Brain Size and Body Size  
Much of the human ability to make and use tools and other objects stems from the large size and complexity of the human brain. Most modern humans have a braincase volume of between 1300 and 1500 cc (between 79.3 and 91.5 cu in). In the course of human evolution the size of the brain has more than tripled. The increase in brain size may be related to changes in hominine behavior. Over time, stone tools and other artifacts became increasingly numerous and sophisticated. Archaeological sites, too, show more intense occupation in later phases of human biological history.

In addition, the geographic areas occupied by our ancestors expanded during the course of human evolution. Earliest known from eastern and southern Africa, they began to move into the tropical and subtropical areas of Eurasia sometime after a million years ago, and into the temperate parts of these continents about 500,000 years ago. Much later (perhaps 50,000 years ago) hominines were able to cross the water barrier into Australia. Only after the appearance of modern humans did people move into the New World, some 30,000 years ago. It is likely that the increase in human brain size took place as part of a complex interrelationship that included the elaboration of tool use and toolmaking, as well as other learned skills, which permitted our ancestors to be increasingly able to live in a variety of environments.

The earliest hominine fossils show evidence of marked differences in body size, which may reflect a pattern of sexual dimorphism in our early ancestors. The bones suggest that females may have been 0.9 to 1.2 m (3 to 4 ft) in height and about 27 to 32 kg (about 60 to 70 lb) in weight, while males may have been somewhat more than 1.5 m (about 5 ft) tall, weighing about 68 kg (about 150 lb). The reasons for this body size difference are disputed, but may be related to specialized patterns of behavior in early hominine social groups. This extreme dimorphism appears to disappear gradually sometime after a million years ago.

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