Recommended Reading: The 10,000 Year Explosion

Almost since the 1871 publication of “The Descent of Man,” in which Charles Darwin applied his theory of natural selection to the human species, biologists have argued over whether the dramatic series of evolutionary events that led to the emergence of Homo sapiens continues to this day.

Some have argued that culture and technology have eclipsed the powerful biological forces that shaped our species in its formative years. In their view the species, no longer faced with a daily struggle for survival, is adrift in an evolutionary Sargasso Sea.

“There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain,” the famed evolutionary biologist Stephen J. Gould once said in an interview.

In their new book “The 10,000 Year Explosion,” anthropologists Henry Harpending and Gregory Cochran argue the contrary position. They claim that in fact, far from grinding to a halt, human evolution has accelerated dramatically since the origins of agriculture about 10,000 years ago.

“We intend to make the case that human evolution has accelerated in the past 10,000 years, rather than slowing or stopping, and is now happening about 100 times faster than its long-term average over the 6 million years of our existence,” they write.

In evolutionary terms, 10,000 years is no time at all — about 400 human generations. Rabbits can go through 400 generations in not much more than a century — can you imagine rabbits being substantially different than they were 100 years ago?

Far from ending the chain of dramatic evolutionary changes that led to upright walking, advanced cognitive abilities and spoken language, Cochran and Harpending argue, the adoption of agriculture so dramatically changed the human environment that a new wave of genetic innovations flourished. These new genetic variants thrived because they helped people cope with the challenges an agricultural way of life presented, such as the shift to a low protein, high carbohydrate diet; the creation of an organized, stratified society and the rise of infectious diseases in response to increased population density.

In fact, many of the genetic variations that 23andMe provides information about are relics of those evolutionary changes. The SNP that confers lactose tolerance, for example, appears to have arisen in Europe about 8,000 years ago among the first people to herd cows and other milk-producing animals. The lactose-digesting variant quickly spread throughout the parts of Eurasia that were ecologically suited to pastoralism.

There are also a number of genetic variations covered by 23andMe that cause physiological problems when two mutated copies are present, but provide protection against infectious disease when a person has one of each version of the gene. For example, the genetic variations that cause sickle cell anemia and G6PD deficiency confer resistance to malaria. Geneticists call this situation balancing selection; over the entire population, the reproductive cost to those who end up with the genetic disease is outweighed by the benefit to others who are resistant to the infectious one.

At the end of the book, Cochran and Harpending make the controversial argument that balancing selection is responsible for the increased incidence of a number of genetic diseases among people of Ashkenazi Jewish descent — and for their higher intelligence relative to other groups.

The authors do raise some interesting points about the anomalously high frequency among Ashkenazi of genetic disorders that stimulate the growth of neurons in the brain. And they cite studies that have shown increased intelligence among people with some of these diseases.

But genetic explanations for between-group differences in intelligence are best taken with a whopping dose of skepticism. Even the definition of intelligence is a matter of intense debate, not to mention the degree to which it can be inherited through genetics. in the end, their case is little more than a just-so story.

In telling it Cochran and Harpending blunt the rest of their book’s powerful message: human evolution is not over by a long sight.

  • vonagan cheeseman

    “But genetic explanations for between-group differences in intelligence are best taken with a whopping dose of skepticism. Even the definition of intelligence is a matter of intense debate, not to mention the degree to which it can be inherited through genetics. ”

    Do you know any of the literature on these issues, beyond Stephen Jay Gould that is? This is easily one of the _least_ controversial topics in the book.


  • Phantom

    Of course such research requires skepticism and high quality proof.

    But it’s obvious that intelligence can be inherited through genetics. We are smarter than apes because of our genes. In humans, you can see from the youngest age that some kids are naturally better at math/English/music/etc.

    Ashkenazi Jewish culture values education very strongly. Obviously that has a huge role in their success. But I also wouldn’t be surprised if, after 5000 years, a slight but discernible effect could be seen in “intelligence” scores.

    I shudder to think what the long term implications are for our Britney Spears obsessed culture. Will these trends be our future?

  • Ben017

    “But genetic explanations for between-group differences in intelligence are best taken with a whopping dose of skepticism. Even the definition of intelligence is a matter of intense debate, not to mention the degree to which it can be inherited through genetics. in the end, their case is little more than a just-so story.”

    Heritability estimtes for intelligence quotient (IQ) range from 0.50 to 0.80. This makes IQ a suitable target for attempts to identify the specific genes involved.” Chorney et al, Role of the cholinergic muscarinic 2 receptor (CHRM@) gene in cognition. Molecular Psychiatry (2003) 8. 10-13.

    “A substantial body of literature from twin, family and adoption studies documents significant genetic effects on human intelligence. Heritability estimates range from 40 to 80% and meta-analyses suggest an overall heritability of around 50%” Dick et al, (2006) “Association of CHRM2 with IQ: Converging Evidence for Genes Influencing Intelligence.” Behavioral Genetics.

    “Multivariate genetic analyses indicate that general intelligence is highly heritable, and that the overlap in the cognitive processes is twice as great as the overall phenotypic overlap, with genetic correlations averaging around .80.”
    Plomin et al (2004) “A functional polymorphism in the succinate-semialdehyde dehydrogenase genes is associated with cognitive ability,” Molecular Psychology 9, 582-586.

    In terms of the Ashkenazi paper:

    Here is a comment from one of the authors, Greg Cochran regarding that paper:

    “Suppose that there are N balls in an urn, of which m are red. The hypergeometric distribution describes the probability that exactly k balls are red in a sample of p distinct balls drawn from that urn.

    There are about 20 surprisingly common genetic diseases among the Ashkenazi Jews, of which 4 affect sphingolipid metabolism (and, incidentally, promote the growth of neural connections). We now know, to a pretty good approximation, how many human genes there are, and how many of those genes affect sphingolipid metabolism. Using a strategy that is an extended version of the simple hypergeometric model (also applying Bonferroni corrections for multiple comparisons, among other complications) you can get an estimate of the probability of such a cluster of mutations occurring by chance. It’s something less than 1 in 100,000. To do this, we used the Gene Ontology database (GO-EBI,EMBL-EBI, 2003), an established tool used to assess the statistical significance of human gene clusters obtained by high-throughput methods such as microarrays.

    Rudyard Kipling wrote the Just-So Stories, which explain how the leopard got his spots and so on. He was not ion fact responsible for our current understanding of the hypergeometric distribution: that was a joke.

    In my opinion, a careful look at the psychometric data, medieval history, and the biochemical effects of the sphingolipid mutations, combined with Gene Ontology calculations of cluster likelihood, combined with millions of simulations of the population genetics of a surprisingly frequency recessive lethal like Tay-Sachs ultimately produces something more than a Just-So Story.

    Posted by: gcochran at Jan 22, 2009 3:29:54 PM”

    Also, there is a series of interviews at 2Blowhards with Cochran:

    “2B: Is part of what enables us to think of culture affecting evolution the fact that we now know that evolution is proceeding rapidly?

    GC: Yes. And because different cultures result in different selective pressures, sometimes quite strong ones….

    2B: How controversial would the thesis that culture influences evolution be among the general run of scientists? What kinds of scientists would try to take issue with you? On what grounds?

    GC: On some points I think I’d get very little argument — lactase persistence for example. On anything that influences personality or cognition, I think they’d hate it, but that kind of evolutionary change is bound to happen. The social world changed and we changed too. There’s genetic evidence for it, and of course lots of evidence of psychometric differences.

    2B: The main example you use to illustrate how cultural developments can influence evolution is agriculture. Why was the development of agriculture such a major event where human evolution was concerned?

    GC: It increased human density by tens of times, which made a big difference in disease risks. It radically changed diet, and it dramatically furthered hierarchy and social complexity.

    2B: You and Henry assert that populations that adopted agriculture were much influenced by the development, and in non-insignificant, deep-in-the-biochemistry sorts of ways. What would your favorite examples of those adaptations be?

    GC: Metabolic/diet changes like lactose tolerance, many changes in genes involved with defense against infectious disease, many changes in genes that affect hearing and smell, changes in neurotransmitters and related genes that most likely influence personality, changes in genes involved with the regulation of nerve connections and brain growth.

    2B: One implication would seem to be that there are striking differences between populations that developed agriculture long ago and ones that encountered it only recently. Fair?

    GC: Yes. Peoples with short histories of agriculture have trouble with alcoholism, diabetes, and generally have a lot of trouble fitting into complex hierarchical societies.

    2B: Can you tell us a bit more about all this? And can you explain why should we think of these differences as more significant than, say, hair or eye color?

    GC: Eye and hair color are somehow involved with genetic changes that gave advantage — Vitamin D looks to have been important, but I don’t think we know the whole story on that. I tend to think things like vulnerability to diabetes or TB are important because they cripple and kill people: generally I’m against that.” archiv…e_1.html#005818

    “2B: Are all population groups evolving at the same rate? Is it possible that some population groups are evolving lickety-split, where others are fairly static evolution-wise?

    GC: If we’re right about big populations under agriculture changing rapidly, hunter-gatherers must have changed less. It would seem that Chinese probably changed a good deal faster than Australian Aborigines.” archiv…e_3.html#005819

Return to top