15-12 million years ago: movement of the earth's crust

6 million years ago: Orroran (first bipedal creature) not confirmed

3.2 million years ago: "Lucy," Australopithecus afarensis: bipedal

2.5 million years ago: Homo habilis: tools

Homo erectus, left Africa 1 million years ago

Homo sapiens first emerged in Africa 500,000 years ago

Homo sapiens neanderthalensis, a regional variety of Homo sapiens arose about 100,000 in Europe and western Asia

Homo sapiens sapiens emerged in Africa 100,000 years ago

45,000-40,000 years ago: Homo sapiens sapiens first moves out of Africa into southwestern Asia and Europe

Australopithecus afarensis -- bipedal creatures, but they retained some anatomical features of tree dwellers. 5-7 million years ago. Did not use tools.

The proliferation of different species of this type is known as adaptive radiation.

"Lucy," 3 million years old, was Australopithecus afarensis. Until Lucy was discovered there was no tangible evidence of bipedalism in a human species older than about 2 million years. Lucy was ape-like, not fully human; trees were still very important to the species. Fossil record does not go back to 7 million years ago; hominids are inferred by molecular biology, which studies the rate of possible mutation. Mary Leakey discovered the trail of very human like footprints made in a layer of volcanic ash some 3.75 million years ago.

15-12 million years ago: the earth's crust was tearing itself apart beneath the eastern part of the African continent. As a result, the land rose blister like in Ethiopia and Kenya, forming great highlands more than 9000 feet in altitude. These great domes transformed the climate. Disrupting the previously uniform west-to-east airflow, the domes threw the lands to the east into rain shadow, depriving the forests of their sustenance. The continuous tree cover began to fragment, leaving a mosaic environment of forest patches, woodland, and scrubland. Open grassland, however, was still rare.

East side story: 12 million years ago, a continuation of tectonic forces further changed the environment, with the formation of a long, sinuous valley, running from north to south, know as the Great Rift Valley. The existence of the valley has had two biological effects: it poses a east-west barrier to animal populations; and it further promotes the development of a rich mosaic of ecological conditions. The population of the common ancestor of humans and apes found itself divided. The western descendants of these common ancestors pursued their adaptation to life in a humid, arboreal milieu: these are the apes. The eastern descendants of these same common ancestors, in contrast, invented a completely new repertoire in order to adapt to their new life in an open environment: these are the humans.

Tools: 2.5 million years ago, humans began producing sharp-edged tools by hitting two stones together. Were the earliest toolmakers employing mental abilities comparable to those of apes, but in a different way? Or did it require them to be of higher intelligence. The brain of the toolmakers was some 50% bigger than that of apes. Later they progressed to hand axes. Only Homo made tools, not Australopithecus, and it is likely that meat-eating by Homo was an important part of that difference. Stone tool making would have been an important part of a meat-eater's abilities; plant eaters could do without these tools. A change in teeth and brain size are both present in the Homo habilis. a jump from 450 to 600 cubic centimeters in h. h.

Homo habilis: handy man. 50% greater brain. Genus homo must exceed 750 cubic centimeters in brain capacity. 2 million years ago.

The ancestral tree bore two main branches: the australopithecine species, all of which became extinct by 1 million years ago, and Homo, which eventually led to people like us, Homo sapiens sapiens.