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 family.

If this accidental variation, or mutation, is helpful in the animal's struggle to survive, it is more likely to be passed on to succeeding generations. In this way, new species can arise.