How do we know about ancient creatures?
Scientists who study the extinct creatures that once roamed the Earth
are called paleontologists. If you were to watch a paleontologist
at work, you would probably see him on his hands and knees, methodically
and painstakingly examining the surface of the ground. This is because he
is looking for fossils, or else he has lost a contact lens.
|By collecting and analyzing fossils, paleontologists have
succeeded in tracing the entire history of life on Earth, from the first
living things to modern man. Of course, there are gaps in the record and
minor uncertainties of interpretation. Nevertheless, the overall picture
is clearly understood. (left)
How does evolution work?
Animals of a given species are alike because they inherit a certain set
of genes from their parents. Every so often, something goes wrong with the
mechanism that transmits the genes, and an animal is born that doesn't
resemble its father and mother. You probably know of examples in your own
The First Living Things
Evolution proceeds from simpler forms to the more complex. The first animals were simple one-celled blobs of protoplasm. Then two-celled animals arose. They were followed by three-celled, four-celled, five-celled, etc. (If you find the mathematics here too difficult, just think of the numbers larger than one as "many.")
It is easy to see how this process led inevitably to more complex forms such as sponges, the worms and so forth. Especially if you don't think about it too much.
The first vertebrates to exploit the rich food resources of dry land were the amphibians (from the Greek amphi-, slimy + bios, dumb). Many species attempted the transition without success. These early forms were hampered by their limited range and mobility on land.
|A true land-dwelling form was made possible by the development of the lung. The amphibians, however, still had to return home to lay eggs and do their laundry.|
This group, with characteristics that foreshadowed those of the later mammals, arose early in the development of the reptiles. However, other reptilian orders became the dominant terrestrial forms, crowding out these forward-looking species. As you may have noticed, often when you arise too early, you're no good for the rest of the day.
The Age of Dinosaurs
The reptiles reached their peak with the rise of the dinosaurs (from
the Greek dino, ugly + sauros, smells bad).
The dinosaurs have traditionally been pictured as slow, stupid and lethargic beasts (above). Recent thinking, however, suggests that they may in fact have been highly intelligent, active and well-adapted to their environment (below).
At the end of the Creosote era, two new classes arose to challenge the
ruling reptiles - birds and mammals.
The first mammals were tiny shrewlike creatures. They seemed to
be no match for the mighty dinosaurs. But though small, they were clever and may
have contributed to the downfall of the giant reptiles.
One of the mammals' evolutionary advantages was that they bore their young alive. As research has conclusively shown, animals that bore their young dead generally got nowhere.
Early mammals were typically small, like the tiny Itsyhippus (left) and the Pond Hippopotamus (right).
After the Creosote extinctions, mammals were able to take
advantage of the fact that they were endothermic (liked flowers),
placental (had bad breath) and quadrupedal (didn't know any
better) to spread and diversify.
In the Tutelary Age, mammals became the dominant class and grew quite large at times. The great Irish bunny, for example, often had antlers eight feet across.
The Universe / Matter & Energy / The Earth / Evolution / The Descent of Man
Appendix / Glossary / Tables & Charts / Further Reading