Social? Ecological? Social-Ecological? —

The evolutionary mystery of gigantic human brains

There’s a lot to know about primate brains—in fact, we’ve barely scratched the surface.

Modern art, eh?
Enlarge / Modern art, eh?

In the Scottish National Gallery of Modern Art is a work that’s simply a list of names: every person the artist, Douglas Gordon, can remember meeting in his life. The list pours on and on in interminable columns, down a corridor and across walls that are multiple storeys high. It's dizzying, yet far from complete, since Gordon is only 52 years old.

The list is an abstract idea about humans made concrete in black paint: we are an intensely, bewilderingly social species. Our brains somehow have a vast vault for storing details about other humans, even when those details entail little more than face or name recognition.

It’s possible that this vast vault, along with a host of our brains' other cognitive abilities, are there precisely because of the intense sociality of our species. One of the most prominent explanations for the evolution of big brains is that large social groups lead to problem-solving challenges, which in turn create an evolutionary pressure for smart, big-brained individuals capable of navigating social situations.

Proponents of the social brain hypothesis point to reams of evidence to back it up, but a string of recent publications raises questions about whether it should dominate thinking in its field. It’s true that in primates, big brains generally go along with big social groups, but that correlation has started to look shakier when researchers have poked at it.

Definitive answers are hard to come by, as, when it comes to primate brains and behavior, what we don’t know dwarfs what we do. The precise combination of factors that drove evolutionary changes are lost to history. That has left the field struggling to test this hypothesis with data that's far less complete than it would like—a challenge shared with many other fields of science.

Yet even in this gray zone, progress can be made—a gradual thinning of the murk, as researchers focus on answering one small question at a time. Even when the most sorely needed information is out of reach, science as a whole can edge forward.

How do you solve a problem like cognition?

Big brains might not seem like such a big mystery. You have a bigger brain, you’re smarter, you take care of your survival needs more easily, and you can start to spend time painting pretty antelopes on cave walls. Boom, solved, it's only a matter of millennia before we’re on the Moon.

The mystery lies in the fact that big brains are hungry. They’re the organ equivalent of a muscle car’s engine, guzzling obscene amounts of fuel, when, for many animals, a Prius seems to do just fine. To justify its existence, the intellectual horsepower produced by the bigger brain had better be worth its weight in calories. And it’s not just humans who have big brains: all our primate relatives seem to be on the brainy side, including our closer great ape cousins like chimps and gorillas. More distant relatives like monkeys and lemurs are quite brainy, too.

Ring-tailed lemurs eat frozen pastry because they have big brains and know frozen pastry is delicious.
Enlarge / Ring-tailed lemurs eat frozen pastry because they have big brains and know frozen pastry is delicious.
VCG / Getty

Primates in general have brains that are weirdly big enough to demand an explanation—what in a lemur’s world makes it worthwhile to cart around a calorie black hole?

Larger social groups are what makes it worthwhile, according to the social brain hypothesis. Big groups are thought to have evolutionary advantages like protection against predators. But living in close social relationships with other individuals brings with it a host of cognitive challenges. Those animals that can handle the cognitive challenges survive and thrive; over generations, that means a species with bigger brains.

And that’s what evidence has pointed toward: primates with bigger social groups also have bigger brains. As evidence has mounted, the idea has become well-established among scientists in the field. Similar results have been found in other families of mammals, like ungulates (deer, camels, and the like) and cetaceans (whales and dolphins). Researchers have reported that individual magpies that are more social are also better at solving puzzles. “It has definitely been the consensus,” said Rob Barton, a researcher who studies brain evolution, in a phone call with Ars.

Revisiting the data

Barton had long been an advocate of the social brain, even co-authoring work on the hypothesis with its main proponent, Robin Dunbar. But he was worried about problems with the data and wanted to take another look. “It occurred to me that this hadn’t really been done fully since the old days,” he explained. He started by taking a cursory look, and when the results were unexpected, he passed the project to a doctoral student, Lauren Powell, to delve into the work more thoroughly.

“I honestly had no agenda about this,” Barton told Ars. “I was expecting to find a correlation with social group size.” But that’s not what they found. Instead, they found that brain size was more closely linked to factors in a primate species’ environment, things like how big their territories were and what kind of food they ate. Unsurprisingly, more calorific diets went along with bigger brains.

Meanwhile, a different group of researchers, led by primatologist Alex DeCasien, published a paper with a similar finding: fruit-eating primates had larger brains than leaf-eating primates. This pattern matched the data on brain size better than the primates’ social lives did.

Should we pivot from the social brain hypothesis to the ecological brain? Not so fast. Powell, Barton, and their colleague Karin Isler compared different data sets and found that the results looked different depending on how they sliced the data.

Small, noisy data sets make a clear answer impossible to pin down. But all the data sets currently available to the field are, well, small and noisy—the data the researchers would love to get their hands on is still well out of reach.

At San Diego's <a href="">Digital Brain Library</a>, each lacy slice of brain is about 70 micrometers thick, or roughly as wide as a human hair.
Enlarge / At San Diego's Digital Brain Library, each lacy slice of brain is about 70 micrometers thick, or roughly as wide as a human hair.

Dream the impossible data dream

Researcher Susanne Shultz, a proponent of the social brain hypothesis, was impressed by Powell’s analysis. “It’s very good. They've tested things rigorously, and their conclusions are very justified,” she said in a phone call with Ars. But, she points out, there should be two different correlations at play in the conversation: there’s the correlation between group size and brain size and a correlation between group size and a region of the brain called the neocortex.

The neocortex receives and integrates information from the external world, organizing it and cooperating with other brain regions to transform that information into behavior. In large-brained animals like primates, the neocortex makes up a disproportionately large part of the overall brain. Scientists are constantly improving their understanding of the neocortex, including figuring out more about how it communicates with other areas, but "the view still clings on that the neocortex is the 'intelligent' bit of the brain," says Barton.

Powell and her colleagues looked at the whole brain and didn’t find support for the social brain hypothesis. The jury’s still out on the neocortex correlation, but Barton points out that the neocortex makes up such a large proportion of the total brain that if the neocortex correlates with another factor (like social group size), then we should expect overall brain size to correlate, too. So, if we don't find the correlation for total brain size, it seems less likely that the correlation would be there for the neocortex.

Getting the verdict requires autopsies—lots of them. And opportunities for primate autopsies are, unsurprisingly, not an everyday occurrence. “Not all of us can go and dissect the brains of every animal that we're doing work with,” said Shultz.

It’s difficult to pin down exactly how many primate species there are, but it’s safe to say it’s currently more than 200; the data set with decent information on brain regions, published by Heinz Stephan and his colleagues in 1981, covers only around 40 species. That’s the data providing support for the neocortex findings.

Barton is actually a cheerleader for the idea that looking at specific brain regions is a better idea than looking at the brain as if it were a big lump—he points to nocturnal species that have enlarged olfactory regions that deal with smell and diurnal species that have bigger visual cortices. If you looked just at the whole brain of those groups, you wouldn’t get the full picture of how it's specialized. But because of the limited data on primates, Powell, Isler, and Barton were able to get whole-brain data on 114 species, so that’s what they used.

When they narrowed their analysis down to just the species used in the Stephan data set, the social brain seemed to be back in the running. The problem is that the data set is skewed in some very important ways—it focuses more on “Old World” African and Asian monkeys, says Shultz, and less on the “New World” monkeys of the Americas. Among those Old World species, there’s a heavy emphasis on the fruit-eaters. Powell also points to a lack of good data on great apes, like gorillas and orangutans, in the data set.

It’s possible that the neocortex correlation shows up because the skew in the data coughs up a false positive. It’s also possible that it’s real. If a primatologist ever won the lottery and funded the dream data set, we’d find out.

Listing image by Kate Shaw

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