HUMANS SHARE more than 95 per cent of their genetic blueprint with that of chimpanzees. What sets us apart from chimps are our brains: homo sapiens means `the knowing man.'

Six million years ago, chimpanzees and humans diverged from a common ancestor and evolved into unique species.

Now University of California, Los Angeles (UCLA) scientists have identified a new way to pinpoint the genes that separate us from our closest living relative — and make us uniquely human.

Evolutionary changes

Dr. Daniel Geschwind, principal investigator, said, "During evolution, changes in some genes altered how the human brain functions," he added.

"Our research has identified an entirely new way to identify those genes in the small portion of our DNA that differs from the chimpanzee's."

The scientists identified networks of genes that correspond to specific brain regions.

When they compared these networks between humans and chimps, they found that the gene networks differed most widely in the cerebral cortex — the brain's most highly evolved region, which is three times larger in humans than chimps, according to a UCLA, Health Sciences press release.

By evaluating the correlated activity of thousands of genes, the UCLA team identified not just individual genes, but entire networks of interconnected genes whose expression patterns within the brains of humans varied from those in the chimpanzee.

"Genes don't operate in isolation — each functions within a system of related genes," said first author Michael Oldham, UCLA genetics researcher. "If we examined each gene individually, it would be similar to reading every fifth word in a paragraph — you don't get to see how each word relates to the other. So instead we used a systems biology approach to study each gene within its context."

Changes at DNA level

Also, the researchers discovered that many of the genes that play a central role in cerebral cortex networks in humans, but not in the chimpanzee, also show significant changes at the DNA level.

Relying on a new analytical approach developed by corresponding author Steve Horvath, the UCLA team used data from DNA microarrays — vast collections of tiny DNA spots — to map the activity of virtually every gene in the genome simultaneously. By comparing gene activity in different areas of the brain, the team identified gene networks that correlated to specific brain regions. Then they compared the strength of these correlations between humans and chimps.

"When we see alterations in a gene network that correspond to functional changes in the genome, it implies that these differences are very meaningful," said Oldham. "This finding supports the theory that variations in the DNA sequence contributed to human evolution."

Brain as body's engine

Many of the human-specific gene networks identified by the scientists related to learning, brain cell activity and energy metabolism.

"If you view the brain as the body's engine, our findings suggest that the human brain fires like a 12-cylinder engine, while the chimp brain operates more like a 6-cylinder," explained Geschwind.

"It's possible that our genes adapted to allow our brains to increase in size, operate at different speeds, metabolise energy faster and enhance connections between brain cells across different brain regions." — Our Bureau

Printer friendly page  
Send this article to Friends by E-Mail