We are the smartest species on the planet, so we have the largest brains in proportion to our body size. That blindingly obvious explanation seems fine until challenged by the relevant facts.
To begin with, having a big brain is no guarantee of brilliance and some brilliant individuals, including French man of letters, Anatole de France, got by with tiny brains, his being around three quarters of the normal size. It is certainly true that, as a rule, large-brained people score higher on IQ tests. Brain size - measured accurately by magnetic resonance imaging - accounts for about a sixth of the differences in IQ scores.
Unfortunately, this does not mean that brain volume causes intelligence. Indeed, when one compares the IQ scores of pairs of sisters, the smarter one is as likely to have the smaller brain as the larger one. This means that the connection between brain size and IQ in the population at large is due to third variables such as prenatal nutrition, maternal health, exposure to toxins, gestation period, and so forth.
So let's forget about the intelligence differences between individuals today and argue that we are all above average - among primates. Primates are very brainy relative to other mammals and this is likely because they have more complex societies. Indeed, the (fore)brain size of primates increases as the size of their social group goes up. Even if large-brained orangutans are solitary today, their ancestors had the primate social boost.
Did our ancestors develop their remarkably bulbous crania about the time when they began doing really smart things? Here is where it makes sense to detour into archeology, specifically ancient tool technology.
By their tools shall ye know them
The human tool kit is impressive. Look at the beautiful kayaks, harpoons, and fish hooks fashioned by the Inuit from skins and bones. The trouble is that such fancy hardware has been around for only about 50,000 years. If one goes back 500,000 years, humans already had very large brains (about 94 percent the size of modern humans for Homo heidelbergensis), but little to show for it in terms of technology.
Going back 1.5 million years, our ancestors, Homo ergaster, had large brains even for primates - more than three times as big as those of other mammals of the same size. Yet, they, and their descendants, were an astonishing bunch of slackers.
The star exhibit here is the Achulean stone hand axe that first appeared a million-and-a-half years ago (Richerson & Boyd, 2004, 1). This tool began as a round stone from which pieces were systematically struck off on two sides to produce a distinctive symmetrical shape. Held by the fat end, it could be used to hack away at meat, bones, or large nuts. The Achulean hand axe is noted for its persistence unchanged over a million years. Equally remarkable, was its proliferation - with no obvious change - over about half of Africa, Europe, and Asia.
Why is the uniformity of the Achulean tools so significant? Human tools are generally assumed to be spread via imitation but imitation always produces some random variation due to copying error. Such variation is simply absent from the Achulean stone industry. As Richerson and Boyd (2004, p. 142, 1) conclude:
How could cultural transmission alone, particularly if based on a relatively primitive imitative capacity, preserve such a neat, formal-looking tool as a [sic] Achulean hand axe over half the Old World for a million years? .... perhaps we need to entertain the hypothesis that Achulean bifaces were innately constrained rather than wholly cultural and that their temporal stability stemmed from some component of genetically transmitted psychology.
To simplify the argument, our ancestors produced uniform tools for much the same reason that funnel spiders always produce the same kind of web - genetics and individual experiences with no copying from others. Over this million-year period, human brain size almost doubled. You would not know this from the crude tools they continued to produce. So why did out crania swell? The saga continues in another post.
1. Richerson, P. J., & Boyd, R. (2004). Not by genes alone. Chicago: University of Chicago Press.
Note Re TV Post
Thanks to readers who expanded on several points, including the fact that children who watch a lot of TV do poorly in school, for complex reasons. The relationship between TV viewing and academic success ia an inverted U. Benefits accrue steadily up to a maximum of about 3 hours per day, suggesting a brain enrichment effect. The interaction between social status and TV viewing is from T. E. Smith (1990/1992) as cited in my study.
