Part IV: Pleistocene Epoch
It is generally thought that by 2.5 mya, there were at least two species of Homo in East Africa, Homo habilis and Homo rudolfensis. The inclusion of those fossils in our genus is not accepted by all and is somewhat arbitrary. Some argue that H. habilis does not differ enough from australopiths to warrant different genus designation. Its inclusion in Homo was prompted by the fact that they are thought to have made and used tools and thus to have been cognitively advanced. H. habilis was more encephalized than the australopiths, and the skull vault is flexed as in Au. africanus, P. boisei, P. robustus, and later species of Homo.
There is debate as to whether H. habilis or H. rudolfensis gave rise to Homo ergaster (African form of the erectus-grade) and hence should be included in our lineage. Both species overlap the more derived H. ergaster in time and geographic space. The size and architecture of the brain of H. habilis make it a contender in the minds of some researchers. However, their limb proportions, i.e. retention of long arms and short legs, do not resemble H. ergaster. According to some researchers, H. rudolfensis possessed more modern femora (plural of femur), limb proportions, and a more orthognathic face, thus making them a better candidate for the ancestor of H. ergaster. However, it has not been determined that postcranial remains can be definitively assigned to the species, because postcrania are rarely found in association with cranial material (from which species designation derives). In addition, the claim to a more orthognathic face has been called into question based upon biometric impossibilities—the reconstruction of the face is impossible in terms of the relative positioning of key features (Bromage et al. 2008). While some claim that H. rudolfensis were more encephalized than H. habilis, others believe that their relative brain size (i.e. brain size taking into account body size) was lower and the temporal region, which is a conservative region of the skull, was more primitive. They also had more robust faces and teeth, unlike H. ergaster.