Over three million years ago, the early primates left the protection of the rainforest, choosing to live in the open plains. This kick started many adaptations to the new environment. They no longer needed to move in a hunched manner as they did under the dense rainforest foliage. Australopithecus began the process of walking upright.
From about 2.4 to 1.5 million years ago, Homo habilis, the first member of the genus called Homo, began using tools. Homo habilis’ first tools were simple one faced stone tools. Using tools eased the strain on the hands, legs and teeth. Teeth had been used to grind and cut through almost anything. Using tools meant that the load on teeth was significantly decreased. The molars shrunk in size and in number. Other teeth also changed shape, as food was now first cut up by the tools. The jaws of the Homo genus began to shrink as the number of teeth lessened. Even now, teeth continue to decrease as a full mouth averages between 28 to 32 teeth. The third molars or wisdom teeth, are slowly being eliminated by processed food which influences smaller jaw sizes.
The Homo genus began to live in organized society. The use of spears influenced a more diverse diet as both hunting and gathering were used to get food. Sexual selection, mutual co-operation, dominance and violence also influenced evolution.
Homo sapiens, the current Homo genus emerged 250 000 years ago, creating more complex tools with greater reasoning which has allowed us to survive in almost any environment.