Friday, February 24, 2006

More on continuous brain growth in humans

While we are thinking about brain evolution, consider this study by Evans et al. in Science. They studied the gene Microencephalin (MCPH), which is known for its severe reduction in brain size coupled with mental retardation. Remarkably, despite this abnormality, there is an overall retention of normal brain structure and a lack of overt abnormalities outside of the nervous system. The MCPH function in healthy humans is less well known, and one can speculate whether it has specific brainy advantages to its carrier. As Evans et al conclude in their article:

“[There] could be several possibilities, including brain size, cognition, personality, motor control, or susceptibility to neurological and/or psychiatric diseases.”

What makes this study interesting is the finding that the MCPH has changed during the past ~37.000 years, and that the spread has been fast. In other words there has been a strong positive selection for this gene, indicating that the brain has continued to evolve even in more recent times. The MCPH is also known to be involved in the evolution of hominids, eventually leading to Homo sapiens. The new finding by Evans et al. demonstrates that this trend has been continuing until more recent times, and is there really any reason to think that the same evolutionary trend has stopped?

So evolutionary psychologists be aware -- don’t ever say that today’s humans minds are the same as that of the stone-age man… Of well, to a large extent, it probably is, but this and other similar reports forcefully tells us that we need to unravel the relative contribution of recent evolutionary trajectories in man. It is also necessary to speculate and study the touchy subject on whether there have been local variations in brain size and function according to the recent brain evolutions that have occurred. After all, evolutionary developments about 40.000 years old indicate that there could be geographical variations in the prevalence of this mutation. I don’t think we’ll see yet another claim of the out-of-Africa pertaining to the past 40.000 years or so.

See also comments by John Hawks and Bob Scheid

Here is the abstract from that article:

Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans

Patrick D. Evans in Science, vol. 309, September 2005

The gene Microcephalin (MCPH1) regulates brain size and has evolved under strong positive selection in the human evolutionary lineage. We show that one genetic variant of Microcephalin in modern humans, which arose È37,000 years ago, increased in frequency too rapidly to be compatible with neutral drift. This indicates that it has spread under strong positive selection, although the exact nature of the selection is unknown. The finding that an important brain gene has continued to evolve adaptively in anatomically modern humans suggests the ongoing evolutionary plasticity of the human brain. It also makes Microcephalin an attractive candidate locus for studying the genetics of human variation in brain-related phenotypes.


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