We have come to the conclusion that, apart from Australopithecus (Zinjanthropus), the specimens we are dealing with from Bed I and the lower part of Bed II at Olduvai represent a single species of the genus Homo and not an australopithecine … But if we are to include the new material in the genus Homo (rather than set up a distinct genus for it, which we believe to be unwise), it becomes necessary to revise the diagnosis of this genus.
With these words, Louis Leakey and colleagues named the species Homo habilis. In so doing, they widened the definition of the Homo genus to include a species with smaller average brain size than that of Homo erectus, which was up until then the earliest known species of our genus. To include this species in the genus Homo, Leakey and colleagues revised the so-called “cerebral Rubicon” dividing Australopithecus from Homo down to 600 cubic centimeters from the 750 cubic centimeters used up until that point (240). This is what Leakey et al. meant by revising the “diagnosis of the genus.” Indeed, over the history of paleoanthropology, definitions of the genus Homo have expanded further, encompassing the varied features of fossils assigned to the genus (177, 241). Thus, how to define the genus Homo – and indeed how to define a genus in the first place – are matters of long-standing and vigorous debate (47, 177).
In a 2014 review paper in the journal Science, paleoanthropologist Susan Antón and colleagues’ employed a broad definition of the genus that included specimens traditionally assigned to the species Homo habilis (e.g., KNM-ER 1813). In their view, these specimens shared with later genus members a suite of characteristics that contributed to their evolutionary success. Namely, these are: “… increases in average body and brain size and changing dental size coupled with increased toolmaking and stone transport” which they contend “suggest dietary expansion, developmental plasticity, cognitive evolution and social investments” (2014: 1236828–8). The present chapter gives an overview of species that have been considered to be members the genus Homo, that to varying degrees partake in the package of adaptations Antón and colleagues describe. Key dental features and possible phylogenetic relationships to anatomically modern human are integrated into this overview.