Though it is often said that humans descended from monkeys, most people today understand that that statement is quite untrue. Modern day humans are descended from a shared common ancestor with modern day apes, with chimpanzees being our closest living relatives. Whenever a new human species is discovered, earlier than those we know of, excitement builds around the idea that this species is the “missing link”, the definitive bridge showing that humans and chimps evolved from the same species. Given that we cannot be certain that there were no other species between any known early hominid and chimpanzees, we can’t say that any species is the chimpanzee-human last common ancestor (often abbreviated CHLCA), but there are a few close possibilities. The separation might have begun to occur around 12 million years ago, though hybridization between chimpanzees and hominids continued to occur for several million years after that.
The most famous “missing link” genus is Austrolopithecus. This species is known to have lived in North Eastern Africa about 4 million years ago. Their hands were adapted to walking on their knuckles, but the morphology of their legs were distinctly different from those of apes and could have supported bipedalism. Even with a group like Austrolopithecus, which has a fortunately large fossil record, things are more complex than we originally thought, as inevitably always is the case. Two species, anamensis and afarensis, were thought to evolve directly one after the other. New discoveries from just the past few years have shown, though, that these two species actually lived at the same time, side by side.
Another possibility of the CHLCA is Sahelanthropus. This species lived between 7 million and 6 million years ago in the same area that Austrolopithecus would live. It is only known from one skull, one which shows similarities to both humans and apes. They could have lived at a time when too much hybridization was occurring to definitively place species in either the human or chimp side. Given that we only have a skull, we can’t determine how much of a biped this species was, either. Until more discoveries are made, Sahelanthropus has to remain a possibility, due to the time, but a tentative one.
Dated to the same time as Sahelanthropus, Graecopithecus is another contender for CHLCA. Scientists are the most skeptical about this possibility. If that is true, it would suggest that the split happened in Europe rather than Africa, as all the other evidence suggests. Graecopithecus shares traits with humans which are not present in chimpanzees, however, and a species this old with such clearly defined traits should be considered when talking about the CHLCA.
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