Fossils and the diversity of life

Why are there so many kinds of living things? Several causes work together over very long times: adaptation to different environments; natural selection, as in Week 12; genetic mutations, random changes in genes that sometimes give a new variation and that add up over generations; and geographical isolation. A barrier such as a mountain range, a wide river or a deep valley stops the groups from mating, so genes no longer pass between them. Each group meets different conditions and changes in its own way, and after a very long time they may become different species. Scientists also compare body structures: this is comparative anatomy. Homologous structures have the same basic plan, inherited from a common ancestor, even when the job differs: the arm of a person, the wing of a bat and the flipper of a whale. Analogous structures do the same job with a different plan: the wing of a bird and the wing of an insect. The more alike the inner structures of two animals are, the closer their relationship; modern birds share hollow bones, posture and feathers with theropod dinosaurs, and the jaw bones of reptiles became the middle-ear bones of mammals. A transitional fossil shows features of two groups: Archaeopteryx, which lived about 150 million years ago, had teeth and a long bony tail like a reptile and feathered wings like a bird. Scientists draw these conclusions from evidence, and they say openly where the record has gaps.