Since the days of Lyell it is common knowledge that organisms go extinct and have gone extinct in the past and sometimes leave evidence of their previous existence in the form of fossils. The fossils that are deepest are the oldest and contain traits that link them as ancestors to later species. The primate fossil record is just one great example. The earliest primates are found in older rocks (65 Mya) than the first fossil monkeys, which are found in older rocks (35 Mya) than fossil apes (20 Mya). Only in the rocks that were formed within the last 6 million years do paleontologists find upright-walking ancestors of modern humans. Through time, as traits for bipedalism and other human characteristics like large brains increase, the ape-like ones decrease.
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