Were human brains bigger in the past

Today's human brain is about 10 percent smaller than the Cro-Magnon brain from more than 20,000 years ago. iStockphoto.com hide caption

When it comes to brain size, bigger doesn't always mean better. As humans continue to evolve, scientists say our brains are actually getting smaller.

The downsizing of human brains is an evolutionary fact that took science writer Kathleen McAuliffe by surprise.

"I said, 'What?  I thought it was getting bigger!'" she tells NPR's Jacki Lyden. That was the story up to 20,000 years ago, she learned. Then, the brains of our ancestors reversed course and started getting smaller -- and they've been shrinking ever since.

Cro-Magnon man, who lived in Europe 20,000 to 30,000 years ago, had the biggest brains of any human species.  In comparison, today's human brain is about 10 percent smaller. It's a chunk of brain matter "roughly equivalent to a tennis ball in size," McAuliffe says.

The experts aren't sure about the implications of this evolutionary trend.  Some think it might be a dumbing-down process. One cognitive scientist, David Geary, argues that as human society grows increasingly complex, individuals don't need to be as intelligent in order to survive and reproduce.

But not all researchers are so pessimistic. Brian Hare, an anthropologist at the Duke University Institute for Brain Sciences, thinks the decrease in brain size is actually an evolutionary advantage.

The Domesticated Brain

"A smaller brain is the signature of selection against aggression," Hare tells Lyden.  "Another way to say that is an increase in tolerance."

Hare says when a population selects against aggression, they can be considered to be domesticated.  And for a variety of domesticated animals like apes, dogs or turkeys, you can see certain physical characteristics emerge. Among these traits are a lighter and more slender skeleton, a flattened forehead -- and decreased brain size.

Hare's studies focus on chimpanzees and bonobos.  In evolutionary terms, they are much like humans, but are physically quite different from one another. Bonobos have smaller brains than chimpanzees -- and are also much less aggressive.

While both have the cognitive ability to solve a given puzzle, Hare says, chimpanzees are much less likely to accomplish it if it involves teamwork.  Not so with bonobos.

"If the food is quite sparse and it's not easy to share, [bonobos] can solve the problem," Hare says. "Chimpanzees, in that same context -- where there's not much food and it's not easy to share -- they just refuse to work together. They can't solve the problem, even though they know how."

Hare does admit that the shrinking human brain could signal an evolutionary dumbing-down, but more important is what the phenomenon tells us about ourselves. Comparing our evolution to that of other animals enriches our understanding of the human condition.

"The nice thing about studying animals and human nature," Hare says, "is that it helps us design or think of some strategies that deal with our darker sides."

Humans have much larger brains than other primates, but it is not clear exactly when and how this difference emerged during evolution. Some scientists believe that the expansion of a part of the brain called the neocortex – which handles sight, hearing, conscious decision-making and language – drove the increase in the size of the human brain. Newer studies have challenged that idea.

One way to learn more about how humans evolved bigger brains is to compare the size of the brain, and specific parts of the brain, between humans and our closest relatives: non-human primates. To make accurate comparisons, scientists must account for many factors. Closely related primates may have more similar traits because they more recently shared a common ancestor. This means the evolutionary relationships between species need to be considered. Larger animals also tend to have larger brains so it is important to consider body size, too.

Now, Miller at al. show that the human brain is much larger than expected even after accounting for these factors, and that increases in brain size accelerated over the course of early human evolution. In the analyses, the brain and skull sizes of different living primate species, like chimpanzees and gorillas, and fossils of extinct primates, including Neanderthals, were compared using mathematical models.

These findings suggest that larger brains provided fitness advantages that led to large brain sizes in modern humans and Neanderthals. These increases in brain size were not driven by disproportionate growth in the neocortex alone, but rather by increases in the size of many parts of the brain. Increases in the relative size of the cerebellum, which is essential for balance and movement, were also important.

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