Saturday, September 17, 2005

Scrambled Eggs and Brains

Two recent papers in SCIENCE (9 September 2005) report on a couple of variants in genes associated with increased brain size have increased in frequency rapidly, too rapidly to be explicable as instances of genetic drift. Statistical tests indicate that one of the variants in the gene Microcephalin arose about 37,000 years ago. The second, haplogroup D of the ASPM gene, is more recent and probably began to occur around 5800 years ago.

The press noted that the two variants occur less frequently in sub-Saharan Africa than elsewhere, which doubtlessly reinforced the belief, nearly universal in America, that blacks are genetically inferior to Europeans. What the newspapers didn’t notice, however, is that the incidence of the genes is highest not in Europe but in places like Pakistan. Some 78.6% of the French population tested possessed the Microcephalin variant, but 98% of the Siberian Ubermenschen. 45% of tested North Italians had haplotype D, but they couldn’t compete with Papua, New Guinea where the sample came it at just under 60%.

Well, the ifs, and, and buts never impress anybody. They are routinely dismissed as liberal piety. It probably doesn’t help to point out that a host of genetic variants affect brain development and that there is no way of determining from purely statistical studies whether the Microcephalin variant or haplotype D are particularly important determinants of better cognitive functions or, indeed, whether they in fact improve cognitive function at all. Natural selection obviously favored these genes, but what phenotypical effects account for their greater fitness is unknown. Genes typically have multiple effects so it is perfectly possible that the variants in question flourished because they boosted immunity to some pathogen or directly increased fertility like another rapidly spreading gene variant recently identified in Iceland.

The authors of the papers go so far as to speculate that the timing of the appearance of the two genetic variants matches up with important milestones in human cultural evolution; but if cave painting or agriculture were somehow made possible by these mutations, they must have been potent indeed since at the outset and for many generations afterwards, only a tiny percentage of individuals possessed them. I’m more inclined to look at things from a reversed perspective. Every cultural accomplishment increases the value of intelligence and makes it likely that any mutation that improves cognitive functioning will spread through the population. What occurs is analogous to a common pattern in the history of technology, the invention of a new practice—extracting motive power from fuel, telephony, flying—inspires an often motley series of secondary inventions that implement or exploit the primary innovation. No point in having more brains if there’s nothing to do with them. It might be that cultural changes circa 38,000 BCE or 5600 BCE made the genetic changes more likely to spread rather than the other way around.

If civilization persists long enough, the genetic basis of human cognitive performance will eventually be elucidated and the results may be ideologically embarrassing to everybody, though probably not in foreseeable ways. In the meantime, it’s a good bet that every isolated research result will be taken as confirmation of the prevailing tribal prejudices. A true statement is validly implied by every other statement, true or false–trust me, it’s logic—so if you are an absolute believer in some tenet, it is perfectly though insanely reasonable to assume that everything you hear is further evidence of its truth. Which is why, incidentally, every surprising result in biology can be instantly seized upon as evidence for intelligent design and people can actually be edified by the sufferings of marching penguins. This sort of thing makes me very dubious about the whole enterprise of popular science.

By the way, there is at least one widely held belief that really is challenged by results like the papers in SCIENCE. I refer to the endlessly repeated notion that the advent of anatomically modern human beings signaled the replacement of genetic evolution with cultural evolution. This commonplace never made any sense at all. Natural selection is inevitable whenever heritable phenotypical characteristics lead to differential rates of successful reproduction. Since not everybody has the same number of live offspring and their relative fertility is surely not a matter of mere chance, evolution will proceed, although its action will often take the form of apparent stasis under the action of what is usually called stabilizing selection, the biological version of quality control. Anyhow, as I previously pointed out, the emergence of culture actually promotes directional evolution—the development of animal husbandry led to the rapid spread of genes that allow adults to drink liquid milk, for example—and I wouldn’t be surprised if ever more complex technological civilization eventually leads to one or more speciation events.

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