Accidental brain evolution suffers a reversal


Early homind skulls, from A Kansan’s Guide to Science (seriously)

A couple weeks ago, the Guardian ran an article in which Oxford neurobiologist Colin Blakemore described “how the human got bigger by accident and not through evolution.” Though I didn’t get to it at the time, I thought that an odd headline, since evolution actually occurs when genetic accidents — those mutation things — grant an advantage. Now John Hawks has written a post addressing what he says is a pretty big muckup by Blakemore:

Thanks 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:

Endocranial volumes of Pleistocene human fossils


Endocranial volume against time for fossil Homo.

Time is in thousands of years before present, running left to right.

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.

There’s more. It’s a nice post, correcting the error in a way that makes clear some basic evolutionary dynamics and clarifies the arc of human (brain) development.

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