Homo is the genus 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, evolving from australopithecine ancestors with the appearance of Homo habilis. Specifically, H. habilis is assumed to be the direct descendant of Australopithecus garhi which lived about 2.5 million years ago. The most salient physiological development between the two species is the increase in cranial capacity, from about 450 cc (27 cu in) in A. garhi to 600 cc (37 cu in) in H. habilis. Within the Homo genus, cranial capacity again doubled from H. habilis to H. heidelbergensis by 0.6 million years ago. The cranial capacity of H. heidelbergensis overlaps with the range found in modern humans. The advent of Homo coincides with the first evidence of stone tools (the Oldowan industry), and thus by definition with the beginning of the Lower Palaeolithic. The emergence of Homo also coincides roughly with the onset of Quaternary glaciation, the beginning of the current ice age. All species of the genus except Homo sapiens (modern humans) are extinct.