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New fossils may be critical missing link in primate evolutionary history

We humans and all ape and monkey species all come from one family stock, but some 25 million years ago, that ancient family split in two, one branch leading to monkeys, and the other leading to us and the other apes. This is according to Ohio University scientists, who base their assessment on the two new—to us, that is—primate species whose fossilized remains the scientists recently dug up in Tanzania.

The two primate species appear to have lived in the Oligocene epoch, which ran from 34 million to 23 million years ago and was marked by, among other things, the global expansion of grasslands and the restriction of tropical broad-leaf forests to Earth’s equatorial zones. The scientists, who published a report of their findings in Nature, have dubbed one species Nsungwepithecus gunnelli and identified it as the oldest member of the primate group that includes the old world monkeys. They classify the other species, Rukwapithecus fleaglei, as the earliest member yet of our group, the hominoids, which includes gorillas, chimpanzees, bonobos, gibbons, and orangutans, as well as us humans. The fossils that turned up include a tooth from the old-world monkey group ancestor and a jawbone from the hominoid ancestor.

The area where the fossils turned up is the site of a rift in the Earth’s tectonic plates. Those plates might have been more active when the species were alive, according to the researchers, who suggest that tectonic-plate activity might have forced shifts in the African landscape, which in turn might have influenced the species to start diverging.

The two newfound species constitute an important hitherto-missing link in the long chain that leads from the very earliest primates apes of many tens of millions of years ago to our own species and the menagerie of ape cousins who share the planet with us today. The pair mark the critical juncture at which apes and monkeys began to diverge along separate paths of evolution.

The new finds also push back the dates of early primate evolution further. Previously, scientists had known of only three species who had been around in the Oligocene, and all three occupied only the late Oligocene at that. Moreover, their living even at that point in time was only a hypothesis, as the oldest fossils ever found were only 20 million years old. Scientists could only extrapolate the species’ earlier existence from DNA patterns. These two new species having clearly lived even earlier, and us having the solid fossil evidence to prove it, will most likely lead researchers to amend some of their existing theories about when key diversifications took place among the various primate groups, according to the researchers.