Convergent Evolution

Convergent evolution is a common phenomenon, in which unrelated plants or animals come to resemble each other because they have adapted to similar ways of life. The similarity of some species of American cacti and certain desert living spurges in Africa is a good example. Seals and Penguins are apparently very different animals, but both are superbly adapted for swimming. Their limbs and general body shapes appear similar when observed in the water.
Few animals have a clear fossil record as the horse. The oldest known fossils of this family, from rocks some 55 million years old, are of a fox sized creature called Hyracotherium. Its teeth indicate that it ate leaves, and it probably lived in the forests. Fossils from successively younger rock layers show that the descendants of Hyracotheium gradually became larger and were represented by several distinct genera before producing the modern horse about two million years ago. Their toes were reduced to one on each foot and their teeth became larger and stronger to deal with tough grasses.