Homo is the genus of great apes that includes modern humans and species closely related to them. The genus is estimated to be about 2.3 to 2.4 million years old, possibly having evolved from australopithecine ancestors, with the appearance of Homo habilis. Several species, including Australopithecus garhi, Australopithecus sediba,Australopithecus africanus and Australopithecus afarensis, have been proposed as the direct ancestor of the Homo lineage. Each of these species have morphological features that align them with Homo, but there is no consensus on which actually gave rise to Homo.
The most salient physiological development between the earlier Australopith species and Homo is the increase incranial capacity, from about 450 cm3 (27 cu in) in A. garhi to 600 cm3 (37 cu in) in H. habilis. Within the Homogenus, cranial capacity again doubled from H. habilis through Homo ergaster or H. erectus to Homo heidelbergensis by 0.6 million years ago. The cranial capacity of H. heidelbergensis overlaps with the range found in modern humans.