The genus Homo almost certainly first evolved in Africa, and it is on that continent that we find the most diversity of species and the greatest evolutionary changes through time. In the mid-20th century, most palaeoanthropologists would have held that, from a palaeontological perspective, our genus contained only a few species, and perhaps only one at any given time. Thus, most would have recognised the existence of perhaps two or three species following one another in time, beginning with Homo erectus in the Early Pleistocene. The past sixty years have witnessed the recovery of a treasure trove of hominin fossils from a variety of temporal horizons in various parts of Africa, Asia and Europe, and of ancient DNA from fossil bones in Europe. These developments, together with more sophisticated analytical techniques, have revealed our genus to have contained considerably more species in the past. At present, we recognise eight or nine species within the genus Homo, with two or more species existing synchronically throughout the Pleistocene (Fig. 1.7.1).

Since the mid-1980s, the lower boundary of the Pleistocene Epoch [i.e. the beginning of the Quaternary Period] has been regarded as corresponding with the base of the Calabrian stratotype, at 1.81 Ma. Recently, however, the International Union of Geological Sciences has recognised the base of the Gelasian stratotype, which corresponds to the Matuyama [C2r] chronozone, or the Gauss-Matuyama boundary, as defining the Pliocene-Pleistocene boundary at 2.588 Ma [Riccardi 2009]. This change is significant for discussion of hominin palaeontology; and pending the outcome of appeals to this ruling we continue here to regard the base of the Pleistocene as 1.8 Ma.