This question is so general that it is difficult to answer. It's a very long and complicated story, and we don't know all of the details yet, although in some cases we have some pretty good guesses. The ancestor of humans included a species of ape that lived approximately six to seven million years ago (we think), that split into a number of branches, one of which led to the modern chimpanzee and another of which leads to modern humans. Most of the other branches became extinct. The branch that led to humans became adapted to a less forested environment than the branch that led to chimps, as the climate where it was living in eastern Africa became increasingly dry. Theevolution of the bipedal nature of humans appears to have had something to do with the loss of trees, although this presents a question because baboons also walk on the ground, and they use their knuckles to walk. However, walking upright freed human hands forcarrying objects, particularly tools, and this seems to have been the beginning of what we would consider "human" evolution.
Now, there are a couple dozen extinct species of ape that are more closely related to modern humans than to anything else currently living. At what point we call these apes humans is a matter of naming definitions, so one can argue about how many extinct species of humans there are. There is only one living species of humans, however, which appeared on the scene apparently about a hundred thousand years ago. What separates our species from its ancestors and relatives, apart from its physical traits of course is up for debate, but our species certainly has the most sophisticated tools of any ape. My personal suspicion is that it was language - which our species can communicate ideas without the item or concept being discussed is actually present to be used to illustrate. As a result, it is possible to transmit knowledge to people whom you have never met, just as we are conversing over email, and thus when a person makes an improvement to a tool or technique, others can copy it without it being potentially lost.
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