The earth is now believed to be over 5000 million years old. Of this man and his nearest ancestors have existed for about 750 thousand years. And, the civilized man, or the man who cultivates his own food and lead some form of organized community life, has been on earth for about 8000 years. 

So, those many millions of years are no less a part of history. And they are of no less importance than the record of those recent few thousand years, since when our history had been written. 

The unwritten history:
Now, imagine the thousands of million years that life has got through, before man or the primates did really evolve. While a handful of the life evolved during these periods, many of those evolved had long gone into oblivion long before man or even his nearest ancestors did make their first appearance. 
How do we come to know about the life in those Pre-historic ages?

Nature has helped us here as well. It has preserved the samples of those life forms in a unique way.   These samples have kept a tab on most, if not all, of the life forms making their appearance and disappearance. This is how nature has written its own documents of history.

These samples are called fossils. Fossils are the remains of prehistoric plants and animals, or their impression, preserved in some form. like embeded in stone, And indeed this is the most authentic source that helps us know about life in those periods. 

Some of the significant fossils:
In 1846, the fossils of a Woolly Mammoth was recovered from the Indegirka River in Siberia. This giant elephant like creature had lived 10,000 years ago, when most of the northern Europe was covered by glaciers. It happened to be a well preserved fossil. For, even the contents of the animal's stomach had been refrigerated in the frozen ground of the arctic country.

Since then numbers of other animals have been found similarly preserved by deep freezing in Siberia and Alaska. In Galicia, Poland, a complete Woolly Rhinoceros was discovered, the soft parts of its body preserved in the oil-soaked ground.

The kinds of fossils found in any rock depends mainly on two things. Partly on the geologic age of the rock, and partly on the environment in which it was formed. In the Jurassic deposits of Lyme Regis, Dorset, the remains of many Ichthyosaurs have been found. It was here Mary Anning, as a girl, discovered the first reasonably complete ichthyosaur skeleton. Later, in 1824, she also found the first complete skeleton of Plesiosaurus.

Spectacular as these examples are, in rare cases the whole of the animals have been preserved.
In parts of western Canada, the delicate patterns of dinosaur skin have been preserved as impressions in sandstone. These were formed from sand that covered the dried out dead animals. Bits of skin impressions in sand have been discovered and so also the nests of dinosaur eggs. All this information is used by the scientists in deducing what these animals looked like. And, also, how they moved and acted in life. 

In the famous Rancho La Brea tar pits in Los Angeles, California, bones of many ice-age animals were preserved by burial in asphalt, which sealed and protected them. Add a little bit of imagination, and you will realize that the cries of a Mammoth, caught in the tar, would bring Saber Toothed Cats or carrion loving condors quickly to the scene. And then they, too, would become engulfed in the clinging asphalt. Even mice, and small birds were trapped in the treacherous seeping tar. The soft tissues of most fossils are preserved only as dark films of carbon or, more often, as black outlines on the rock. Again some small marine animals made up the mighty coal-forming forests are preserved in this way.
Again this type of a dramatic disaster are very rare. Other kinds of indirect evidence of prehistoric life are footprints and trails. Also, wastes of some animals and stomach stones, called gastroliths, of others are occasionally found.

Now here are some pre-historic animals based on the period or geological ages for which fossilized evidences have been procured. 

Pre-Cambrian:
Not much have been know about the life of this era, where life is said to have originated. Rocks of Pre-Cambrian age had been found to contain fossils of algae, fungi, jellyfish, worms and some sponge-like animals. All were very simple but already far advanced from the only single cells which 
might be called the first living organisms. Fossils of any of such organisms are yet to be discovered.

Cambrian:
In the first phase of the Paleozoic Era, life thrived in ancient seas. During the whole of the
Cambrian and Ordovician periods and most of the Silurian, totaling 175 million years, many kinds of creatures thronged the shallow seas. These periods show a great variety of animal and plant life. 
All major groups of animals without backbones, called invertebrates, were in existence before the end of Cambrian times, the earliest of the Palaezoic Era starting about 570 million years back. But life was apparently confined to the seas. Most abundant were small brachiopods or lamp shells and trilobites. Trilobites were enormously varied. Most were about 2 inches in length. The largest grew to 18 inches. Many had long spines, giving them a rather shaggy appearance; others were oval and nearly smooth. The snail like trilobites existing during this time have long been extinct. All creatures of the Cambrian were simple compared to their present day descendants.                     

Fishes, the first animals with backbones, became abundant during the Devonian, beginning some 395 million years ago. Thus Devonian is sometimes called the Age of Fishes. Some of the lobe finned fishes, such as the large scaled Osteolepis ventured onto land as the forerunners of land dwelling amphibians. While sharks have survived till these days, the jawless ostracoderms, placoderms have gone into extinction. The Dinichthys of around 30 ft. was the largest animal of its day, some 300 million years ago. All of them showed considerable advancement over Pre-Cambrian life. Possibly till then the land was not inhabited by living things, not even in the lakes and streams. 

Animals came up in land during the later part of the Paleozoic Era. This part is known as the Carboniferous Period that began some 345 million years ago. It lasted for about 120 million years and is subdivided into three parts. During this period, amphibians had their presence on land increased in numbers and diversity. Some of the labyrinthodonts were 10 ft long. The oldest reptiles belonged to the Pennsylvanian age. Clams and fishes abounded in streams and bays. And cockroaches, spiders, scorpions, and dragonfly-like insects with the wingspread of 30 inches also existed in this age.
Many of the marine invertebrates of the lower Paleozoic became extinct during the Permian period. Beginning about 280 million years ago and lasting for some 35 million years this age was dominated marine vertebrates like primitive variety of bony fishes and sharks. Reptiles also increased in numbers and variety. Most of them seem to have lived in relatively dry regions and included pareiasaurs, reptiles with bony skin-plates, and theriodonts, the mammal-like reptiles. Those who became extinct could not stand the geographical and climatic conditions which turned extreme in many parts of the world. In parts of the northern hemisphere shallow landlocked seas evaporated forcing many of the marine animals to adapt themselves to the land environments.

This was followed by the Mesozoic Era, the era of middle life. It lasted about 160 million years, beginning from about 225 million years. Among the marine invertebrates of this era, the most conspicuous features were the increase of foraminifera, echinoderms, and mollusks. Ammonite and belemnite cephalopods were the most distinctive of all the invertebrates. Their coiled and cigar-liked skeletons are found in abundance in Mesozoic rocks throughout the world. Mesozoic can be divided into three stages.

The oldest period of the Mesozoic era is the Triassic, beginning about 225 million years ago and lasting about 30 million. Both plants and animals were significantly different from those of the preceding Permian era. Animals included mammal-like reptiles and insects, lung-fish and amphibians.

Reptiles included the early dinosaurs like saltoposuchus and procompsognathus. These were swift and scampering creatures like the size of the present day turkeys. The giant dinosaur of the day was Plateosaurus, a 20 ft vegetarian. It belongs to a group that gave rise to the dinosaur giants of later periods. Equally as large was the carnivorous variety, Phytosaur. A crocodile like offshoot from the thecodont reptiles. 
Jurassic period, the second part of the Mesozoic period, witnessed the emergence of the first mammals and birds. Dinosaurs which means terrible reptile, were the supreme animals on land. The largest was almost 90 ft long and probably weighed about 50 tons. Others stood 20 ft high, about as tall as a two storied house. Others, much smaller and more delicately built, were very much like birds. Still other had armor of fantastic shapes. Some were savage predators; others were ponderous vegetarians.

One group had hip bones similar in structure to those of modern reptiles. The other group had bird-like hips. But all were reptiles, and shared the reptilian characteristics of cold blood and a relatively feeble brain. Of all dinosaurs the monument necked,  Around 20 ft long, spike backed Stegosaurus, weighed 10 tons. Yet, they had a brain about the size of a walnut.   

Among the marine reptiles Plesiosaurs were widespread and common in Jurassic and later part of the Mesozoic, called Cretaceous times. They were good swimmers and fed on fish which they caught by rapid thrusts of their long, thin neck. Most kinds had broad, flat bodies with long necks and small heads. Their legs were heavy paddles and their tails were long. They have been described as a snake threaded through the body of a turtle. The largest were 50 ft long.
Ichthyosaurs, which lived at the same time, moved with fish like body movements and used their fin like paddles chiefly for steering. 

Cretaceous is the most recent period of the Mesozoic era lasting about 65 million years. In most parts of the world thick deposits of both marine and non-marine sediments accumulated during the Cretaceous period, indicating repeated rising and falling of land and sea. 
And it wasdinosaurs again who dominated the age. Some of the significant variety present then included the horned dinosaurs, Triceratops and Styracosaurus, the spike back lizard. The tank like armored Ankylosaurus also belonged to this age. Among the many kinds of carnivorous dinosaurs Tyrannosaurus was one of the most savage variety. It stood 20 ft high and its skull was over 4 ft long. 

Reptiles continued to be common in the seas throughout the Cretaceous. Ichthyosaurs, the fish like reptiles, declined in numbers and importance. Sea turtles had shells 12 ft long; plesiosaurs were up to 50 ft; and serpent like Mosasaurs were also common. Related to lizards and snakes, many of these were over 20 ft long. They were among the most savage of all marine reptiles in upper Cretaceous times and ate fish. 
The end of the Cretaceous, and hence the Mesozoic, was marked by widespread extinction of many groups of animals. For instance, the ammonites, dinosaurs, plesiosaurs, and pterosaurs. 

Cenozoic Era, meaning the era of recent life, followed the Mesozoic, and lasted about 70 million years. It is also known as the age of mammals because of their dominance. Mammals which had existed as a small, rather insignificant group for at least 100 million years of the Mesozoic, replaced the extinct dinosaurs and spread to almost every environment. The era is divided into Paleocene, Eocene, Oligocene, Miocene and Pliocene epochs.  The more recent and the shortest of all includes the Pleistocene and the Recent epochs. Plants and animals of these times resembled those of today than did those of earlier eras. Early in the Cenozoic, mammals were divided into two groups: plant eaters like condylarths and flesh eaters like creodonts. Though hoofed animals were predominant, mammals of the ancient variety became extinct at the close of the Eocene. These include carnivores like oxyaena, the creodonts, condylarths, uintatheres and amblypods. But reptiles like the crocodiles and turtles survived and were common in the lakes and rivers of many regions.

Many of the grazing mammals developed on the grassy prairies that became widespread during the 
dry Miocene epoch of the Cenozoic era. Among them were Dinohyus, a 6 ft primitive pig. Syndyoceras, a deer like animal that had an extra set of horns on its nose. A small rhinoceros like Diceratherium also known to have existed in those days.

Alticamelus was a long necked primitive camel without a hump. Moropus was an ungainly chalicothere. Merychippus was a nearly modern horse that had a hoofed middle toe and a small useless toe on each side.

Pliocene followed the Miocene and lasted about 5 million years. A variety of elephants lived during this time. So did pronghorn antelopes, the bizarre Synthetoceras with its forked horn nose, and the earlier horse Hipparion.

The marine animals of this time were similar to those still living today, although they include some species now extinct.

The Pleistocene is the most recent age stretching over the last two million years. The primitive horse evolved into its modern form. Pliestocene witnessed the last of the ice ages. It started about 3 million years ago and ended about 11000 years ago. The remains of many Ice Age animals have been found in the La Brea Tar Pits in Los Angeles, California. These include the American Mastodon. It was the extinct relative of modern elephants. It foraged in the swamps of North America in Ice Age some 10000 to 15000 years ago. It was a browser. Modern elephants are largely grazers, as were the mammoths from which they developed. These mammoths also became extinct during the later part of the Pleistocene. So did the carnivorous, saber-tooth(s) and the Dire Wolves.