No quantum leap in human brain size?

March 30, 2010 • 1:47 pm

A few days ago I posted about neurobiologist Colin Blakemore’s thesis that, about 200,000 years ago, human brain size underwent an instantaneous increase in volume by 30%.  Blakemore suggested that this was the result of a macromutation sweeping to fixation, but not through natural selection (he suggests no alternative, but perhaps he meant genetic drift).

Over at john hawks weblog, Hawks shows that there was no sudden increase in human brain size. Here’s the chart that Hawks provides which plots brain volume over time (the data appear to come from Lee and Wolpoff, 2003, but I haven’t checked):

Fig. 1. Change in hominin brain size over time.  Time (thousands of years before present) on x axis, brain volume (cc) on y axis.

Now none of us have done any statistics on the supposed 30% “leap” 200,000 years ago, and I haven’t fitted a line to this to see if the increase has accelerated over time.  Hawks says that this:

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).

Well, yes, there is certainly a “single” pattern of increase (whatever that means), but perhaps a hint that the rate of change became a bit faster 200,000 years ago. But it certainly wasn’t an instantaneous increase, as one would expect with the rapid fixation of a macromutation that increased brain size by 30%.  There seem to be a fair few intermediates!

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Lee S-H, Wolpoff MH. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196.

13 thoughts on “No quantum leap in human brain size?

    1. Moreover, what warrants linking the brain volumes across species? It does not make sense to link the increases in volume. All one can say it that habilis cast volume is smaller than erectus than sapiens so forth. My hypothesis (unpublished, although i did have a poster at consciousness, arizona, 2006) is that you can model different volume “slopes” for each species that get “steeper” as you approach sapiens. Early sapiens cast volume is smaller than “modern” sapiens.. Of course, we need lots more crania to have a landscape. I said something about this while commenting the Dr Blakemore talk. This linear increase thinking is flawed, I suggest.

  1. Also, this does not tell us how brain size evolved relative to body size. Is there is not evidence that homo body size increased somewhat over the past 2 million years? How much of this brain size increase is scaling up?

  2. Thanks for the link.

    “Single pattern” means that there’s no two-epoch (or more-epoch) fit that is significantly better than a simple geometric fit.

    Re: multiple species — the major regions (China, Java, Africa, Europe) exhibit a common trend. We tested this in 2001 (Hawks and Wolpoff, Evolution 55:1474). The higher data points there after 200k are mostly Neandertals.

    Today’s population means are mostly between 1200-1400 ml, which is in the middle of the interval of volumes represented among humans 200k ago.

    1. Just as a matter of curiosity, has Prof. Wolpoff abandoned his hypothesis that Homo Sapiens evolved from Neanderthals? It is my understanding that he is/was one of the last holdouts for this hypothesis.

      1. He doesn’t say that H. sapiens evolved from neanderthals, but rather that neanderthals and H. sapiens aren’t different species and that there was significant interbreeding between the two groups.

        There are still a great many in the field who suspect this to be the case, although it is not currently the mainstream view. I imagine that these issues will be largely resolved in the next decade or so as the nuclear genome of neanderthals and their paleolithic H. sapiens peers are published and compared.

        1. Then it would appear that he has shifted his position as I have read that at one time he proposed that Homo Sapiens evolved from Neanderthals and adhered to that position after virtually all of his colleagues had abandoned it.

          1. Nope, not true. It might just be a semantic misunderstanding, as “evolved from” could be considered accurate if humans have any neanderthal genes at all, but it can also refer to neanderthals being the primary ancestors of modern humans, which he has never claimed.

            His work has certainly been distorted by lots of different people trying to argue against straw men, so that might be the source of the misunderstanding.

  3. Any graph that purports to reveal a change in a rate of change should be constructed with a vertical axis that is logarithmic. Otherwise what you finish up with is an optical illusion. If that were done in this case, it would be clear that there is no “sudden increase” requiring explanation.

  4. A quantum leap is the smallest leap possible, so shouldn’t the title be something like “Lots quantum leaps in human brain size?” After all, isn’t evolution the accumulation of many quantum leaps into macroscopic differences?

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