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The small human groups—like virtually every other mammalian omnivore— adapt to seasonal change, migrating to areas of optimal foraging and hunting at the appropriate time of year. Everyone agrees that the increasing human cranial capacity was accompanied by an increase in intellect— memory, symbolic manipulation, learning capacity, etc.Somewhere in the history of this—and whether it started 2 mya or 20,000 ya is anybody’s guess—the mating structure changed. The largest anatomical differences between human and chimp brains are in the frontal lobes—those areas associated with executive functioning, evaluation, and reason.
Irrespective of whether amh replaced and/or interbred with Homo erectus and Neanderthals, there is considerable agreement on the particulars of very recent human evolution. erectus gave way to the large, vertical forehead of modern humans, permitting the brain to increase in size.
The skeletal structure attained a gracile form very close to modern humans.
Over the course of 2 million years, several varieties and species of Australopithecines may have cohabited together and possibly even competed with one another in the savanna and woodlands that border the savanna.
Little is known about the behaviour of the Australopithecines other than they probably used tools and were a highly social species as chimps and we are today.
It is also possible that the causes for increased brain size shifted over time, say from social communication to symbolic and rational thought to competition.
Whatever the reason(s), they must have been quite important.The discovery of new fossils, the sequencing of the human genome, and the completion of the Human Genome Diversity Project will provide new insights that alter currently cherished beliefs.This has been the history of science in human evolution, so there is no reason to suspect that the trend will change.Still, we are stuck in the present and must do the best we can with the available data.Most biologists suspect that humans and chimpanzees split off from a common ancestor as recently as 4 to 5 million years ago. One of the first evolutionary developments that distinguish human ancestors from chimps was upright posture.The upper cranium of the skull expanded and assumed a more rounded shape, permitting brain size to increase from about 450 cc to between 8 cc.Other evolutionary trends continued—the protruding jaw receded a bit, teeth became smaller, and height increased.One of the earliest of the human genera, the Australopithecines, who walked upright and had modified hands, but in many other ways resembled a chimp.They were small (between 3 and 4 feet tall), had curved fingers, and a skull with a protruding jaw, a recessed cranium, and heavy ridges behind what are now the eyebrows.The reason for the development of upright posture is unknown, but it certainly permitted the Australopithecines to travel long distances, freed their hands to carry objects, and may even have allowed more efficient thermoregulation.Because of these advantages conferred by upright posture, many anthropologists suspect that the early Australopithecines were adapting to life on the savanna while the ancestors of today’s chimps remained in the forested areas of central Africa.