8 May 2007

Gene Mutation Responsible For Human Intelligence Tracked Down?

by Kate Melville

Human and chimpanzee genomes are 99 percent the same, but clearly, that 1 percent difference is hugely significant. Exactly why our mental and linguistic capabilities are so far ahead of our chimp cousins now looks closer to being explained, thanks to a new study in the journal Human Mutation. It shows that a certain form of neuropsin, a protein that plays a role in learning and memory, is expressed only in the central nervous systems of humans. Importantly, it seems that it originated less than 5 million years ago and scientists believe they now know the mechanism behind its production.

Carried out by researchers from the Chinese Academy of Sciences in Kunming, China, the study analyzed the DNA of humans and several species of apes and monkeys. Previous work had shown that type II neuropsin, a longer form of the protein, is not expressed in the prefrontal cortex of lesser apes and Old World monkeys. In the current study, they tested the expression of type II neuropsin in the prefrontal cortex of chimpanzees and orangutans, and found that it was not present. Since these two species diverged most recently from human ancestors, this finding demonstrates that type II is a human-specific form that originated relatively recently, less than 5 million years ago.

Subsequent gene sequencing revealed a mutation specific to humans that triggers a change in the splicing pattern of the neuropsin gene, creating a new splicing site and a longer protein. Introducing this mutation into chimpanzee DNA (a la Planet of the Apes) resulted in the creation of type II neuropsin. "Hence, the human-specific mutation is not only necessary but also sufficient in creating the novel splice form," said study leader Dr. Bing Su.

Interestingly, the results also showed a weakening effect of a different, type I-specific splicing site and a significant reduction in type I neuropsin expression in human and chimpanzee when compared with the rhesus macaque, an Old World monkey.

This leads the researchers to suggest that before the emergence of the type II splice form in humans, the weakening of the type I splicing site already existed in the common ancestor of humans and chimpanzees, implying a multi-step process that led to the dramatic change of splicing pattern in humans. They noted that the region of the chimp sequence that has a weakening effect on the splicing site is also likely to be found in humans. "It is likely that both the creation of novel splice form and the weakening of the constitutive splicing contribute to the splicing pattern changes during primate evolution, suggesting a multi-step process eventually leading to the origin of the type II form in humans," explained Dr. Su.

Future studies into the exact biological function of type II neuropsin could shed light into why it was positively selected for during human evolution. "The present results underscore the potential importance of the creation of novel splicing forms in the central nervous system in the emergence of human cognition," concluded Dr. Su.

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Source: Human Mutation