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Evidence fossil records and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominin ancestor about 6 million years ago. Many species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin is used to refer to species that evolved after the split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. Hominins, who were bipedal in comparison to the other hominins who were primarily quadrupedal, includes those groups that probably gave rise to our species. These are: Australopithecus africanus, Homo habilis, and Homo erectus, along with non-ancestral groups such as Australopithecus boisei. Determining the true lines of descent in hominins is difficult. In past years, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans. In the past several years, however, many new fossils have been found. It is possible that there were often more than one species alive at any one time and that many of the fossils found represent hominin species that died out and are not ancestral to modern humans. However, it is also possible that too many new species have been named. There have been three species of very early hominins which have made news in the past few years. The oldest of these, Sahelanthropus tchadensis, has been dated to nearly seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics. To date, it is unclear how this fossil fits with the picture given by molecular data. The line leading to modern humans and modern chimpanzees apparently bifurcated about 6million years ago. It is not thought at this time that this species was an ancestor of modern humans. It may not have been a hominin. A second, younger species that was around 5.7 million years ago, Orrorin tugenensis, is also a relatively-recent discovery, found in 2000. There are several specimens of Orrorin. It is not known whether Orrorin was a human ancestor, but this possibility has not been ruled out. Some features of Orrorin are more similar to those of modern humans than are the australopiths, although Orrorin is much older. A third genus, Ardipithecus ramidus  from 4.4 million years ago was discovered in the 1990s. The scientists who discovered the first fossil found that some other scientists did not believe the organism to be a biped thus, it would not be considered a hominid. In the intervening years, several more specimens of Ardipithecus, including a new species, Ardipithecus kadabba from 5.6 million years ago, demonstrated that they were bipedal. Again, the status of this genus as a human ancestor is uncertain, but, given that it was bipedal, it was a hominin. All of this information has been accessible to us today because of anthropologists who use fossilized skeletal remains of early human ancestors to reconstruct and learn from the past.


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