Cenozoic Biological Evolution (by Colin Groves)

Abstract

Ancestors of mammals separated from reptiles and birds during the Carboniferous. Early members of the mammalian lineage, called “mammal-like reptiles” as they lack mammalian specializations, flourished during the Permian. Survivors of the Permian-Triassic boundary mass extinction progressively developed mammalian characters. The basic characteristic whereby the Mammalia are defined is the structure of the middle ear. In “reptiles”, including mammal-like reptiles, the lower jaw consists of several bones, one of which, the dentary, contains the teeth, another, the articular, forms a joint with a bone called the quadrate in the cranium, and there is only a single bone, the stapes, in the middle ear. Through the Miocene and Pliocene, mammalian lineages seem to have undergone diversification and extinction at rates that had characterised most of the Cenozoic. It was only the Pleistocene that saw elevated extinction rates, mainly affecting large mammals. The human lineage may have separated from that of chimpanzees some 6–7 million years ago, but about 4 million years ago there was an episode of interbreeding between the two lineages, leaving humans with an X chromosome that is markedly more chimpanzee-like than the rest of the genome. Comparative morphology and DNA analysis agree chimpanzeesare the closest living relatives of humans, followed by gorillas, then orangutans and then gibbons. Most molecular clock calculations indicate that the human and chimpanzee lineages separated some 6 million years ago. Sahelanthropus tchadensis is plausibly promoted as the earliest member of the Hominini.