The evolutionary history of humanity begins at least 6-7 million years ago with the fossil ape Sahelanthropus tchadensis, which displays both human and ape-like features. Sahelanthropus may be the common ancestor of chimpanzees, gorillas, and/or humans, an early ancestor of humans, an early ancestor of chimps, an early ancestor of humans, or a completely different lineage to all of the above. In any case, it pushed back the likely date of chimp/human divergence by several million years, which based on early molecular studies was though to be 3-5 million years ago. Such a late divergence is no longer accepted among the anthropological community.
Shortly after 6-7 million years ago, or whenever the evolutionary history of human ancestors split from chimps, the fossil record continues with Orrorin tugenensis (6.1-5.8 mya), the earliest human ancestor with evidence of bipedal locomotion; Ardipithecus (5.5-4.4 mya), another upright-walking species that nonetheless had a brain and body similar to that of a chimpanzee; the famous Australopithecus (4-2 mya), a "gracile australopithecine" represented by the fossil "Lucy;" Kenyanthropus (3-2.7 mya), one of the first known apes in evolutionary history with a flat face; and Paranthropus (3-1.2 mya), a "robust australopithecine," with a sturdy build and brain size approaching 40% of modern humans.
Around 2.2 million years ago, the genus Homo appeared in evolutionary history, coexisting with Paranthropus and other human-like apes that lived at the time. This genus was a huge intellectual improvement on what came before it, and one of its earliest members, Homo habilis, has a name that means "handy man." That's because this was one of the first animal species to master stone tool technology, though there is evidence that Australopithecus garhi, dated to 2.6 million years ago, was probably a skilled stone tool-user as well. This marked the beginning of the Stone Age, which continued for millions of years until the Bronze Age began just 5,300 years ago.
The most important species in the evolutionary history of humans are our immediate relatives: the members of genus Homo. The word "Homo" simply means "man" in Latin, and these beings were indeed close to man, with large brains, an upright posture, social natures, and tool-using capability. Unfortunately, they are all now extinct, so we'll never know what they were really capable of, or how they communicated with one another. These important human relatives include Homo habilis, Homo rudolfensis, Homo ergaster, Homo georgicus, Homo antecessor, Homo cepranensis, Homo erectus, Homo heidelbergensis, Homo rhodesiensis, Homo neanderthalis, Homo sapiens idaltu, and Homo floresienses. Genetic material left behind by some of these species is being studied and will shed important insight on their relationship to present-day humanity.
Michael is a longtime WiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.