The Genus Homo  
Although scientists do not agree, many believe that after the evolutionary split that led to the robust australopithecines, A. africanus evolved into the genus Homo. If so, this evolutionary transition occurred between 1.5 and 2 million years ago. Fossils dating from this period display a curious mixture of traits. Some possess relatively large brains—several almost 800 cc (about 49 cu in)—and large, australopithecine-sized teeth. Others have small, Homo-sized teeth but also small, australopithecine-sized brains. A number of fossil skulls and jaws from this period, found in Tanzania and Kenya in eastern Africa, have been placed in the category H. habilis, meaning "handy man," because some of the fossils were found associated with stone tools. H. habilis possessed many traits that link it both with the earlier australopithecines and with later members of the genus Homo. It seems likely that this species represents the evolutionary transition between the australopithecines and later hominines.

The earliest evidence of stone tools comes from sites in Africa dated to about 2.5 million years ago. These tools have not been found in association with a particular hominine species. By 1.5 to 2 million years ago, sites in various parts of eastern Africa include not only many stone tools, but also animal bones with scratch marks that experiments have shown could only be left by humanlike cutting actions. These remains constitute evidence that by this time early hominines were eating meat, but whether this food was obtained by hunting or by scavenging is not known. Also unknown at present is how much of their diet came from gathered vegetable foods and insects (dietary items that do not preserve well), and how much came from animal tissue. It is also not known whether these sites represent activities by members of the line leading to Homo, or if the robust australopithecines were also making tools and eating meat.

A new species of large brained, small-toothed hominines emerged in Africa around 1.7 million years ago. A similar species appeared in Asia around the same time or slightly later. Until the 1990s scientists classified both species as Homo erectus. However, most scientists now believe that they actually constitute two separate hominine species. The African specimens are now usually referred to as Homo ergaster, while the designation H. erectus is reserved for the Asian fossils. Most scientists believe that H. ergaster arose first in Africa and then spread rapidly into Asia, where it evolved into H. erectus. The relationship between these two species remains controversial, however. Recent discoveries and new dating methods have led some paleoanthropologists to claim that H. erectus inhabited China between 1.7 million and 1.9 million years ago. Some scientists have suggested that the oldest hominine fossils found in Asia may be evolutionary predecessors to H. erectus.

Throughout the time of H. erectus and H. ergaster the major trends in human evolution continued. The brain sizes of early H. ergaster fossils are not much larger than those of previous hominines, ranging from 750 to 800 cc (45.8 to 48.8 cu in). Later H. erectus skulls possess brain sizes in the range of 1100 to 1300 cc (67.1 to 79.3 cu in), within the size variation of Homo sapiens.

A number of archaeological sites dating from the time of H. erectus reveal a greater sophistication in toolmaking than was found at the earlier sites. Evidence found at the cave site of "Peking Man" in northern China,  suggests that H. erectus used fire. Fossils of bones from large mammals such as elephants found near H. erectus sites suggest that H. erectus had developed sophisticated hunting skills. Such data suggests that many hominine behaviors were becoming more complex and efficient.

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