9 The Earliest Putative Homo Fossils

Abstract

The earliest fossil remains of the genus Homo have been discovered in eastern, southeastern, and southern Africa. The sample comprises about 200 skeletal fragments attributable to about 40 individuals and assigned to two species: Homo rudolfensis (2.5–1.8 Ma) showing a combination of primitive dentition with Homo-like locomotion and Homo habilis (2.1–1.5 Ma) exhibiting a progressive reduction of tooth roots but resembling great apes rather than humans in the postcranial skeleton. Another significant difference between early Homo and the australopithecines is the brain size, which was larger in early Homo than Australopithecus but smaller than in Homo erectus. Endocasts of H. habilis from Olduvai Gorge and Koobi Fora reveal a number of distinctive features some of which are recognized as Homo autapomorphies. Differences in tooth wear between H. rudolfensis, with megadont teeth and more horizontal tooth abrasion, and H. habilis, with more gracile molars and higher relief in worn teeth indicate significant differences in diet and ecology of early Homo species. The origin of the genus Homo coincided with the onset of material culture. Between ca. 2.8 and 2.5 Ma, extensive open habitats comprising more arid-tolerant vegetation developed in Africa. The selective pressures of this habitat change resulted in the increased survival of more megadont species varieties. Megadonty allowed these species to feed on tougher fruit and open woodland–open savannah food items resulting in the phyletic splitting of Australopithecus afarensis into Paranthropus and Homo lineages by ca. 2.5 Ma. An evolutionary scenario that complies with both the Habitat Theory and early hominid biogeography is provided. It delineates the association between faunal turnover and climate change, and suggests a single origin for the Paranthropus lineage but separate origins for H. rudolfensis and H. habilis from A. afarensis and A. africanus ancestors, respectively.