The Human Genus

With the genus Homo we are given the earliest fossil evidence for dispersal outside of Africa (Figure 3.10). As with all lineage beginnings, the identification of the first member of the genus Homo in the fossil record is difficult. There are several specimens from Ethiopia, Kenya, and Tanzania that are worthy candidates. The problem lies in the definition of Homo. The characteristics that warrant inclusion in the exclusive club of the human genus are not generally agreed upon.

It is no longer fashionable to assign hominins to Homo based on stone toolmaking ability, since it has been difficult to conclusively demonstrate association between the earliest tools and a specific species. There is considerable overlap in geography and age of Homo, Paranthropus, and Australopithecus during the late Pliocene and early Pleistocene, so we cannot definitively exclude Australopithecus or Paranthropus from the stone toolmakers' guild.

Other researchers rely on anatomical rather than behavioral evidence for inclusion in the genus Homo. The majority of researchers consider Homo habilis to be the earliest member of the human genus based on its differences from Australopithecus, such as its smaller and flatter face, less sloping forehead, more rounded skull, smaller jaws and teeth, thinner cranial bones, and lack of cranial crests. The long-standing brain size requirement is still generally followed: skulls that have cranial capacities over 600 cc are safely considered to belong in the genus Homo. As long as skulls with that cranial capacity do not have Australopithecus or

Paranthropus features, that rule is supported and none have been found to contradict it so far.

Based on its more ape-like limb proportions reconstructed from fragmentary skeletal remains, and based on many of its similarities to aus-tralopiths as opposed to later H. erectus, H. habilis is increasingly being placed in the genus Australopithecus.

One reason behind this taxonomic shift is the partial skeleton OH 62, a H. habilis from Olduvai Gorge. The fragmented remains of this old female were found near a dirt road after they had been unintentionally driven over. When the limbs are reconstructed and proportions are estimated, OH 62's body is very ape-like, sometimes even more ape-like than some australopiths. Some fossils, like this one in particular, cause more trouble than provide answers. Until better skeletons of Australopithecus, Paranthropus, and early Homo are discovered, the classifications of many fossil hominins around this crucial period of human evolution around 2 Mya are best considered tentative. In the end Homo is just a name. We know that at some point Homo evolved from earlier australopiths. The more fossils we find, the blurrier the division between "apeman" (australopiths) and "human" (Homo).

Fossil hominins clearly located within the early part of the genus Homo are also undergoing identity crises. H. habilis was established in 1964 by Louis Leakey, Philip Tobias, and John Napier. They designated the new species based on hand bones and skull fragments of a juvenile (OH 7) that they excavated from Bed I at Olduvai Gorge. H. habilis literally means "handy man"—a name that is based upon the inferred humanlike grip capabilities of the fossil hand. The species was deemed responsible for the Early Stone Age tools (Oldowan Industry) found near the site.

H. habilis is a species of small-brained, small-bodied hominins that is usually kept separate from another species that spanned the same time range (2.3-1.8 Mya) but had a larger brain, larger teeth, and a flatter face, H. rudolfensis. The two fossils that symbolize the differences between H. habilis and H. rudolfensis are the very small skull KNM-ER1813 (500 cc) and the significantly larger skull KNM-ER 1470 (775 cc) (Figure 3.11). For some paleoanthropologists (splitters), the size difference between the two is too large to keep them in one species, but others (lumpers) consider it normal variation worthy of a single variable species. In 2006, new dates for the sediments at Koobi Fora were reported by Patrick Gathogo and Frank Brown. Now the smaller skull (1813) is deemed 0.25 Mya younger (1.65 Mya) than the larger one (1.9 Mya). If these dates are confirmed, there may no longer be an issue.

Figure 3.11 Both found in the early 1970s at contemporaneous sites at Koobi Fora, Kenya, these skulls (right: KNM-ER 1470, left: KNM-ER 1813) typify the diversity of size and shape in early Homo. Photographs by Alan Walker.

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