Misinformation about brain evolution29 Mar 2010
Due to Jerry Coyne, I encountered an interview in the Guardian with Colin Blakemore: “Colin Blakemore: How the human brain got bigger by accident and not through evolution.”
The headline is a misnomer, as Blakemore is not denying evolution, he is denying selection. But Blakemore’s argument is based completely on a false presentation of the facts. Consider:
The question is: why is it so big compared to the brains of our predecessors, such as Homo erectus? Until 200,000 years ago, there had been a gradual increase in brain size among hominins, starting three million years ago. Then, abruptly, there was a remarkable increase of about 30% or so.
That’s Blakemore. Now, here’s a chart of endocranial volumes of Pleistocene human fossils:
As you can see, there’s no sudden jump 200,000 years ago, or at any other time. The data, such as they are, are consistent with a single pattern of increase over time, as pointed out by Sang-Hee Lee and Milford Wolpoff (2003).
Heck, it’s the lack of a sudden jump that has gotten all the attention. Because if “modern” humans suddenly showed up in Africa 200,000 years ago, and all of a sudden had vastly larger brains than any other hominins, wouldn’t that be a simple and tidy story? Don’t you think we’d all be talking about the sudden origin of modern humans as reflected by their larger brains?
It just didn’t happen.
Well, it’s one thing to be empirically wrong. That’s a simple error that’s easily corrected. But Blakemore, relying on the erroneous assumption of a single shift in brain size, asserts that neutral macromutations must be an important mode of human brain evolution:
Genetic studies suggest every living human can be traced back to a single woman called "Mitochondrial Eve" who lived about 200,000 years ago. My suggestion is that the sudden expansion of the brain 200,000 years ago was a dramatic spontaneous mutation in the brain of Mitochondrial Eve or a relative which then spread through the species. A change in a single gene would have been enough.
I hope that the empirical pattern is enough to convince you that this hypothesis is false. The “sudden increase” simply did not happen.
But in case you need more persuasion: Blakemore here assumes that the increase in brain size had no negative consequences. Otherwise it couldn’t proceed neutrally. Here is his argument:
The environment of early humans was so clement and rich in resources that this greedy new brain, which would have absorbed even more of the body's energy, could be sustained without danger. Later, when times got hard, during droughts or climate changes, it helped us deal with these crises, which could otherwise have killed us off, by dreaming up novel ideas to problems.
You see the outline: Life was easy, and humans could grow fat-brained, like so many sheep. Fortunately, our fat brains were then useful when times were tough. Blakemore describes this as somehow different from the idea that brains were adaptive – it’s in fact just another adaptive story for larger brains.
But it falls apart, when we consider that assumption – life was easy. I put it to my students this way: Suppose you lay a lot of sugar beets out on your land. What will happen to the deer population?
The answer is not that the deer will grow fat-brained and later evolve to conquer humanity. The answer is that there will be a lot more deer.
Population growth is much faster than adaptation, and it’s hundreds of times faster than a neutral gene can transit through the population. Humans in the past were not a static population, living in peace with an abundant environment. They were repeated faced with Malthusian crises – on submillennial timescales. That’s why a close understanding of climate variability is so relevant to our evolution. The fact that tools and behaviors change so slowly in the Middle Pleistocene is informative – it shows that humans weren’t coming up with dramatically new ways to track shifting ecologies.
And that means that the selection pressures of the energetic and life history constraints on the brain were repeatedly imposed on human populations. A substantial increase in brain size should have immediately been disadvantageous – if it had no compensatory benefits to fitness.
What remains is testing the hypotheses about those benefits to fitness. Blakemore actually is presenting one such hypothesis – that a larger brain mostly was adaptive because of its ability to transmit traditions. That’s testable, and is consistent with the greater transfer of information apparent in recent archaeological traditions compared to Middle Pleistocene ones. But there are other hypotheses as well, and it is quite difficult to compare them with the available record.
That’s why it’s so important to state the empirical record accurately.
UPDATE (2010-03-29): A reader points out that Malthusian crises, in terms of resource or food availability, may have been avoided by warfare or predation – people kill each other instead of starving. I see that point, particularly where we consider the way that epidemic disease can relax competition for food until population growth resumes. Performing well under predation or competition would be one way that brain size might have had compensatory benefits to fitness beyond its energetic and life history costs.
Lee S-H, Wolpoff MH. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196.