Controversial Study Suggests …

Now, what subject might be automatically controversial? Intelligence of course! Here’s the full headline:

Controversial study suggests human intelligence peaked several thousand years ago and we’ve been on an intellectual and emotional decline ever since (in The Independent here)

It is reporting an article by Gerald Crabtree in Trends in Genetics.

His argument is based on the fact that for more than 99 per cent of human evolutionary history, we have lived as hunter-gatherer communities surviving on our wits, leading to big-brained humans. Since the invention of agriculture and cities, however, natural selection on our intellect has effective stopped and mutations have accumulated in the critical “intelligence” genes.

It seems to be based on two basic ideas.

One: there is quite a high mutation rate in human DNA from generation to generation.

A comparison of the genomes of parents and children has revealed that on average there are between 25 and 65 new mutations occurring in the DNA of each generation. Professor Crabtree says that this analysis predicts about 5,000 new mutations in the past 120 generations, which covers a span of about 3,000 years.

Some of these mutations, he suggests, will occur within the 2,000 to 5,000 genes that are involved in human intellectual ability, for instance by building and mapping the billions of nerve cells of the brain or producing the dozens of chemical neurotransmitters that control the junctions between these brain cells.

So, a quite a high rate of mutation means some are likely to be harmful to intelligence. But, by the same token quite a few could be helpful to intelligence. A high mutation rate could be the basis for fairly swift increases in intelligence, if it was selected for.

Two: there was stronger selection for intelligence among foragers than among farmers.

Life as a hunter-gatherer was probably more intellectually demanding than widely supposed, he says. “A hunter-gatherer who did not correctly conceive a solution to providing food or shelter probably died, along with his or her progeny, whereas a modern Wall Street executive that made a similar conceptual mistake would receive a substantial bonus and be a more attractive mate,” Professor Crabtree says.

I would think that selective pressures for intelligence would vary quite a bit among both foragers and farmers, as compared to pressures for other traits. In any case, this all seems like conjecture. Is there any evidence that actual living foragers are more intelligent than farmers?

More here: Gerald R. Crabtree, Our Fragile Intellect (pdf)

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