Between 2.7 and about 1.8 million years ago, the fossil record reveals an adaptive radiation of hominins. At least three lineages emerge: the robust forms (genus Paranthropus), the gracile members of the genus Australopithecus, and the genus Homo, which may or may not be derived from one of the gracile Australopithecus.
The robust australopithecines, genus Paranthropus, existed from about 2.7 to 1.0 million years ago. They are characterized by massive chewing adaptations that result in robust skulls. They were bipedal and had hands similar to human hands. Their brains were slightly larger than those of earlier hominins.
Two kinds of gracile australopithecines occurred at the end of the Pliocene. Australopithecus africanus, a biped with a slightly divergent big toe, occurred in South Africa between about 3.0 and 2.4 million years ago. Australopithecus garhi, associated with animal bones that show evidence of modification by stone tools, is found at about 2.5 million years ago in east Africa.
Fossils of hominins with absolutely and relatively larger brains than earlier forms are found in east Africa and South Africa between 2.4 and about 1.6 million years ago. These forms are classified as early members of the genus Homo and are placed in two species, H. habilis and H. rudolfensis.
The first stone tool industry, the Olduwan, appears in the archeological record about 2.6 million years ago. Olduwan tools are simple choppers and flakes; they added substantially to their users' ability to process foods.
Approximately 2 million years ago, members of the genus Homo began to move around Africa and out of Africa into Eurasia. Compared with earlier hominins, these first humans have larger brains, larger bodies, reduced sexual dimorphism, shorter arms relative to legs, longer legs relative to the body, and reduced absolute and relative postcanine tooth size.
There is a great deal of contention over the number of species represented by fossils in the genus Homo between 1.8 and about 0.3 million years ago, but there is no debate that these fossils represent members of the genus Homo.
It is not fully clear why humans spread around Africa and into Eurasia about 1.8 million years ago, but they were probably able to do so because of their anatomy, cognitive abilities, social cooperation, and material culture.
From at least about 2 million years ago—maybe as early as about 2.5 million years ago—biocultural evolution became a primary mechanism by which members of the genus Homo coped with environmental challenges.
