The origins of humanity may be traced to the tropical African Pliocene, around 6 million years ago (Myr). Genetic evidence has for some time predicted the existence of a common ancestor to chimpanzees and humans around 5–6 Myr (Takahata & Satta, 1997; Gagneux & Varki, 2001). Recent discoveries of African fossils that are claimed to be close to this common ancestor have been dated to between 6 and 7 Myr (Brunet et al., 2002).

From this point until the emergence of Homo erectus 1.9 Myr ago and its rapid subsequent range expansion (Aguirre & Carbonell, 2001), hominids were confined to sub-Saharan Africa. The estimated number of species that lived during this long period in the Pliocene varies among authors. If we follow a conservative approach (Klein, 1999) we observe a pattern of increasing hominid species richness from about 4.6 Myr with a peak between 1.9 and 1.6 Myr and a sharp decline thereafter (Fig. 1.1). The decline after 2 Myr ago is correlated with increasing climate instability.

The peak in diversity coincides with the first appearance in the fossil record of H. erectus. Recently this early African member of the genus Homo has been separated from contemporary Asian forms. The name H. erectus has been retained for the Asian forms and the name H. ergaster for the African (Klein, 1999). Recent evidence suggests, however, that the two significantly overlap in morphology and that they should form part of a geographically diverse species H. erectus (Asfaw et al., 2002).