Increased rate of encephalization

By Razib Khan | April 4, 2010 11:16 pm

A week ago I pointed to a controversy about the rate of growth of human cranial capacities over the past few million years. I asserted that the rate of growth was gradual, with no major discontinuity. Over at Genetic Inference Luke Jostins’ has done a more formal analysis.
linearHe finds:

The model shows a definite speed-up of brain size increase recently, and fits the data significantly better than a simple trend line (F(1,90) = 15.8, p < 10^-5). I estimate that the speed-up occured 252kya, and can say with 95% confidence that it lies between 203 and 377 kya. This result is pretty robust to exactly what model we use; I also tried using a model where brain size grew exponentially with time, and this gave a similar break-point: 250kya, with a 95% interval of 167-402 kya (see this graph).

Read the whole thing. I personally don’t find an increased rate of encephalization 200-300 years ago that implausible; the emergence of behavioral modernity about 50 thousand years ago resulted in much more rapid cultural evolution than before. But perhaps John Hawks could add some context here. It may be that Neandertals are oversampled in this dataset within the last few hundred thousand years vis-a-vis other archaic H. sapiens, distorting the trend line somehow. To me it still seems that the secular trend of increase over such a long period is somewhat puzzling, especially in light of relative stasis in toolkits. It makes me almost wonder if modern humans in their present highly cultural form were almost inevitable barring extinction due to some deep evolutionary positive feedback loop which was set in motion ~2 million years ago with the emergence of the hominin lineage.

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  • toto

    It’s even more impressive when you consider that, across species, brain size is expected to grow sub-linearly with body weight (more precisely, as a lower-than-1 power of it). When
    allometry is trumped, you must have a large effect (or a large artifact).

  • http://po-ru.com/ Paul Battley

    “an increased rate of encephalization 200-300 years ago” – I think you mean 200-300 thousand years ago, right?

  • miko

    Maybe at some time point a barrier to increased encephalization (which does look like it is being selected for) was removed: e.g. an adaptation in mothers allowing earlier birth, giving birth to bigger heads (I guess pelvis data should exist), or perhaps even the ability to provide care for babies born earlier (could be a purely cultural adaptation). I still think it’s possible there is some kind of sampling bias at work, as well.

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  • http://backseatdriving.blogspot.com/ Brian Schmidt

    “It makes me almost wonder if modern humans in their present highly cultural form were almost inevitable barring extinction due to some deep evolutionary positive feedback loop which was set in motion ~2 million years ago with the emergence of the hominin lineage.”

    You could go back even further and argue a technological species was almost inevitable due to an increasing encephelization loop set in motion when monkeys evolved from lemur-like ancestors. Some people don’t like that argument, though.

  • http://www.genetic-inference.co.uk/blog/ Luke Jostins

    Thanks for the link.

    Re: Neandertals, I also repeated the analysis after removing all Neandertal specimins, and this didn’t significantly affect the fit of the model; the conclusions are still the same. Interestingly, this is also true if you remove Sapiens but keep Neandertals, suggesting that the speed-up occured in both lineages around the same period (give or take a 100ky). This makes it seem more likely that some sort of pan-Homo environmental or cultural change occured, though it also increases the chance that it is some sort of measuring or sampling artefact.

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  • ruth

    I wonder what the graph would look like if all the highly encephalized classical Neandertals and fossil modern humans were removed and a worldwide selection of (less big-brained) present humans inserted instead. Would anyone who has the data like to try that? Also, I think large males are way overrepresented in the Neandertal (and possibly also fossil modern human?) sample, though that may be true for later erectus samples as well so I have no idea whether it affects the trend in any way.

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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