Razib Khan One-stop-shopping for all of my content

December 27, 2010

Slouching toward idiocracy?

Filed under: Brain,Cranial Capacity,Evolution,Human Evolution — Razib Khan @ 1:32 am

The September issue of Discover Magazine had an interesting piece, If Modern Humans Are So Smart, Why Are Our Brains Shrinking? It’s now online, though to read the full article you’ll have to have a print subscription, or, pay 99 cents to get a digital copy of that issue. John Hawks is described as “a bearish man with rounded features and a jovial disposition.”

The background to this phenomenon is rather simple. For several millions years up to ~200,000 years ago there was a study increase in hominin cranial capacities. I say hominin because it seems that this increase was evident in all branches of the human lineage. Neandertals were increasing in cranial capacity, just as African humans were. Then there was a leveling off and stabilization. Finally, over the past 15,000 years or so there has been a decline, from a median of 1,500 cubic centimeters (cc) to 1,350 cc.

You can read the article for an elaboration on the various hypotheses. But roughly, some think we’re getting less intelligent, while others believe that the brain is reorganizing its structure and development. Remember that the brain uses about ~20% of our caloric intake. It’s a metabolically expensive organ.

I would like to add that even if the median human intelligence is decreasing, the current generation has the largest absolute number of very bright people alive at any given time. This is a natural function of the large human population. If the stability of civilization rests not on the median human, but the coordination and mobilization of large numbers of cognitively gifted humans, then perhaps we should not worry too much in the short to medium term. Even with stabilizing world populations we’ll have a generation or two of large numbers of brights before differential fitness of the smart and dull really start eroding the numbers of the former.

April 4, 2010

Increased rate of encephalization

Filed under: Anthroplogy,Cranial Capacity,Encephalization,Evolution,Human Evolution — Razib Khan @ 10:16 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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