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DNA hints at roots of brain evolution

Scientists identify areas of human DNA that have undergone relatively rapid evolutionary change, hinting at their role in the development of bigger brains.
/ Source: Reuters

They could be key pieces in the puzzle of human genetic evolution — areas of human DNA that changed dramatically after the evolutionary division from chimpanzees, though they had remained almost unchanged for millennia before.

Scientists from the United States, Belgium and France identified 49 “human accelerated regions,” or HARs, showing a lot of genetic activity.

In the most active, identified as HAR1, they found 18 out of the 118 nucleotides had changed since evolutionary separation from chimps 6 million years ago, while only two had changed in the 310 million years separating the evolutionary lines of chimps and chickens.

“Right now we have very suggestive evidence that it might be involved at a critical step in brain development, but we still need to prove that it really makes a difference,” said team leader David Haussler from the Howard Hughes Medical Institute and the University of California at Santa Cruz.

Other members of the team came from the University of Brussels and Universite Claude Bernard in France.

“It is very exciting to use evolution to look at regions of our genome that haven’t been explored yet,” Haussler said.

“It is extremely unlikely that the evolution of just one region in the genome made the difference between our brains and the brains of non-human primates,” he said. “It is much more likely to be a series of many, many small changes, each very important, but none doing the entire job by itself.”

HAR1 is part of a novel RNA gene HAR1F that is produced during the key formative period for the human brain from seven to 19 weeks of gestation.

Not only that, but the RNA is produced by the Cajal-Retzius neuron, which plays a crucial role in the six layers of neurons in the human cortex.

“We still can’t say much about the function. But it’s a very exciting finding because it is expressed in cells that have a fundamental role in the design and development of the mammalian cortex,” Haussler said, noting the need to investigate the remaining 48 HARs.

The findings were published in Thursday's issue of the journal Nature. Chris Ponting of Oxford University wrote in the same issue hailing it as a possible major step forward.

“Previously, the hunt for changes in DNA that are causally linked to human-specific biology had concentrated on differences that would alter the amino-acid makeup of the encoded protein,” Ponting wrote. “Now it would seem that searches within the functional non-coding ’dark matter’ might be more enlightening.”