Why do humans have such huge brains? Scientists have a few hypotheses.

Our ancient ancestor Lucy, the famous Australopithecus afarensis fossil, lived between 3 million and 4 million years ago and had a body in the realm between modern human and animal. The Australopithecus pelvis and leg bones are similar to ours, and A. afarensis likely stood upright, like us. But they were extremely different in a hugely important way: They had relatively tiny brains — nearly three times smaller than today’s Homo sapiens version.

This is one of the great questions in human evolution: What happened to make the small, early brains of Lucy and her kind balloon into the smarty-pants, socially adept noggins we have today?

Large brains are the defining characteristic of our species. Not only are they the most complicated organs in all the animal kingdom, but some scientists have proclaimed them the “most complicated object in the known universe.”

But human brains are much larger than you’d expect considering our body size. Figuring out why evolution favored such big brains will help us answer the biggest question in all of anthropology: What does it mean to be human, and what makes us unique?

There are some hypotheses about our brains. It could be that our environments were challenging, and we evolved big brains to conquer it. Or it could be that because we survive best as social creatures in groups, we need the big brains for our complicated social lives.

These and other theories likely played a role in our evolution, but we still don’t really know which came first or which was more important. Though, as a Nature paper out Wednesday describes, scientists are coming up with new ways to sort it all out.

Three explanations for why we have such big brains

First off, why should we care about this question? We have big brains now. What does it matter how we got them?

One answer, explains Mauricio Forero, an evolutionary biologist at University of St. Andrews in Scotland, is that understanding the evolution of the brain may help us better understand how the brain works.

“If you want to understand how a racing car works, you could save a lot of resources if you read the designers’ notes,” Forero, the lead author of the Nature study, says. “And if we want to understand how the brain works, you could probably save a lot of resources by understanding how it was designed.” In the case of the brain, that means understanding the forces of natural selection that worked upon it.

So far, evolutionary anthropologists have laid out three broad categories of explanations for why the human mind grew so large (there are many other, more specific sub-theories). They are:

  • Environmental: Physical challenges — like finding, hunting, or remembering sources of food — provided selection pressure for bigger brains.
  • Social: Interacting with others — either cooperatively or competitively — favored people with brains large enough to anticipate the actions of others.
  • Cultural: People who were able to hold on to accumulated knowledge and teach it to others were most likely to reproduce. (One of these cultural factors could have been cooking. As biological anthropologist Richard Wrangham famously argued in his 2009 book Catching Fire, when we learned to cook food, we got access to more easily digestible calories, which freed up energy and time develop bigger brains.)

It’s likely all three factors played a role, and influenced each other. But the mix has to be precisely right to create the human brain in its current form. For example, if natural selection pressures favored a high degree of cooperation, that would actually favor a smaller brain. Think about it: If you rely on others to a high degree, you don’t need to use your own brain as much. Ants, an incredibly cooperative species, don’t have much when it comes to brains. Same goes for bees.

Also, it turns out, there are huge tradeoffs involved in having a big brain. Mostly: They need a lot of fuel. Today, about 20 percent of the energy produced by our body’s metabolic activity goes to our brains.

“The brain is a really expensive tissue that requires a lot of energy,” writes Simon Neubauer, an evolutionary anthropologist at the Max Planck Institute for Evolutionary Anthropology. There “has to have had an advantage that drives this process.”

If the selection pressures were slightly different, evolution could have favored a different course: using that energy for increased reproduction (or to get us to sexual maturity at an earlier age), or to build bulkier bodies. But instead we have these energy-hogging brains. So how do we know which factor was the most important for their evolution?

Forero’s paper suggests the environment played a bigger role than social factors

Typically this question of “why are humans brains so big” is tested from correlational studies. Anthropologists and biologists study the brain sizes of different species, and try to deduce what behavioral changes a larger brain allows for. For instance, in many primates, greater social learning is correlated with larger brain size.

This approach is kind of messy, and it’s hard to draw firm conclusions from it. (Alternatively, it’s impossible to do experiments on human evolution. You can’t just take a group of people, put them on islands with differing environmental stresses, and watch them evolve.)

Instead, Forero and his co-author in the new Nature paper turned to math. They set up a computer model that played out different scenarios of human evolution: one where all the selection pressure was from the environment, and one where all the selection pressure was from social life. (They did not model cultural selection pressure but hope to do so in a future study.) The model took into account the metabolic costs and benefits of developing a larger brain versus other components of the human body. And they ran the program to see: What mix of factors generates a brain that’s most like the ones we have today?

According to their model, the environmental factors are the strongest explanation for why we have big brains. That challenges in our environment, and less so challenges in our social groups, lead to the rapid expansion of of noggins. It’s suspected that much of our evolution occurred on the African savannah, where food and water are harder to find than in a forest. We’d also have to be clever to avoid harsh, punishing heat.

But questions still remain. “If we find that difficult environments favor human brain expansion, this raises the question, why humans? Why not other species?” Forero says.

Plenty of species live in challenging environments (there are even species adapted to living in the perfect darkness of caves) but have never evolved to a point where they can order pizza from a smartphone (truly the pinnacle of human ingenuity and evolution). Perhaps, this is where culture — the ability to learn and teach others — matters. We found it easier to survive in a challenging environment when we could accumulate knowledge about the environment.

This is not to say other social factors didn’t play a role in our development. We are, after all, highly social and group-obsessed creatures. And it’s likely evolution made us this way. For one, it’s hard to think about how to share cultural knowledge without also fostering social cooperation.

The ultimate answer to the size of our brains is elusive

The conclusions we can draw from this model is really limited, says Amy Bauernfeind, a comparative neurobiologist at the Washington University School of Medicine. Models are always going to be a simplification of reality. They’re a mathematical way of understanding if our hypotheses about evolution make sense. But they’re not a direct observation of evolution itself. So it’s hard to use them to make definite conclusions.

The reality is that a lot of the factors that lead to the evolution of our big brains are hard to disentangle from one another. And the question of what’s more important for evolution — a challenging environment, or a stimulating social life — is a bit oversimplified to begin with, says Matthew Rossano, who studies psychological evolution at Southeastern Louisiana University, in an email.

“All the factors feedback on each other,” Rossano explains. “Learning how to make a new tool [to conquer the environment] leads to dietary changes, which affects brain development, which affects gestation and birth, which has social implications, which increases social complexity, which affects the brain, which allows for greater technical intelligence, which means new more complex tools, which means more dietary changes, which means cooperative hunting, which is a social change.” And so on.

That the evolution of the human brain is so hard to model speaks to an intriguing fact, however: The brain expanded under very specific evolutionary conditions. Our intelligence wasn’t a given, but the result of a intensely complicated process, which could have played out very differently. Understanding the forces that shaped the human mind could, one day, help us understand how rare our intelligence is, not just on planet Earth, but possibly in the broader universe.

For now, the ultimate answer to the size of our brains is elusive. But scientists suspect there is an answer. The human brain is so unique, and so densely complicated, it’s unlikely to have just developed by truly random chance.