Life Past & Present

The earth teems with life. Mountains, prairies, deserts, beaches, lakes, rivers and sees—every part of lend, sea end air is inhabited by living things. The number of different species of living things is enormous. More then 350,000 species of plants end 1,120,000 species of animals are known.

How did these many species originate? Has life always been the same es it is now? Men have asked these questions for thousands of years. To answer them we must turn to fossils and to a knowledge of living organisms end their structure. Only an understanding of living animals can put life in the fragments of bones and shells millions of years old.

The elephant is the largest living land animal in the world today. But the study of fossils shows not only that elephants area recent group in the long history of living things but also that early elephants looked more like hogs. As geologists trace elephant fossils from older to younger rocks they piece together the history of elephant evolution. Fossil bones and teeth reveal the structure of early elephants, but by studying these fossils in the light of the anatomy of living elephants, complete reconstructions of extinct elephants can be made with reasonable accuracy. Some unusual occurrences ot mastodon fossils with crude flint weapons prove that these elephants were hunted by our ancestors.

All forms of life have evolved from early beginnings, some three billion years ago. From relatively few primitive forms, the major groups of plants and animals developed. Living things became more complicated and adapted to many different ways of living. The number of different species gradually increased until they reached the tremendous diversity of today. The study of fossils (paleontology) traces the various paths by which animals and plants evolved to their present forms. Some, like elephants and horses, have changed greatly through the ages. Others, like the horseshoe crab and cockroach, have not changed in hundreds of millions of years. Still other fossils show lines of development that came to a dead end. Giant Sloths, once plentiful, are known only as fossils.

The Glyptodont, a 9foot armored mammal from the late Cenozoic, is a fossil that shows spectacular and obvious adaptation. This relation of modern day armadillos was protected against carnivores and other enemies by a thick, solid, domed armor, which reached almost 5 feet in length in some forms. The head and tail were also armored, and in some species, the tail terminated in a spiked, mace-like club. Yet despite, or because of, these unusual adaptions glyptodonts became extinct.
 

Most plants and animals exist only because they are successfully adapted to their environments. Each distinct environment such as a desert, pond or mountain top supports a more or less distinct population of animals and plants. Those which, over long periods of time, have become fitted to cope with local conditions have survived. All the rest have become extinct. Many living things are uniquely adapted to particular environments. The streamlined shape of a fish and the structure and function of its fins and tail are adaptations to life in the water. The fleshy stems of a cactus are adaptations that conserve water in the desert. Such adaptations succeeded, but the fossil record is strewn with the remains of those that failed. The slogan of life may well be—adapt or perish.

Survival in animals depends on adaptations as varied and as intricate as the animals themselves. Virtually every structure at a plant or animal may be regarded as adaptive. Many animals have protective coloring and a few forms, such as the bottom-living flounder are able to change their color to conform to their background. Such an intricate adaptation in rarely discernible in fossils. However, if the adaptation affects bone or shell, it may show op clearly in the fossil record.

Development of modern animal life is difficult to trace because the fossil record is incomplete. Enough is known to suggest the general pattern of evolution and to reconstruct in some detail history of groups in those areas where fossils occur abundantly. The chart below shows some of the relationships among major groups of vertebrates:

Cladogram of select vertebrates showing



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