The Incredible Shrinking Brain

Among the DVDs on my shelf is a box set of the The Outer Limits (original series), the early ‘60s science fiction TV show. The budget for each Outer Limits episode was too spare even for shoestrings, so the producers employed cheap off-the-shelf special effects that are laughable to young 21^st viewers accustomed to state-of-the-art CGI. Nevertheless, many of the scripts were clever and imaginative, so it remains a fun series to sample on occasion. 

My sample from the set two nights ago was the episode “The Sixth Finger” in which a very ordinary fellow (David McCallum) is given an evolutionary push forward in a scientist’s new handy-dandy evolution-speedup-device. As is common in scifi tales (and, to the annoyance of paleontologists, in the minds of many people), there is an assumption that human evolution has a particular direction and that the direction is toward ever more brainpower. Accordingly, the machine gives McCallum’s character a hugely enlarged cranium and an extra digit on each hand.

The assumption is not only wrong, it is very nearly backwards. Neither Homo sapiens nor any other species has any inherent predisposition to evolve in one direction rather than another, but it is true that external environmental pressures can sustain trends over long periods of time. So, for some two million years of hominin evolution, our ancestors’ brains indeed became progressively larger. Then a funny thing happened. Brain size peaked 20,000 years ago; since then, brains have shrunk. Some of this decrease can be attributed to the drop in human stature that accompanied the start of agriculture. On every continent, regardless of what crops were domesticated, whenever people shifted from hunting and gathering to farming they became substantially shorter and less healthy; farming reduces the variety and quality of diet. (People have been getting taller for the past 100 years as diets got better again.) Naturally, head size shrank along with the rest of the body. Yet, the decline in cranial capacity outpaced the decline in height. The typical 21st century human has a smaller brain than a Cro-Magnon of equal overall body size, and the difference isn’t negligible. All else equal, the average cranial volume has dropped from 1500 cubic centimeters for a Paleolithic male to a modern 1350 cc. Anthropologist John Hawks at the University of Wisconsin comments that if the rate of decline seen in the past 20,000 years continues for the next 20,000, we will be back in the cranial neighborhood of Homo erectus.

Why did this happen? Paleontologists David Geary and Drew Bailey (University of Missouri) speculate that people don’t need as much brainpower to survive and reproduce as societies grow larger, and, in biology, if you don’t use something it goes away. Geary and Bailey examined the skeletal evidence and concluded that the biggest drop in brain size occurred between 15,000 and 10,000 years ago as population densities rose everywhere, not just in agricultural settlements but also in regions dominated by hunter-gatherers. Apparently it becomes easier for dim bulbs to get along if there are enough other people around to help them out.

This raises the question of whether brain size and intelligence are actually connected, particularly as a ratio of overall body size. Between any two random individuals of the same species, the answer is no. Cranial volume doesn’t predict much of anything – big-headed dummies and little-headed geniuses abound. Leaving all non-anatomical considerations of intelligence aside for the moment, the physical substructure of a brain matters, too, and a smaller brain can be more complex than a larger one. Statistically, however, the answer is yes. Of course there is a correlation. On the bell curve of intelligence distribution, there are more big heads on the right side of the curve than on the left. So, too, when comparing different species.

It seems that recent advances in artificial intelligence are coming at just the right time. If Hawks’ projection holds, we’re going to need every teraflop.

In “The Sixth Finger,” by the way, (spoiler follows) McCallum gets back in the evolution device and asks his girlfriend to send him even further ahead. Jill Haworth, though, doesn’t like her men too brainy. So, she reverses the controls and returns him to one step above caveman. If Geary and Bailey are right, caveman would have been better.

Big-Headed and Temperamental (from “The Sixth Finger”)