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A gene that makes us human?

Aug. 16, 2006
Special to World Science

Scientists say they’ve found a gene that evolved extremely quickly in human ancestors, and that may partly explain why our brains are dramatically bigger than those of other animals.

The gene may have “caused our brains to evolve to be much larger and have more functions than the brains of other mammals,” said David Haussler of the University of California, Santa Cruz. Its precise function remains unknown, he added, but it seems to be active in brain development.

The study, published in the Aug. 16 advance online edition of the research journal Nature, was conducted by an international team of neuroscientists led by researchers at the university.

In a computer-aided analysis, they compared the genomes of humans, chimpanzees, and other animals to identify parts of the human genome that have evolved unusually quickly. 

Evolution is a continual change in a species that occurs because individuals who are best at surviving and reproducing spread their genes more effectively than the others, who tend to die off. The repeated spread through the population of these genes, over time, leads to changes in the whole species.

Thus, scientists assume that if a particular part of the genome is changing relatively quickly, it is evolving rapidly. 

The researchers in the new study identified the most drastically altered human DNA region—thus the one that has changed the most rapidly—as an area called HAR1.

They found that this region is part of two overlapping genes. One of these, called HAR1F, is active in special cells that appear early in embryonic development and help form the human cerebral cortex, the brain’s outer region. This is the most dramatically expanded area of the brain in humans as compared to other animals.

The cells also produce a molecule called reelin that guides the growth of and connections among brain cells, the scientists said. HAR1F is active at the same time as the gene that makes reelin, so it might somehow interact with reelin, the researchers added.

“The evidence is very suggestive that this gene 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,” Haussler said. 

The analysis found HAR1 is similar in all mammals except humans, the researchers said. There were only two differences in “letters” of the gene between the chicken and chimp genomes, but 18 differences between chimps and humans.

That’s “an incredible amount of change to have happened in a few million years,” said the university’s Katherine Pollard.

HAR1F is unusual in another way, the researchers said. Most genes produce a type of molecule called proteins, which carry out the bulk of the body’s day-to-day activities. But genes don’t produce proteins directly. They typically first produce a molecule called RNA, which carries a version of the genetic code. This, in turn, is used to produce the proteins.

But researchers are increasingly learning that some important genes produce only the RNA, which seems to have useful functions on its own. HAR1F appears to be one of these, the scientists in the study said. Interestingly, Pollard added, the human and chimp versions of this gene seem to produce RNA molecules with quite different shapes.

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