The second key human character was the increase in relative brain size that occurred much later, only about 2Myr ago with the origin of the genus Homo. The early bipedal humans still had rather ape-like heads with brain sizes of 400-550 cm3, similar to apes, and by no means comparable with modern humans, who have a brain size of 1000-2000 cm3 (mean, 1360 cm3), a value approached by 500,000-year-old fossil Homo.
Various anatomical characters changed as a result of the increase in brain size. The back of the head became enlarged to accommodate it and the face became less projecting and placed largely beneath the front of the brain, rather than in front of it. Thus, the projecting face of the apes was lost with increasing brain size in the human line and this led to a shortening of the tooth rows. The rounded tooth row with a continuous arc of teeth and no gap (diastema) between the incisors and canines (Figure 11.5(d)) is a human character.
Present fossil evidence then suggests that human evolution followed a 'locomotion-first' pattern, with bipedalism arising before 6 Myr ago and the enlarged brain less than 2 Myr ago. During the first half of the twentieth century, though, many experts held to the more comforting 'brain-first' theory, and the fossil evidence seemed to confirm their view.
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