Why Brain Size Matters

If bigger really is better, why are large brains rare in nature? Brain cells have huge appetites, and the trade-off for a huge head is a weaker body. 

By Carl Zimmer|Saturday, September 01, 2012

Morkel Erasmus/Shutterstock

In 1758 the Swedish taxonomist Carolus Linnaeus dubbed our species Homo sapiens, Latin for “wise man.” It’s a matter of open debate whether we actually live up to that moniker. If Linnaeus had wanted to stand on more solid ground, he could instead have called us Homo megalencephalus: “man with a giant brain.”

Regardless of how wisely we may use our brains, there’s no disputing that they are extraordinarily big. The average human’s weighs in at about three pounds, or 1,350 grams. Our closest living relatives, the chimpanzees, have less than one-third as much brain—just 384 grams. And if you compare the relative size of brains to bodies, ours are even more impressive.

As a general rule, mammal species with big bodies tend to have big brains. If you know the weight of
a mammal, you can make a fairly good guess about how large its brain will be. As far as scientists can tell, this rule derives from the fact that the more body there is, the more neurons needed to control it. But this body-to-brain rule isn’t perfect. Some species deviate a little from it, and a few deviate a lot, but we humans are spectacular rule breakers. If we were like other mammals, our brains would be about one-sixth their actual size.

Competing theories seek to explain the value of a big brain. One idea, championed by psychologist Robin Dunbar of the University of Oxford, is that complicated social lives require more intelligence, and therefore more mental material. A baboon, an animal with a relatively large brain, can make a dozen alliances while holding grudges against several rivals. Humans maintain far more, and more complicated, relationships.

Managing a social network can yield significant benefits: When a fight breaks out, it pays to know who will be at your back. But keeping tabs on social life is challenging. Dunbar and his colleagues have found that it takes longer for people to answer questions when they have to think about what’s going on in other people’s minds. And the more “mind reading” a question requires, the more mental activity it demands.

An alternate theory comes from Daniel Sol of the Center for Ecological Research and Forestry Applications in Barcelona, Spain, who studies what happens when animals move into new habitats. In both birds and mammals, big-brained species are more likely to be successful than are smaller-brained ones. The research suggests that having a larger brain particularly helps in novel environments to solve problems, like finding food, that are important to survival.

Neither theory answers a puzzling question: If big brains are so useful, then why are they relatively rare? The answer is that nothing in nature comes for free—and where the brain is concerned, the cost can be enormous. In fact, scientists are discovering that human biology reorganized itself to cope with the burden of an oversize brain.

Why Are Big Brains Rare? 

Leslie Aiello, then of university College London, and Peter Wheeler of Liverpool John Moores University offered the first possible reason for the rarity of big brains in 1995. Neurons, they pointed out, have a voracious appetite. They require lots of energy to produce their voltage spikes and to release neurotransmitters. A three-pound human brain burns up to 20 times as many calories as three pounds of muscle. Twenty-five percent of all the calories you eat each day end up fueling the brain. For a newborn infant, with its little body and relatively large and fast-growing brain, that figure leaps to 87 percent.

More than 6 million years ago, our ancestors did not face such relentless demands. Our forebears stood only about as tall as a chimpanzee, with brains the size of a chimp’s. It wasn’t until about 1.8 million years ago when Homo erectus emerged from our lineage, that things really changed. The first members of our genus to look like us, H. erectus stood about as tall as a modern human, with a brain that weighed around 900 grams. A half-million years ago, the brains of our ancestors began growing again; 200,000 years ago they reached
roughly the same weight as ours today.

Aiello and Wheeler noted that this dramatic increase in brain size would seem to have required a dramatic increase in metabolism. Yet once size is taken into account, humans burn the same number of calories overall as do other primates of similar size. Somehow, Aiello and Wheeler argued, our ancestors found a way to balance their energy budget. To compensate for this major increase in energy requirements, something else had to be cut.

The scientists compared the sizes of other organs in humans and other primates. Relatively speaking, our liver and heart are roughly the same size as would be expected in comparison with other primates. But our guts have shriveled. They weigh only 60 percent of what you’d expect in a primate of our size. Intestinal cells also need a lot of energy, because they are highly innervated. Losing such a big portion of their guts could have allowed our ancestors to compensate for much of the brain’s extra energy demand.

Smaller Guts, Bigger Brains

Aiello and wheeler christened their idea “the expensive tissue hypothesis.” To test it, they compared the size of brains and guts in a range of primate species. They found that the bigger a primate’s brain relative to the species’s overall body size, the smaller the guts tend to be. This consistent trade-off suggested that trimming our guts was essential to supersizing our brains.

Biological anthropologist William Leonard at Northwestern University put the expensive tissue hypothesis to a broader test, comparing brain and gut sizes across a wide range of mammal species. In species besides primates, there was no connection between the size of the brain and that of the gut, he found, suggesting that the link Aiello and Wheeler found might not fully account for our enormous brains.

Leonard thinks that the key is to be found in what we eat. After tallying the quality and quantity of food in the diets of many primate species, Leonard found that brainier species eat more high-energy foods such as meats, tubers, and seeds, while creatures with smaller brains consume primarily bark and leaves, which aren’t as energy-dense. As brain-to-body ratio increases, presumably, the additional calories from these richer foods supply the additional fuel.

The human genome might also provide some clues to our big brains, finds Greg Wray, an evolutionary biologist at Duke University. A gene called SLC2A1, which builds a protein that transports glucose from blood vessels into cells, is vital to the brain’s well-being. Mutations that reduce the amount of protein created by this gene lead to brain disorders such as epilepsy and learning disabilities. If one copy of the SLC2A1 gene is completely dysfunctional, the results are devastating: The brain develops to only a fraction of its normal size. If neither copy of the gene works, a fetus simply dies.

Power of Mutations

Wray and his colleagues compared SLC2A1 in humans and other animals. They discovered that our ancestors acquired an unusually high number of mutations in the regulatory elements of the gene. The best explanation for that accumulation of mutations is that SLC2A1 experienced natural selection in our own lineage, and the new mutations boosted our reproductive success. Intriguingly, the Duke team discovered that the mutations didn’t alter the shape of the glucose transporter protein—a change that would probably alter its function. Rather, the mutations increase the level of gene expression in us in comparison with, say, chimpanzees.

Wray guessed that these mutations changed the total number of glucose transporters created in human brain tissue. In order to make glucose transporters, the cells must first make copies of the SLC2A1 gene to serve as a template. Wray discovered that human brains had 2.5 to 3 times as many copies of this template as did chimpanzee brains, suggesting that we have more glucose transporter proteins as well.

Our bodies have another glucose transporter, one that specializes in delivering the sugar to muscles. Wray found that the gene for these muscle transporters, called SLC2A4, also seems to have undergone recent natural selection, but in the opposite direction, so that our muscles have fewer glucose transporters than chimps’ muscles. Wray’s results support the notion that our ancestors evolved extra molecular pumps to funnel sugar into the brain, while starving muscles by giving them fewer transporters.

Becoming Homo megalencephalus was hardly simple. It was not enough for evolution to shrink our gut and shift our diet. It had to do some genetic engineering, too.

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