A recent study he conducted with a colleague, Drew Bailey, led Geary to this epiphany. The aim of their investigation was to explore how cranial size changed as our species adapted to an increasingly complex social environment between 1.9 million and 10,000 years ago. Since that period predates the first alphabets, the researchers had no written record with which to gauge the social milieu of our predecessors. Consequently, the Missouri team used population density as a proxy for social complexity, reasoning that when more people are concentrated in a geographic region, trade springs up between groups, there is greater division of labor, the gathering of food becomes more efficient, and interactions among individuals become richer and more varied.
Bailey and Geary found population density did indeed track closely with brain size, but in a surprising way. When population numbers were low, as was the case for most of our evolution, the cranium kept getting bigger. But as population went from sparse to dense in a given area, cranial size declined, highlighted by a sudden 3 to 4 percent drop in EQ starting around 15,000 to 10,000 years ago. “We saw that trend in Europe, China, Africa, Malaysia—everywhere we looked,” Geary says.
The observation led the researchers to a radical conclusion: As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive. As Geary explains, individuals who would not have been able to survive by their wits alone could scrape by with the help of others—supported, as it were, by the first social safety nets.
Geary is not implying that our beetle-browed forebears would have towered over us intellectually. But if Cro-Magnons had been raised with techno-toys and the benefits of a modern education, he ventures, “I’m sure we would get good results. Don’t forget, these guys were responsible for the ‘cultural explosion’”—a revolution in thinking that led to such startling new forms of expression as cave paintings, specialized tools, and bones carved into the first flutes. In terms of raw innate smarts, he believes, they probably were as “bright as today’s brightest” and might even have surpassed us.
Still, Geary hesitates to use words like genius or brilliant in describing them. “Practically speaking,” he explains, “our ancestors were not our intellectual or creative equals because they lacked the same kind of cultural support. The rise of agriculture and modern cities based on economic specialization has allowed the very brightest people to focus their efforts in the sciences, the arts, and other fields. Their ancient counterparts didn’t have that infrastructure to support them. It took all their efforts just to get through life.”
SMALLER BUT SMARTER
When I follow up with Hawks, the anthropologist who first tipped me off about our missing gray matter, I assume that his interpretation of the trend will be like Geary’s. But even though Hawks does not doubt the findings of the Missouri team, he puts a completely different (and, in his view, more uplifting) spin on the data.
Hawks spent last summer measuring skulls of Europeans dating from the Bronze Age, 4,000 years ago, to medieval times. Over that period the land became even more densely packed with people and, just as the Missouri team’s model predicts, the brain shrank more quickly than did overall body size, causing EQ values to fall. In short, Hawks documented the same trend as Geary and Bailey did in their older sample of fossils; in fact, the pattern he detected is even more pronounced. “Since the Bronze Age, the brain shrank a lot more than you would expect based on the decrease in body size,” Hawks reports. “For a brain as small as that found in the average European male today, the body would have to shrink to the size of a pygmy” to maintain proportional scaling.
Hawks chose to focus on Europe in the relatively recent past, he explains, because there is an exceptionally large number of complete remains from that era. That allowed him to reconstruct a detailed picture of what was happening during our downsizing. The process, he discovered, occurred in fits and starts. There were times when the brain stayed the same size and the body shrank—most notably, he says, from the Roman era until medieval times. But more frequently, the brain got smaller while the body remained the same. Indeed, Hawks says, that is the overarching trend for the thousands of years he studied.
The image of a brain dwarfed by its body conjures up dinosaurs, a group not exactly known for their intellectual prowess. But Hawks sees nothing alarming in the trend. Quite the contrary, he believes the startling decrease in our brain volume—both in absolute terms and relative to our stature—may be a sign that we are actually getting smarter.
As complex societies emerged, brains shrank because those previously unable to survive by wits alone could now scrape by with the help of others.
This upbeat perspective is shaped by Hawks’s focus on the energy demands of the brain. The organ is such a glutton for fuel, he says, that it gobbles up 20 percent of all the calories we consume. “So although a bigger brain can presumably carry out more functions, it takes longer to develop and it uses more energy.” Brain size probably depends on how those opposing forces play out.
The optimal solution to the problem, he suggests, “is a brain that yields the most intelligence for the least energy.” For evolution to deliver up such a product, Hawks admits, would probably require several rare beneficial mutations—a seeming long shot. But a boom in the human population between 20,000 and 10,000 years ago greatly improved the odds of such a fortuitous development. He cites a central tenet of population genetics: The more individuals, the bigger the gene pool, and the greater the chance for an unusual advantageous mutation to happen. “Even Darwin knew this,” he says. “That’s why he recommended that animal breeders maintain large herds. You don’t have to wait so long for desirable traits to arise.”
Hawks notes that such changes would be consistent with the many brain-related DNA mutations seen over the past 20 millennia. He speculates that the organ’s wiring pattern became more streamlined, the neurochemistry shifted, or perhaps both happened in tandem to boost our cognitive ability.
A TAMER BREED
Other researchers think many of their colleagues are barking up the wrong tree with their focus on intelligence as the key to the riddle of our disappearing gray matter. What may have caused the trend instead, they argue, is selection against aggression. In essence, we domesticated ourselves, according to Richard Wrangham, a primatologist at Harvard University and a leading proponent of this view.
Some 30 animals have been domesticated, he notes, and in the process every one of them has lost brain volume—typically a 10 to 15 percent reduction compared with their wild progenitors. Domesticated animals also have more gracile builds, smaller teeth, flatter faces, a more striking range of coloration and hair types—and, in many breeds, floppy ears and curly tails. Except for those last two traits, the domesticated breeds sound a lot like us.
“When you select against aggression, you get some surprising traits that come along with it,” Wrangham says. “My suspicion is that the easiest way for natural selection to reduce aggressiveness is to favor those individuals whose brains develop relatively slowly in relation to their bodies.” When fully grown, such an animal does not display as much aggression because it has a more juvenile brain, which tends to be less aggressive than that of an adult. “This is a very easy target for natural selection,” Wrangham argues, because it probably does not depend on numerous mutations but rather on the tweaking of one or two regulatory genes that determine the timing of a whole cascade of developmental events. For that reason, he says, “it happens consistently.” The result, he believes, is an adult possessing a suite of juvenile characteristics, including a very different temperament.