Sex, IQ & ET: How We Got Big Brains

Sep. 26, 2002

by Seth Shostak - Senior Astronomer

Relative to their size, humans have the biggest brains on the planet. Check out the guy sitting next to you on the bus: hunkered beneath a fringe of moussed hair and a few millimeters of skull are three crinkly pounds of brain -- the only substantive difference between you and species you regard as food or pets.

But how did this happen? What special circumstance, what unperceived evolutionary force, nudged our hulking, hairy ancestors toward intelligence, and silently trebled the size of their brains in two million years or less?

This question is seductive not only because it tells us something exquisitely interesting about ourselves, but also because the answer could give us insight into whether other intelligent beings really exist.

Many researchers have considered how the processes of natural selection might encourage higher IQ. For example, predator-prey relationships can do this. When a lioness bags an antelope, shes more likely to snag one of the dumb ones. Result? The lioness has a meal, but the average ability of the antelopes has been raised. The next night, the dumber lions will have a harder time getting dinner, and will preferentially drop out of the leonine gene pool. In this animal arms race, the IQ of both species is ratcheted upward.

This is survival of the functionally fittest, and intelligence is certainly one component of making it in a competitive environment. But theres another type of mechanism that might have been more important in creating our cerebral machinery. Its called signaling for fitness, and its a widespread component of beastly behavior.

The way to understand signaling for fitness is to consider a canonical example: peacock tails. When choosing a mate, peahens prefer males with long, bright tails. This isnt simply due to a quirk of their little pea brains; its good reproductive strategy. Those impressive tails are metabolically costly, and require both good health and success in finding food. In addition, a flashy tail can attract predators, which only the wily can avoid. So when a peahen spies a male with a well-endowed tail, she can be sure he has good genes.

If she chooses to get to know him better, their offspring will have a survival advantage.

For most species, males court and females choose. So its usually the males who are using these fitness signals as semaphores to indicate that theyre up to snuff. In the words of Geoffrey Miller, an evolutionary psychologist at the University of New Mexico, "evolution is driven not just by survival of the fittest, but reproduction of the sexiest."

So how does this dovetail with the evolution of intelligence?

Miller points out that musty theories claiming we developed our impressive cerebra from, for example, tool use, dont seem to fit the facts. The stone axes chipped by our one-pound-brained ancestors were about as good as those made by their three-pound-brained successors. Instead, Miller suggests that the ramping up of IQ was the result of 100,000 generations of pre-human courtship operating on fitness signals made possible by brain power.

"The brains a really good indicator of fitness because its growth depends on at least half of the genes that humans have," he says. "A brain, after all, is very complex, very sensitive to genetic mutations, and costs a lot of energy to run."

If you have a good brain, you have good genes. Theyre our flashy tail.

But its a messy business to literally display your brain. So how can a quality cranium signal its superiority to a mate? It does so with behaviors such as speaking well, or by demonstrating musical ability, a sense of humor, or creativity. These activities depend upon many parts of the brain, and consequently are reliable indicators of mental merit.

Brains are like the Dow Jones index, which is an accessible and generally trustworthy gauge of a complex system because its rooted in a number of important stocks.

So males strut their stuff by crooning, being witty, and speaking well, while the females use these clues to sort out the best one to take home to mom and dad. You might wonder why this mechanism would ever lead to women who are also clever. But thats a natural consequence of the long maturation time of humans.

"Human males invest a whole lot more in being parents than any other primates," says Miller, "so the more they invest in their offspring, the choosier they should be about their long-term mate. This puts the females under evolutionary pressure; early in courtship to attract a high-fitness guy, but after that to keep him interested, to keep him from wandering off with some other woman. Theyre strongly selected to tell interesting stories, to be continually amusing and charismatic in order to maintain the males commitment and investment in their children."

Millers ideas will no doubt affect your view of significant others. But what insights do they give into the likelihood of intelligence on other worlds?

"I think this is one of the key evolutionary processes that can produce intelligence anywhere in the universe," he says. "And its universal because any evolutionary system will have some kind of genetic code. That code will be vulnerable to mutations of some sort -- and fitness indicators reveal just how many mutations you have."

"Clearly, one of the best ways to signal is through behavior that can only be produced by a highly complicated nervous system whose growth depends on most of your genes. So in my view its a very powerful evolutionary process. Theres good evidence that its important in just about every other animal species. And we have one example -- us -- where its led to intelligence." And on other worlds around other stars? Miller laughs: "I think aliens who are obsessed with showing off their brain power to the opposite sex will be fairly common out there."

So if and when we discover the extraterrestrials, you shouldnt be surprised to find that they are slick talkers, good musicians and -- unlike their Hollywood ciphers -- able to tell a decent joke.