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I've read that humans gained about 200cc of brain capacity some 100,000 years ago. Is this true? Also, how did human brains get to be so big?

-A curious adult from California

December 11, 2008

I haven't seen any data that shows that the human brain increased in size over the last 100,000 years. Obviously the human brain is bigger than it used to be. But it increased in size over the last few million years. Not 100,000 years ago.

I have seen reports that human brains went from about 1250 cc to 1400 cc a couple of hundred thousand years ago. But I don't know how significant that is because brain size alone isn't what is important. What matters is brain size compared to body size.

Ancient humans were a lot shorter and their smaller brains may have just been proportional. In other words, they may have had smaller brains but it didn't matter. Sort of like men and women's brains.

Women's brains are on average about 100-150 cc smaller than men's. This of course doesn't matter and has more to do with body than brain size. Women are as smart as men despite their smaller brains.

However, in the past 2 million years, human brains nearly doubled in size. Our bodies have certainly not doubled in size. So our brains have gotten much larger compared to our bodies. This is quite striking and leads to your second question: why do humans have such big brains?

I'll split this one up into two sections. First I'll talk about some of the changes that have happened in our genes since we split from chimps. Not surprisingly, genes involved in brain development have changed the most.

In the second section I'll talk about some of the reasons for why these genes ended up being so different. Ideas range from a total accident because of a DNA change that caused smaller jaws to a big advantage in being so smart.

Chimp vs. Human Brain Genes

Humans and chimpanzees shared a common ancestor 6-8 million years ago. Over that time, one group evolved into humans and another into chimps.

An obvious difference between the two species is the size of their brains. Modern human brains are about three times larger than chimpanzee brains. Changes like this happen at the gene level.

A gene is a part of our DNA that can control a trait. Genes control things like hair and eye color. Or height. And yes, even brain size.

There isn't just one gene that controls something as complicated as brain development. Lots of genes are involved.

A human brain is three times the
size of a chimp brain because
of genes.

Given the difference in brain size, these genes are bound to be different between humans and chimps. So scientists have started to compare these genes in people and chimps to find the differences responsible for making human brains so big.

For example, scientists have found genes in humans that are there just to make our brains bigger. These -- brain size' genes allow our brain cells to grow faster and increase in size.

These genes seem to be different in chimps, not allowing their brains to become as large as ours. Scientists have found many other changes in brain genes between humans and chimps too.

Not only did a lot of our brain genes change, but they also changed rapidly. Scientists can tell this because the number of differences in brain genes between chimps and humans is much higher than would be predicted for two species that split only 6-8 million years ago.

All of this means that for some reason, humans ended up with many differences in their brain genes in a very short amount of time. The tricky question now, is, why this happened.

Why Humans Evolved Big Brains

Before getting into the whys, I wanted to first mention that there is more than just genetic evidence that our brains got bigger. The fossil evidence supports it as well.

Scientists have been able to find remains of early humans preserved in the Earth's crust. By comparing these skulls, they have been able to trace the gradual increase in brain size over the past two and a half million years.

So human brains are definitely bigger than they used to be. And this is most likely because of differences in genes.

Now the hard part. Why did humans end up with these genetic changes? Was there an advantage to having a big brain? Or was it purely an accident?

One idea is that a bigger brain could have made us smarter. This would definitely be an advantage for a social animal like us.

Scientists have indeed found that a bigger brain roughly translates to being more intelligent. However, this is a very rough relation. Human brain size varies considerably, just as body size does. Neanderthals, who preceded modern humans, actually had larger brains than we did at 1500cc. So it is hard to know if the bigger brain actually makes us smarter.

We do know that as the brain became bigger, it also needed to find new ways to coordinate between all its different parts. This required changes in its shape and structure to go along with the increase in size. These changes, especially in the front part of our brains, are mostly responsible for our high intelligence.

From a survival point of view, a big brain is not without its drawbacks. For example, a large brain requires a lot more care and feeding. About a quarter of the food we eat is used just to maintain our brain function. Our brains also require a diet rich in proteins.

It may be that a larger brain had to wait until our diets had more meat. According to Dr. Stephen Cunnane at the Universite de Sherbrooke in Quebec, Canada, eating clams and shellfish may have been essential for our brains to grow to their current size.

There may be another reason that human brains were able to get bigger. Brains may have increased in size because they could.

In our primate relatives, the size of their jaws keeps them from being able to have big brains. Humans have much smaller jaws than other primates. And this change in jaw size happened around the same time that our brains got bigger.

Scientists have found that this difference in jaw size comes from a single DNA change. Studies show that this change happened about 2 million years ago.

As a result of this DNA change, our ancestors lost their bulky jaw meant for chewing nuts and leaves. This may have not only allowed human skulls to get bigger and thus allow a bigger brain, but may also have played a role in humans switching to a more meat-based diet. Now they could grow a big brain and feed it.

Here is a possible scenario. Some of our ancestors had a change in their jaw size gene. These ancestors needed to eat meat because they couldn't eat nuts and leaves anymore.

Now this combination of smaller jaw and more meat meant that the brain could get bigger. If there was an advantage to having a big brain, then those individuals who had genes that would let their brains get bigger would prosper. In a relatively short time, humans would have big brains!

Of course, all of this is conjecture at this point. We don't know exactly why we ended up with bigger brains. But as I talked about in the first section, we are starting to figure out the brain genes that make us different from a chimp. And once we figure this out, we'll at least know how our big brains get made!

Sandeep Ravindran

Small human jaws
may allow for
a big brain.

Scientists aren\'t sure why
humans evolved such
big brains.