The large human brain is clearly the distinguishing feature separating us from the African apes, our closest relatives. Although early 19^th century investigations were drawn to the large brain as the first human trait, the fossil evidence that was emerging would prove otherwise. It was our habitual upright stance—bipedal posture and locomotion, that came first. This habitual bipedalism that marks our family seems to have evolved in response to a fluctuating environment of both forest and open plains. These earliest ancestors were essentially small, bipedal apes. The earliest primates consisted of 3 main groups: one, giving rise to the modern prosimians, a second produced the anthropoids, and the third which became extinct.

The primates are one of the earliest of the mammal groups to evolve after the mass extinction that took place 65 mya. They appear to have arisen first in what are now North America and Europe, but the success of their adaptations allowed them to radiate over the Old World and back into the New World after the hemispheres separated. About 23 mya, primitive apes first appeared, and from one group of African ape our family, Hominidae, branched off around 5 mya or earlier.

About 3 mya, a further climate change led to two new hominid adaptive responses. One gave rise to another small, bipedal apelike form with massive chewing bones and muscles adapted to the tough, gritty vegetation of the plains. The other response was the evolution of larger-brained hominids. These hominids, the first members of genus Homo, survived by inventing stone tools, which, among other things, allowed them to scavenge the meat of the vast herds of grass eaters.

Stone tools have been a crucial element of human evolution and the major technology for 99 percent of human cultural history. The evolution of the human brain allowed early humans to problem solve and form social relationships, but it is the material remains of early hominids that provide physical evidence of how the brain was put to use.

With the evolution of early hominids, stone tool technology evolved as well. Skills improved, and more control over material allowed for refinement of manufacture, the introduction of new materials, and stylistic change. With these technological improvements occurring in parallel with environmental changes, subsistence patterns shifted from scavenging to cooperative hunting and a widening environmental exploitation. Early Homo lived in small, cooperative groups, foraging and scavenging. Larger groups later gave them a better understanding of their surroundings, and then, evolving stone tool technology helped provide them important new food resources. Bipedalism, larger brains, and an increasingly complex social organization, coupled with this technology conditioned future primate evolution.

Successful early Homo adaptations accelerated hominin evolution about 2 mya. Taxonomy and relationships remain controversial among anthropologists who work to articulate and refine the emerging fossil record. The discussion centers on three ‘stages’ based on this record: Homo erectus, archaic Homo sapiens, and anatomically modern Homo sapiens.

From this adaptive base, the evolution of our genus accelerated. The Homo erectus stage, with its basically modern bodies and even larger brains, migrated all over the Old World, encountering the climatic changes of the Pleistocene, improving stone tool manufacture, and, at least in some areas, taming fire. Modern-sized brains were reached 750,000 ya, although crania retained some primitive features. These archaic Homo sapiens, first seen in Africa and southern Europe, also spread across the Old World, and exhibited such typically human behaviors as burial of the dead and care of the elderly and infirm. The Neandertals are one of the best-known forms of this stage.

The anatomically modern Homo sapiens stage, first appearing in Africa around 300,000 ya, is characterized by further advances in tool making, the clear practice of big-game hunting, and the first expressions of art. Members of this stage entered Australia and nearby islands and reached the New World. Farming, cities, writing, and all the cultural features we associate with modern humanity follow.