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First chordates

"Primate teeth reported in late Cretaceous deposits. Paleocene primate identification is controversial. There is consensus recognizing early Eocene tarsier-like species, middle Eocene lemur-like species, and recently discovered late Eocene simian species.

"Primate teeth reported in late Cretaceous deposits. Paleocene primate identification is controversial. There is consensus recognizing early Eocene tarsier-like species, middle Eocene lemur-like species, and recently discovered late Eocene simian species.

tions. The Cenozoic was more variable in every way, with more diverse and sometimes chilling climates and periods of major mountain building. A burgeoning animal and plant life is evident in fossils from sediments laid down during all of these periods.

Second, there were several mass extinctions, with the greatest, at the end of the Permian Era, signaling the beginning of the Mesozoic. The most famous extinction, attributed to impact by a small asteroid, occurred at the end of the Mesozoic (the K-T, or Cretaceous-Tertiary boundary) 65 million years ago. Niches, emptied of their otherwise well-adapted organisms that could not survive the environmental catastrophe, could then be filled by suitably adapted birds, mammals, teleost fish, and snakes.

Third, although mammals were present during much of the 185 million years of the Mesozoic Era, all were small-bodied, none larger than living cats. They were probably nocturnal in their habits. Only during the Cenozoic did very large mammal species appear, and even today the average mammalian species is about cat size and nocturnal. Humans are giant vertebrates, physically larger and heavier than

90% or more of living species. Anthropoid primates are an unusual group of mammals; species of the suborder Anthropoidea (monkeys, apes, and humans) are diurnal and are well-adapted for color vision.

Finally, a major environmental event in human history may have been the Pliocene drying of the Mediterranean about 5 million years ago, which probably contributed to natural selection among chimpanzee-like primates for a species that became the earliest hominid. Extensive glaciation characterized the Pleistocene Epoch and may have driven the evolution of the human species to its present grade.

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