions have to be before the most primitive life find it impossible to survive. There is already evidence that bacteria frozen in the deep ice of Antarctica for tens of thousands of years can be revived at room temperature in the laboratory today. So why not millions or hundreds of millions of years?
With the possibility that such bacteria could theoretically survive in the ice for a much longer period of time, perhaps for many billions of years, NASA has taken the idea so seriously that they have now sent into space a US$158 million (A$300 million) spacecraft called Contour (short for COmet Nucleus TOUR) designed to analyse the icy material in the nucleus of two comets - Comet Encke in 2003 and Comet Schwassmann-Wachmann 3 in 2006 - to help confirm or deny the possibility that life on Earth could have began from one of these comets crashing to our planet over 4.6 billion years ago.
The idea of the Earth being seeded by (well, effectively extraterrestrial) organic matter that could have kick-started life on Earth is known as the theory of panspermia.

Crawford 2000, p.28.

Iron is an important element in the making of chlorophyll. Some scientists believe the oceans and shallow seas of the early Earth had much higher concentrations of iron than available today. However, oxygen eventually binds with iron. So as soon as the free oxygen were released by the chlorophyll-loving organisms, iron concentrations in the oceans can go down and with it less and less of these organisms existing in the oceans of the world. With the current high levels of greenhouse gases now existent in the Earth's atmosphere and the great concern shown by people living near the ocean as well as some businesses who depend on the shoreline staying where it is, scientists are experimenting with the possibility of introducing thousands of tonnes of iron into the ocean to help build up the population of chlorophyll-loving organisms like algae and so combat the greenhouse effect.

Predators are actually plant-eating animals evolved to become meat eaters. They came about in the harsh and unloving environment to help deal with the problem of hunger when the plant foods they originally consumed weren't around in sufficient numbers.
Predators serve an important job in the world by controlling the population levels of plant-eating animals and therefore allow plant life to exist in the environment. Otherwise plants could be eating out of existence or reduced so significantly that many plant-eating creatures would starve to death.
Unless plant-eating animals can think about the consequences of their procreation actions to the environment in a holistic and global manner and not just in an immediate sense (eg. can they survive while supporting an offspring) and can decide when it is appropriate to procreate, predators are needed to bring balance to the rest of the animals on Earth and to ensure the survival of the plant kingdom.

Scientists have studied a primitive multi-cellular organism known as Volvox carterii to help trace the gene responsible for cooperation between cells leading to multi-cellularity. During the study, the scientists discovered 16 of the 2,000 cells linked to form a globule appear to defy the trend of trying to fight among other cells to be the stongest and fittest for the task of reproducing themselves as expected in Darwin's evolutionary theory. Rather the cells have learned to give up on reproducing themselves in favour of helping to keep the entire group of cells alive and if necessary commit evolutionary suicide where necessary if it means a better chance for the group to continue surviving.
And now scientists believe they may have found this "altruism gene" known as RegA. The gene itself may have developed over millions of years during lean times when some cells were not required to perform certain tasks. Repeat the process long enough and eventually some cells may have learned to live without having to, say, reproduce themselves in favour of more altruistic goals for the group.
Further details of this research can be found in the 23 May 2006 online issue of the research journal Molecular Biology and Evolution.