The earliest hominins predating 4.0 million years ago, such as Sahelanthropus and Ardipithecus, generally had smaller teeth and small brain size—comparable to those of chimpanzees. With the appearance of the genus Australopithecus about 4.2 million years ago, the teeth, particularly the molars, became much larger while brain size remained small. The shift to larger teeth has been related to the decrease in forest cover and woodland areas—habitats that were preferred by their predecessors—and expansion of grasslands, which forced early human ancestors to change their adaptive strategy—from softer food resources such as fruits to tougher resources requiring more chewing—and conform with the changing environment. After the emergence of Homo, we start seeing coevolution of reduction in dental size and consistent increase in brain size contemporaneously with the earliest clear evidence of stone tools around 2.6 million years ago.

The increase in brain size and subsequent reduction in dental size in Homo are associated with behavioral changes such as tool use and incorporation of meat as a dominant component of their dietary resource. One of the reasons, especially for the reduction of dental size in the genus Homo, has been connected to tool use and food that was processed and reduced into smaller pieces before it was ingested. However, the use of fire to cook food and the sophistication of tool kits throughout the evolution of Homo did not result in further reduction in dental size even though the brain continued to be larger and larger.

Image credit David L. Brill. From Lucy to Language, Simon & Schuster, 2006.