Oxford neurobiologist Colin Blakemore, a distinguished researcher and author of several popular-science books, gave the Ferrier Prize Lecture at the Royal Society on March 15 and was just interviewed by The Guardian. His topic: the evolution of the human brain. Although the lecture precis implies that Blakemore talked about several topics, the Guardian singled out one: Blakemore’s apparent assertion that a rapid increase in human brain size a few hundred thousand years ago was due to a “macromutation”—a single change in the DNA of large effect:
In a recent lecture, the Oxford neurobiologist argued that a mutation in the brain of a single human being 200,000 years ago turned intellectually able apemen into a super-intelligent species that would conquer the world. In short, Homo sapiens is a genetic accident.
The question is: why is [our brain] 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.
Blakemore suggests that this big mutation might have occurred in “mitochondrial Eve,” the woman from whom, he says, we all descend:
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.
Finally, Blakemore suggests that the spread of this mutation through our ancestors, and its fixation, was not due to natural selection!:
How have scientists explained this jump in brain size?
Many have argued that if there was a dramatic increase in brain size, there must have been a fantastic advantage that came with it: improvements in tool construction, more complex language and other cultural changes. In other words, they say simple natural selection explains what happened.
So what is your take on this view?
I think they’re fooling themselves. There was very little change in human behaviour at this time, as far as we can see from the fossil record – certainly not one that is explained by a sudden jump in the size of the human brain. These hand-waving arguments about tiny changes in culture explaining the emergence of such a huge change in brain structure just doesn’t hold water. It’s like arguing that a reptile suddenly developed fully formed wings and then sat around for 200,000 years before suddenly saying: Oh my God, I’ve discovered I can fly. It’s ridiculous. . .
What effect did this have?
Very little at first. 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.
Okay, here are just some of the problems with Blakemore’s thesis:
1. First of all—and this has been endlessly repeated by geneticists—it’s misleading to say that “every living human can be traced back to a single woman called ‘Mitochondrial Eve’ who lived about 200,000 years ago.” What we know is that all the DNA in one of our cell organelles, the mitochondrion, descends from a female that lived then. But the rest of our DNA, the vast majority of it, descends from other ancestors, so that each bit of our genome comes from (to use the technical jargon “coalesces back to”) a different ancestral individual. Our genome is a mosaic of bits of DNA from different people.
Mitochondrial “Eve” was the ancestor of only our mitochondrial DNA. It’s extremely improbable (I’d say the chances are zero) of any other gene not in the mitochondrial DNA descending from this same woman. Blakemore does allow that the big-brain mutation could have occurred in a “relative” of Eve, but that’s not necessary either, except in the sense that all humans living at that time were relatives because they shared a common ancestor.
2. There’s not the slightest bit of evidence that this large increase in brain size resulted from a single mutation in the DNA. Yes, the increase in brain size may have been geologically sudden, but we can get things that would look instantaneous in the fossil record through a sudden bout of natural selection that fixes several or many genes over a relatively short period, say thousands of years. A punctuated pattern of evolution does not necessarily point to the occurrence of single “macromutations.” Steve Gould sometimes made this mistake when talking about punctuated equilibrium.
Blakemore has every right to theorize about macromutations, but in a public lecture (or interview) it would be seemly if he mentioned the problems with his “macromutation” idea.
One is that a big change in brain size without a corresponding increase in skull size probably would have been maladaptive, if not fatal. It’s much more likely that brain and skull co-evolved gradually due to the accumulation of several to many mutations, which is normally how the evolution of complex traits occurs in nature. Too, the brain is plastic, but is it plastic enough to suddenly accommodate a 30% increase in volume without evolutionary changes in wiring? Finally, the usual finding when you look at the genetic basis of “complex” adaptations in the wild (and I’m talking about things other than color, or the disappearance of traits like stickleback spines), it’s almost always due to more than one gene.
3. Finally, perhaps the biggest problem with Blakemore’s suggestion is this: if the mutation wasn’t favored by selection, how did it get fixed? He says that the big-brain mutation was initially “neutral”—that in fact it normally would have been disfavored because a bigger brain eats up too much metabolic energy, but was tolerated because there were lots of foods and resources around. Then, when the environment became leaner and meaner, that big mutation would show its adaptive effects. To repeat from above:
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.
This scenario invokes a big brain as a preadaptation: a trait that just happened to be hanging around but then became useful when the environment changed. Note that Blakemore is claiming more than just that the macromutation stayed around at low frequencies because it was tolerated during fat times and then later became fixed through selection when times got tough. Rather, he’s asserting that the mutation swept to fixation without natural selection. Again:
In other words, they say simple natural selection explains what happened.
So what is your take on this view?
I think they’re fooling themselves. There was very little change in human behaviour at this time, as far as we can see from the fossil record – certainly not one that is explained by a sudden jump in the size of the human brain.
I find this scenario pretty implausible. Implausible not just because you need other changes in the body (like a bigger cranial capacity and perhaps some new wiring) to accommodate a brain that is suddenly 30% larger, but because Blakemore gives no explanation for how a big mutation that has no effect on fitness (or even a slightly negative effect) swept through the population in the first place (i.e. gets “fixed” as we geneticists say). A mutation can be “tolerated” in a species, but that doesn’t explain why it spreads through the population until everybody has it. It seems much more likely that changes in brain size, since they were sustained over millions of years of evolution, resulted from natural selection.
Granted, we don’t know what kind of selection. There are as many theories for the evolution of bigger human brains as there are theorists. Suggestions have included the advantages of big brains for hunting, for living in social groups, for attracting females, for trying to suss out the intentions of your fellows, for using tools, and so on. This is one of the questions whose answer we may never know. All scientists must live with the idea that some answers are simply beyond our ken. (That’s why it’s easier for us to not invoke gods to explain the unknown.)
But I think that Blakemore’s own suggestion is deficient in several ways. The Guardian can’t point them out in an interview, but that’s what websites like this one are for.
I have to say that although lots of people concoct evolutionary stories about humans, many of those folks could benefit from a little acquaintance with population and evolutionary genetics.