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"The Evolution of Life." A World Evolution Overview
Text mostly by Doug Silke, main reference is the Open Learning television series

Our planet is teeming with different life-forms, many new species are discovered. Much to learn about how this living world evolved. Fossil evidence indicates a world once dominated by microscopic life forms.

The Pre-Cambrian Era
(The part in italics was based upon notes taken directly from another educational TV science series on evolution and genetics. Sorry, a great show but unsure of it's name.)

3.5 billion years ago, the earth cooled sufficiently to form a crust, then clouds formed in the atmosphere of water vapour, CO2 and nitrogen notable with absolutely no oxygen. Various molecules formed in the lakes and oceans, and accumulated gradually over a very long long time without any bacteria to break down these molecules they were free to form over millions of years into a myriad of complex molecules. The high temperatures and pressures in lightning strikes have been known to form molecules that are similar to basic amino acids, the building blocks of life.

Eventually into this world full of molecules of many different types, emerged one that could divide and duplicate itself. The start of life itself. This may be unimaginable today, but without bacterial degradation of these molecules, millions of years may do things quite naturally that seem extraordinary to us now.

This life-form had no living predators though no doubt conditions made it touch and go for a while, but a huge soup of food lay waiting patiently in the world seas. Ultimately the life-forms of the time was free of constraints and could explode to fill this ecosystem niche for the first of many times throughout time.

Humans today are only just starting to realise that the earth is finite in size. And no species is immune, not even these ancient indescribably simple precursors to single celled life. They too immediately began totally polluting the atmosphere with an extremely dangerous, previously unheard of and highly toxic chemical that was the by-product of life.

The system of consuming CO2 to make energy that could be stored in the form of sugars evolved. This process liberated Oxygen. Gradually the oxygen level built up in the atmosphere.

Oxygen is highly reactive as we know... and it was easily capable of attacking these early life forms. No doubt the generation of oxygen from life-forms reached a stable plateaux for some millions of years, as the "plants" that generated it were killed by the higher levels... Until one species evolved a tough outside membrane (an early form of single celled life) that could keep out the oxygen, allowing only controlled amounts of oxygen at much more tolerable levels to penetrate within itself through holes in this membrane. (perhaps taking hours of the 24 hour geological clock's time to evolve)

Suddenly again a species was again unlimited, with huge food availability, and again off it went in a populating frenzy... As with the present human population explosion that is occurring now (only during last few seconds of 24 hour geological time), the consequences for the other partners on the finite planet was dramatic... extinction. With this increase in plant population, the poisonous pollutant "oxygen" levels in the atmosphere increased to unprecedented levels, way out of control...as those species that fluked the necessary membranes or other mechanism able to cope evolved to thrive in line with the rate of increase.

No doubt most species were simply unlucky with the turn of events, their potential adaptations were simply set to evolve and thrive only in other conditions. They were totally fried to a crisp into extinction by oxidation. One of the many mass extinctions that has occurred and like one that is now occurring again by human hand.

The story continues... as oxygen levels built up and the densities of single celled life increased in the oceans, another life-form evolved that admirably suited these conditions. It no longer needed to photosynthesise, because it could obtain unlimited food supply by devouring the plentiful numbers of single celled plants, it then consumed oxygen to liberate the energy and CO2 from the devoured sugars. Early animals had evolved.

As with most evolution this new species suddenly had unlimited scope for population increase, with oxygen and plants plentiful. Again as always, most life forms could not compete. This time they were eaten into extinction, only those few early plants that could develop the most suitable defence mechanisms could survive.

Only by these fluky evolutionary processes did the oxygen levels continue to rise. Some 800 million years ago this rise in oxygen levels became sufficient to form an ozone barrier in the atmosphere. This barrier now did what previously a few inches of water did for all the life-forms in the sea, it stopped ultraviolet radiation from penetrating to the land surfaces.

Only by this, and many other chance evolutionary processes was life able to colonise the land.

We last shared common ancestry with the sulphur eating bacteria more than 3.5 billion years ago.

After billions of years of evolution, during which the incredible complexity of the single celled life evolved, according to one hypothesis it is thought that the genome of a single celled life-form was accidentally duplicated. The redundant set was then free to mutate with far more chance of the resultant cell surviving. This type of event is thought to be a major factor enabling the evolution of multicellular beings some 600-700 million years ago.

The "Imunoglobulin" superfamily's evolution 600-700 million years ago does seem to be part of this evolution of multicellular beings.

From fossil evidence we know algae has been on earth at least 1.8 billion years ago.

Older and more primitive life forms have been found in rocks from all over the world, the probable fossil remains of blue-green algae and bacteria found in rocks nearly 3 billion years old in Australia, Africa and America.

We have identified, thousands of plants and animals that once inhabited the earth. But the story is far from complete. Especially life's origins itself.

Fossil types: moulds are the most common. Bone and shells may remain intact in sedimentary rock for millions of years, or is replaced by minerals from the rock. Thirdly casts preserve the original forms, and are nearly as common. Finally footprints etc. can be seen, burrows and nests.

Fossils are best preserved in the oceans, millions of the species occur and many may be preserved. Chemical weathering etc, of land based fossils means that the record is far less complete. So the study of fossils is skewed towards the remains of sea living animals.

Animals with bones or shells are most likely to be preserved. rarely only do softer body parts leave records. Vertebrate animals are themselves poorly represented.

Stromatolites are an exception, the layers of sediments and calcium carbonates trapped in the algae hairs remain, not the actual (with cyanobacteria) algae imprint itself. they used to be very dominant, now existing in only a few areas with warm shallow water. Stromatolites added immense amounts of oxygen to the earth's atmosphere, with important consequences for the evolution of life.

Fossils of stromatolites date back as much as 3.5 billion years.

Early "prycariote" cells divide making exact clones of themselves. Asexual reproduction can only genetically alter by genetic mutation. There is very little chance of evolution in pryocariotes. Later eucariote cells emerged. These reproduce sexually, resulting in many variations in the progeny's genetic makeup. A whole new ability for these organisms to change and evolve was now possible.

About 600 million years ago many complex eucaryote life forms developed. No-one knows exactly why this change occurred, but the first appearance of these fossils in rocks of this age marks the end of the Precambrian era and the beginning of the Paleozoic era. This is the Cambrian explosion. Life for the first time involved the use of hard shelled organisms.

Prior to this, no doubt hard shelled life forms evolved, but they did not survive. Probably this had something to do with the absorption of oxygen. Prior to this time there was not much oxygen in the atmosphere, and the absorption of this was difficult through hard shells. However in the Cambrian it possibly became necessary for animals to have hard parts to avoid being eaten.

The Paleozoic Era

During the Cambrian period in the early Paleozoic, conditions were very good for the evolution of life. Within 50 million years all the major groups of organisms which still survive today had evolved, including the first vertebrates.

During the Paleozoic continents were drifting towards the equator transforming cold glaciated continents into warm shallow seas, organisms thrived. The bottoms of these seas were literally carpeted with invertebrate animals and primitive corals , with the first fish swimming above in the shallow seas.

By the middle Paleozoic time plants and animals had made their way out of the ocean, and have begun to populate the land. This emergence was possible in part only because the atmosphere had become oxygen rich from stromatolites, and the ozone layer formed and protected against ultraviolet light. This process was well under way in the Precambrian, but it was not until the Mid-Paleozoic that life forms were sufficiently developed to take advantage of the protected dry land.

At the same time as oxygen increased, CO2 decreased because life forms used it up. CO2 is trapped in organisms in sea water, then turned into limestone. So organisms are an integral part of the carbon cycle, playing a pivotal role. The ocean, the sea, sedimentary rock, organisms, plants, atmosphere, all take part.

The late Paleozoic was a time of great global change. Continents now formed into a great continent called Pangea. This great continental mass nearly bridged the earth's oceans from pole to pole, severely disrupting the earths climate. These continental collisions transformed shallow oceans in to mountain ranges, similar to the alps and Himalayas. The sea currents altered dramatically resulting in changes in climate and conditions that had a great effect on the ecosystem, triggering the largest mass extinction in our history, a tragic end to this paleozoic era, when 90 % of all species of life became extinct.

The surviving life forms now had free rein, with much reduced competition. The fossil record suggests that individual species gave rise to a multitude of new life forms, each well adapted to survive in this new world.

Mass extinctions have occurred throughout the history of life, taking out a lot of organisms, allowing a new set of organisms to develop and repopulate the world with new species. The arrival of new species after the Paleozoic extinctions heralds the next major era. The Mesozoic.

The Mesozoic Era

The Mesozoic was a critical time, when life forms evolved from their ancient to their more modern condition. Reef building corals reappeared on continental margins, and large reptiles joined the fishes as swimming predators. Later in the Mesozoic the flowering plants and deciduous trees first evolved on land, as well as flying reptiles and the first birds. Mammals were also present, but they were small and inconspicuous. Despite this proliferation of life the Mesozoic is best known as the age of the dinosaurs. They dominated the land for some 140 million years.

Dinosaurs have captured our imagination. Piece by piece a spectacular view of earths past inhabitants unfolded. Not all are the cold blooded sluggish reptiles that were first imagined, but many seem to run fast, and to inhabitate cool climates. This suggests that they were warm blooded. Some even lived in nest building herds. They might have still dominated our world today except for a change 60 million years ago that caused another dramatic change.

It is suggested that dust in the atmosphere cooled the climate, triggering another mass extinction.

Whatever the cause... volcano or asteroid, this extinction not only killed the dinosaurs, but it ushered in the Cainozoic, the present era of time.

The Cainozoic Era

The Cainozoic is marked by intense tectonic changes that helped to direct evolution. EG. Australia became more remote from other land masses allowing evolution to here take a course that allowed Australia's platypus, kangaroos and the spiny ant eater to appear only here. This set the stage for mammals to dominate the land.

The last 2,000,000 years have generally been considered the ice ages. During the last half of the last glacial period fossils accumulated at Rancho Labreya in Los Angeles. Natural tar pools have trapped thousands of animals in especially good condition from the late cenazoic era…. The oldest specimens are terrestrial vertebrates 38,000 years old, the most recent are 10,000 years old. From microscopic plants and pollen forms to giant bisons and mammoths have all been fossilised.

Microfossils are really needed to reconstruct the microclimate. Small species probably spent their entire life right where fossilised, like small mice etc. Larger animals may have arrived there from a long distance away.

40,000 years ago, LosAngeles was a sagebrush plane, with groves of cypress and pine. Many larger animals lived there: deer, sabretoothed cats, dier wolves, ground sloths and mastedons. Most of the larger mammals in this ecosystem have since become extinct.

Other Cainozoic deposits show fossils spanning millions of years. The horse is especially stunning. Appearing near the beginning of the Cainozoic, the first horses are no larger than dogs, and have four toes on each foot. They lived in woodlands. Then about 40 million years ago a new ecosystem developed, the grasslands. Moving in to forage in these open lands the horse grew larger with time, and evolved hooves to replace toes. This permitted horses to run swiftly, with a better chance of survival.

150 years ago people knew that species changed, but the mechanism, or why, was not known. The idea that species change to suit their environment, was first described by Charles Darwin. A set of various traits are always present in a set of animals of the one species, and they evolved gradually to suit slow changes in their environment.

But when looking for these slow changes, fossil hunters did not find them. The hypothetical intermediates between species were not found. Many thought that there were gaps in the fossil record. But then came the realisation that they do not exist.

Punctuated equilibrium has been added to Darwin's theory. A species comes into existence and lasts for a long time unchanged. Then suddenly, in terms of geological time, a new species comes into existence, with none if any evidence of intermediates.

Not only can life evolve gradually, but in a world where rapid change can occur, rapid evolution can also take place. Adapt or die out.

During the Pleistocene, ice age mammals dominated the continents, including: mastadons, mammoths, ground sloths and sabre toothed cats. They suddenly disappeared about 8000 years ago. The cause of this is not known, but one theory is that the increasing efficiency of human hunters brought these to extinction.

Human fossils are quite rare. It is felt that they originated from Africa, where they first evolved.

It is sobering to realise that it all began with tiny simple organisms. The resilience of life is amazing considering the extinctions that have occurred to date. Indeed the latest mass extinction may be occurring right now. The impact of humans on earth is causing species of both plants and animals to become extinct at an astounding rate. Many scientists estimate that a larger percentage of the earths species will become extinct within our lifetime than occurred during the last mass extinction of the dinosaurs.


It is estimated that the world currently contains some 3-40 million species. Conservatively for each of these another 50 to 600 species have inhabited the world but exist no longer. All in the 3.5 billion years since life first began.

For example, Hawaii's largest island is only 400,000 years old, but hosts hundreds of different species, found nowhere else.

E.G. the Silversword plants of Hawaii seem to have evolved into several species from one Silversword species that probably was carried by birds from the American continent. Even the most different of Hawaii's Silversword plant species vary by only 95% in DNA. These are nearly as closely related as are diverse groups of humans. Indeed all silverswords do seem to hybridise, although some are very rare.

Some people think of humans as a final product in a long process of organic evolution. But our species is only one branch on the evolutionary tree of life. Organic evolution has been operating since life first appeared on earth. Always the geological process continuously changes the earth's surface. We can only imagine what life might be like in the distant future.

Based totally on other references, text mostly by Doug Silke
Open Learning television series: "The Evolution of Life", 1992
The part in italics was based upon notes taken directly from one educational TV science series on evolution and genetics. A great show but unsure of it's name.
Pathophysiology; McCance, Katheryn; Huether, Sue E;1990; CV Mosby Company, Missouri.
Cracking the Code; Suzuki, D; Levine, J; 1994; Allen and Unwin, Marrickville

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