What Makes us Different?

Not very much, when you look at our DNA. But those few tiny changes made all the difference in the world

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What causes changes in both the dark matter and the genes themselves as one species evolves into another is random mutation, in which individual base pairs--the "letters" of the genetic alphabet--are flipped around like a typographical error. These changes stem from errors that occur during sexual reproduction, as DNA is copied and recombined. Sometimes long strings of letters are duplicated, creating multiple copies in the offspring. Sometimes they're deleted altogether or even picked up, turned around and reinserted backward. A group led by geneticist Stephen Scherer of the Hospital for Sick Children in Toronto has identified 1,576 apparent inversions between the chimp and human genomes; more than half occurred sometime during human evolution.

When an inversion, deletion or duplication occurs in an unused portion of the genome, nothing much changes--and indeed, the human, chimp and other genomes are full of such inert stretches of DNA. When it happens in a gene or in a functional noncoding stretch, by contrast, an inversion or a duplication is often harmful. But sometimes, purely by chance, the change gives the new organism some sort of advantage that enables it to produce more offspring, thus perpetuating the change in another generation.


A striking example of how gene duplication may have helped propel us away from our apelike origins appeared in Science last month. A research team led by James Sikela of the University of Colorado at Denver and Health Sciences Center, in Aurora, Colo., looked at a gene that is believed to code for a piece of protein, called DUF1220, found in areas of the brain associated with higher cognitive function. The gene comes in multiple copies in a wide range of primates--but, the scientists found, humans carry the most copies. African great apes have substantially fewer copies, and the number found in more distant kin--orangutans and Old World monkeys--drops off even more.

Another discovery, first published online by Nature two months ago, describes a gene that appears to play a role in human brain development. A team led by biostatistician Katherine Pollard, now at the University of California, Davis, and Sofie Salama, of U.C. Santa Cruz, used a sophisticated computer program to search the genomes of humans, chimps and other vertebrates for segments that have undergone changes at substantially accelerated rates. They eventually homed in on 49 discrete areas they dubbed human accelerated regions, or HARS.

The region that changed most dramatically from chimps to humans, known as HAR1, turns out to be part of a gene that is active in fetal brain tissue only between the seventh and 19th weeks of gestation. Although the gene's precise function is unknown, that happens to be the period when a protein called reelin helps the human cerebral cortex develop its characteristic six-layer structure. What makes the team's research especially intriguing is that all but two of the HARs lie in those enigmatic functional noncoding regions of the genome, supporting the idea that much of the difference between species happens there.


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