Monday, August 20, 2007

ASPM and human evolution

A year ago, Science published an article showing that the ASPM and microcephalin genes continued to evolve even after the emergence of Homo sapiens, being more prevalent in some populations than in others. This finding raised many eyebrows because the evolution of these genes is closely tied to increase in brain size between lower primates and humans.

Eyebrows have been lowering over the past year. Two independent research efforts have failed to find any relationship between ASPM variants and performance on IQ tests. Today, the general feeling is that it is time to move on to other things. This finding, like so many others, seems to have been just a flash in the pan.

Yet some, like me, think otherwise. Two British researchers, Dan Dediu and D. Robert Ladd have recently argued that the latest ASPM and microcephalin variants are associated with populations that speak non-tonal languages (PNAS "Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin"). Their hypothesis might explain the latest microcephalin variant, which is estimated to have arisen 37,000 years ago. The latest ASPM variant, however, is much younger, being only 5,800 years old (95% confidence interval between 500 and 14,100 years ago). It is probably too young to account for the emergence of non-tonal languages. Neither Indo-European languages nor Uralic languages are tonal, so it is likely that a non-tonal language was ancestral to Proto-Indo-European (8,000 - 6,000 years ago) and Proto-Uralic (10,000 - 7,000 years ago).

What cultural change, then, might have favored the latest ASPM variant? This is the subject of an article I have recently written for Medical Hypotheses:

The spread of alphabetical writing may have favored the latest variant of the ASPM gene


ASPM, a gene that regulates brain growth, has evolved considerably in the primate lineage that leads to humans. It continued to evolve even after the emergence of modern humans, with the latest ASPM 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%). Despite its apparent selective advantage, this variant does not seem to improve cognitive performance, at least not on standard IQ tests. At present, we can only say that it 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 alphabeticalwriting, 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.). This task was largely delegated to scribes of various sorts who enjoyed privileged status and probably superior reproductive success. Such individuals may have served as vectors for spreading the new ASPM variant.

1 comment:

MacGregor said...

15 lines down- "alphabeticalwriting" typo?

Scribes in premodern conditions faced higher demands on mental processing that could lead to privileged status and reproductive success whereas Talmudic scholarship, you might think, was/is conducted under relatively modern conditions.

The truth is, rather surprisingly, that Talmudic scholarship is conducted in a "complex argot" which it takes "many years" to master, let alone conduct scholarly disputes in. (A People that Shall Dwell Alone) The section about argot stuck in my mind and now I see its special relevance, the rigors of disputation may be secondary to the difficulty inherent in its special form of expression.

Another key to understanding from Evo and Proud.