How did modern humans develop from their ape-like ancestors? When did they start walking on two legs and making tools?
The familiar image of human evolution, in which we see a chimpanzee-like creature on one side, while on the other we find modern humans with a series of individuals found between them, each more upright and a bit less hairy than the one before him, is an abstraction that falls short of properly describing a very complex process.
What we do have instead are bones and bone fragments that belong to about 20 different species, the precise number of which is also under debate. Some researchers may think that three fossils found in relatively close proximity could belong to the same species, while others will argue that each one represents a separate species.
More and more fossils are continually being uncovered: one of the last found fossils, Homo naledi, was first described in the scientific literature in late 2015. Many of the species co-existed over tens and hundreds of thousands of years. In fact, it appears that the current situation, in which only a single species of humans lives on Earth, is the exception.
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