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9.5: Earliest Bipeds

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    Three species of very early hominins have made news in the past few years. The oldest of these, Sahelanthropus tchadensis, has been dated to nearly 7 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” or “Chad Man,” is a mosaic of primitive and evolved characteristics, and it is unclear how this fossil fits with the picture given by molecular data, namely that the line leading to modern humans and modern chimpanzees apparently bifurcated about 6 million years ago. While the phylogeny of S. tchadensis  is unknown, some researchers believe that it may represent a stem  or basal hominin, i.e. one of the earliest members of our tribal tree. 

    Figure \(\PageIndex{1}\): “Chad Man”

    A second, younger species, 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 Orrorinis much older.

    A third genus, Ardipithecus, was discovered in the 1990s, and 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, classified as two different species, demonstrated that the organism was bipedal. During the early 1990s, fossils were unearthed at the site of Aramis  in the Middle Awash region  of the Afar Triangle of Ethiopia (see Figure). Since that time, material from more than 50 individuals has been recovered, in particular the famous “Ardi” skeleton that is ~50% complete. Prior to the discovery, all or most early African hominin fossils were considered to be australopiths. Tim White and his colleagues determined that the material was distinctive enough to warrant new genus classification.

    Figure \(\PageIndex{2}\): Digital reconstruction of Ardipithecus ramidus specimen. “Ardi” by T. Michael Keesey is licensed under CC BY 2.0.

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