Remember the kerfuffle over ASPM two years ago? ASPM is a gene that regulates brain growth. It evolved considerably in the primate lineage leading to humans and continued to evolve even after the emergence of modern humans, with the latest variant arising about 6000 years ago somewhere in the Middle East. The new variant then proliferated within and outside this region, reaching higher incidences in the Middle East (37–52%) and in Europe (38–50%) than in East Asia (0–25%).
Interest died down when it was found that this variant, despite its apparent selective advantage, does not seem to improve cognitive performance, at least not on standard IQ tests (Mekel-Bobroy et al., 2007; Rushton et al., 2007). Nor do ASPM variants correlate with human brain size variability (Rushton et al., 2007).
Now, new light has been shed on this puzzle by a paper on ASPM in other primates. This gene was initially linked to overall brain size because non-functioning variants cause microcephaly in humans. A comparative study of primate species, however, has shown that evolution of ASPM does not correlate with major changes in whole brain or cerebellum size:
Particularly striking is the result that only major changes of cerebral cortex size and not major changes in whole brain or cerebellum size are associated with positive selection in ASPM. This is consistent with an expression report indicating that ASPM’s expression is limited to the cerebral cortex of the brain (Bond et al. 2002). Our findings stand in contrast to recent null findings correlating ASPM genotypes with human brain size variation. Those studies used the relatively imprecise phenotypic trait of whole brain instead of cerebral cortex size (Rushton, Vernon, and Bons 2006; Woods et al. 2006; Thimpson et al. 2007). Although previous studies have shown that parts of the brain scale strongly with one another and especially with whole brain (e.g., Finlay and Darlington 1995), evidence here suggests that different brain parts still have their own evolutionary and functional differentiation with unique genetic bases. (Ali & Meier, 2008)
This is a point I raised a year ago. If we look at how the new ASPM variant spread geographically and temporally, it seems to match a very specific mental ability, and not general intelligence:
At present, we can only say that it [the new variant] probably assists performance on a task that exhibited the same geographic expansion from a Middle Eastern origin roughly 6000 years ago. The closest match seems to be the invention of alphabetical writing, specifically the task of transcribing speech and copying texts into alphabetical script. Though more easily learned than ideographs, alphabetical characters place higher demands on mental processing, especially under premodern conditions (continuous text with little or no punctuation, real-time stenography, absence of automated assistance for publishing or copying, etc.).
…How well are these tasks evaluated by standard IQ tests? Although most tests involve reading, transcribing, and taking dictation, these abilities are not evaluated over long, uninterrupted time periods. If we look at the two studies that discounted a cognitive advantage for the new ASPM variant, neither tested its participants for longer than 82 min and the tests themselves involved a mix of written and verbal tasks.
It seems premature to conclude that the new ASPM variant is unrelated to cognitive functioning. Current IQ tests do not adequately evaluate mental processing of alphabetical writing, particularly under premodern conditions. Yet this is the cognitive task whose origin and spread most closely coincide with those of the new ASPM variant in human populations. It is also a demanding task that only a fraction
of the population could perform in antiquity, in exchange for privileged status and probably superior reproductive opportunities. (Frost, 2007)
Is this cognitive task localized in a specific part of the brain? There seems to be evidence for such localization … which I will review in my next post.
Ali, F. and Meier, R. (2008). Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size. Molecular Biology & Evolution. Advance access
Frost, P. 2007. “The spread of alphabetical writing may have favored the latest variant of the ASPM gene”, Medical Hypotheses, 70, 17-20.
Mekel-Bobrov, N., Posthuma D., Gilbert S.L., et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Hum Mole Genet, 16, 600–8.
Rushton, J.P., Vernon, PA.., Bons, T.A. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK, 3, 157–60.