Determining how the human brain differs from nonhuman primate brains is central to understanding human behavioral evolution. There is currently dispute over whether the prefrontal cortex, which mediates evolutionarily interesting behaviors, has increased disproportionately. Using magnetic resonance imaging brain scans from 11 primate species, we measured gray, white and total volumes for both prefrontal and the entire cerebrum on each specimen (n = 46). In relative terms, prefrontal white matter shows the largest difference between human and nonhuman, whereas gray matter shows no significant difference. This suggests that connectional elaboration (as gauged by white matter volume) played a key role in human brain evolution.

The human brain is one of the most intricate, complicated, and impressive organs ever to have evolved. Understanding its evolution requires integrating knowledge from a variety of disciplines in the natural and social sciences. Four areas of research are particularly important to this endeavor. First, we need to understand basic principles of brain evolution that appear to operate across broad classes of organisms. Second, we need to understand the ways in which human brains differ from the brains of our closest living relatives. Third, clues from the fossil record may allow us to outline the manner in which these differences evolved. Finally, studies of brain structure/function relationships are critical for us to make behavioral sense of the evolutionary changes that occurred. This review highlights important questions and work in each of these areas.

No abstract

Hominid brain size increased dramatically in the face of apparently severe associated evolutionary costs. This suggests that increasing brain size must have provided some sort of counterbalancing adaptive benefit. Several recent studies using magnetic resonance imaging (MRI) have indicated that a substantial correlation (mean r ‫؍‬ Ϸ0.4) exists between brain size and general cognitive performance, consistent with the hypothesis that the payoff for increasing brain size was greater general cognitive ability. However, these studies confound betweenfamily environmental influences with direct genetic͞biological influences. To address this problem, within-family (WF) sibling differences for several neuroanatomical measures were correlated to WF scores on a diverse battery of cognitive tests in a sample of 36 sibling pairs. WF correlations between neuroanatomy and general cognitive ability were essentially zero, although moderate correlations were found between prefrontal volumes and the Stroop test (known to involve prefrontal cortex). These findings suggest that nongenetic influences play a role in brain volume͞cognitive ability associations. Actual direct genetic͞biological associations may be quite small, and yet still may be strong enough to account for hominid brain evolution. Hominid brains have roughly tripled in volume in less than 3 million years, and little of this can be explained by body size increases (1-4). Because larger brains take longer to mature (5), have very high metabolic costs (6), and reduce the efficiency of bipedal locomotion (because the pelvic aperture must still allow birth) (7), increasing brain size must therefore have provided counterbalancing adaptive benefits (8). Seven recent studies using magnetic resonance imaging (MRI) to estimate brain volume have shown a substantial correlation (mean r ϭ Ϸ0.4) between brain size and general cognitive performance (9-16). Although these findings are consistent with the hypothesis that the payoff for increasing brain size was related in some way to greater general cognitive ability (g), they do not control for potentially confounding between-family (BF) environmental influences.Family background, socioeconomic status (SES), crossassortative mating, and cultural influences potentially affect both anatomy and behavior in the same direction independently, which could thereby result in noncausal associations between brain size and cognitive ability. These BF influences can be controlled for by asking whether WF sibling differences in brain size are associated with WF sibling differences in cognitive performance. This technique has been used to show that height͞IQ correlations are probably not genetically mediated (17)(18)(19)(20), but that myopia͞IQ correlations apparently are genetically influenced (21). To date, there is only one brief abstract of a WF MRI study of brain͞cognitive ability associations in the literature (16), which found no significant WF correlation between general cognitive ability (g) and MRI brain volume in 40 male siblings (no d...