Human brain probably is the most exquisite structure in the world. During evolution, its size increasing has experienced several hops. About 2.5 million years ago, hominids started out with brains weighing approximately 400-450 grams, while, around 200,000 to 400,000 years ago, its size already had beat all the other primates. Now, human walk around with brain so big (1350 to 1450 grams) and so sophisticated that some scientists are certain that human have reached an evolutional apex and had done with evolving. But as the size of the brain may correlate with intelligence, the debating of if our brains could get bigger or smarter had never been deceased.
However, to address the underlying mechanisms of brain enlargement with genetic tools has been difficult, because as a hallmark of human evolution, this dramatic change is human-specific. As a result, one and probably the only option left to scientists is to figure out when and why human brain stopped growing to the expected size, for example, in the case of human autosomal primary microcephaly syndrome. In this rare brain developmental disorder, the size of the brain is roughly similar to that of an early hominid, and through genetic screening, at least two genes, ASPM and microcephalin (MCPH1), have been uncovered. If mutations occur, brain size will be affected. Mice carrying a truncated ASPM protein were shown to have reductions of both brain and testis size, while the transgenic mouse carrying human ASPM could rescue this phenotype, but did not cause any additional enlargement of the brain.
Meanwhile, ASPM and MCPH1 both are evolved rapidly under Darwinian positive selection during evolution of human and non-human primates. According to previous reports, a new allele of ASPM emerged 5,800 years ago, roughly correlates with the development of written language, spread of agriculture and development of cities.By exploring the allele distribution among various human populations, scientists claims that it is now present in 50 percent of the populations of the Middle East and Europe, but to a much less extent in the populations of East Asia and Africa. Whereas, the allele associate with MCPH1 is believed to have developed about 37,000 years ago and has spread to become the most common form throughout the world except Sub-Sahara Africa. Based on the allele emerging time point, MCPH1 has been considered to correlate with tonal language.
Although some scientists grant that human brain may still be evolving as the alleles’ presence variations could be taken as the evidence to support natural selection, the evolutionary pressures that may have caused the spread of these mutations haven’t been identified and the nature of this relationship is far from clear. Like African population, who seems do not carry either gene in great frequency, may have other genes on duty, or ASPM or MCPH1 may have nothing to do with brain size in other populations.
Meanwhile, some evolutionary studies of MCPH1 have demonstrated a rapid change in protein sequence associated with the brain enlargement during the two key taxonomic transitions in primates, i.e. between lesser apes and great apes, and between great apes and humans. So, it is still possible that the divergence of MCPH1 protein sequences among primates may have caused functional changes that contribute to brain enlargement.
Most recently, Dr. SU Bing’s team of Kunming Institute of Zoology, Chinese Academy of Sciences has examined the activating and repressing effects of MCPH1 on a set of its down-stream genes and then compared the functional outcomes of a series of mutant MCPH1 proteins that carry mutations at the human- and great-ape-specific sites. They proposed that during primate evolution in general and human evolution in particular, the divergence of MCPH1 protein sequences under Darwinian positive selection led to functional modifications, and the functional changes of MCPH1 are likely executed by regulating several key down-stream genes.
Dr. SU’s study support that MCPH1 plays a role in brain growth, but what are the interactions of it with other possible genes. More importantly, are those genes portending smarter brain and bigger IQs? Obviously, we don’t know yet.
Get more details of Dr. Su’s study by the following link: http://www.biomedcentral.com/1741-7007/11/62/abstract
(By Su-Qing Liu)
