The fastest evolving gene in the human genome is one linked to brain development, researchers say.

A study of differences between the human and chimp genomes has identified a gene associated with neural growth in the cerebral cortex – the part of the brain involved in processing thoughts and learning – as having undergone “accelerated evolutionary change”.

Katherine Pollard and colleagues at the University of California Santa Cruz, US, suggest that the fast-changing gene may help explain the dramatic expansion of this part of the brain during the evolution of humans.

They identified the rapidly evolving region of DNA – called human accelerated region 1 (HAR1) – after carrying out an extensive computational comparison between the genomes of humans, chimpanzees and other vertebrates.


Critical role

There are only two changes in the 118 letters of DNA code that make up HAR1 between the genomes of chimps and chickens. But chimps and humans are 18 letter-changes apart. And those mutations occurred in just five million years, as we evolved from our shared ancestor.

“That is an incredible amount of change to have happened in a few million years,” Pollard notes.

Subsequent experiments looking at the brains of human and primate embryos revealed that HAR1 is part of two overlapping genes. One of these genes, called HAR1F is active in nerve cells that appear early in embryonic development and play a critical role in the formation of the layered structure of the human cerebral cortex.

The role of the other gene, called HAR1R, is less clear, but it also appears also to be involved in cortex development.

“Very suggestive”

The researchers point out that these genes do not appear to code for any proteins, but are what is known as an RNA genes.

“We don’t know exactly what it does, but the evidence is very suggestive that HAR1F is important in the development of the cerebral cortex, and that’s exciting because the human cortex is three times as large as it was in our predecessors,” says David Haussler, director of the Center for Biomolecular Science and Engineering at the University of California Santa Cruz, who assisted with the study.

“Something caused our brains to evolve to be much larger and have more functions than the brains of other mammals,” he points out.

Journal reference: Nature (DOI: 10.1038/nature05113)

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