Thursday, November 11, 2010

Parallel evolution on different continents


The distribution of animal groups was influenced by land routes that were, in their turn, determined by continental drift. A land bridge between the Americas enabled more advanced mammals to invade the south while the armadillo and opossum moved north. Before the desert barrier was established in northern Africa animals now typical of the plains moved in from the north while African animals such as the elephant migrated north. In the east, some oriental and Australian species reached a transitional area between Asia and Australia, while others, such as the squirrel and the tree kangaroo, were unable to.

 The first mammals appeared 216 million years ago, although there was a setback in their evolution with resurgence 114 million years ago. Early mammals were small and probably laid eggs. Hoofed mammals, carnivorous bats and rodents had all diverged from the primate line before the Cretaceous catastrophe. After this period there was rapid development and diversification. Most modern mammals developed around 35 million years ago.  The ice age saw the emergence of man giant mammals, most now extinct. More extinction was to follow due to indiscriminate hunting by human.

Dinosaurs still roamed the Earth when the first primate like mammal appeared: a tree shrew called pergatorius. By 55 million years there were tarsier like primates with grasping hands and feet, binocular vision and relatively large brains. By 30 million years ago the hang nose Old World monkeys and the broad nose New World monkeys had split; ten million years later the ancestral apes split off. Eight million years old Sivapithecus, once thought to be ancestral to man, was probably closer to the orangutan. Molecular evidence suggests human ancestors split from those of chimpanzees 5 million years ago.

Hominid fossils are rare but the best candidate for our early ancestor is probably Australopithecus afarensis which lived in east Africa about 4 million years ago. It was small but had legs that could each be placed under its center of gravity, allowing it to walk upright. Foot prints in Tanzania suggest it did so. Homo habilis, the first member of our genus, made simple stone tools and had a bulge in its brain corresponding to the area we use for speech.  Walking upright may have been the key to human success, allowing the brain cavity to expand without obscuring vision, freeing the hands and putting a bend in the windpipe that we now employ in speech.