Study finds differences in human, chimp brain

Despite similarities, gene activity in people faster


NEW YORK - Philosophers and theologians may speculate about the essence of human nature, but biologists have a test that should, in principle, deliver an exact definition: compare humans and their close cousin, the chimpanzee, at the finest level possible, and the differences will identify the special ingredient that must be mixed into animal clay to make it human.

Researchers have now compared the genetic activity of the chimp and human brain, the organ that presumably holds the vital difference.

Despite reports from anatomists that the two species' brains seemed to differ only in size - the human brain has more than three times the volume - the gene chip has brought to light numerous differences in how the brain cells of the two species operate at the genetic level.

The finding is reported in the journal Science by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues in Germany, the Netherlands and the University of California at San Diego.

On the genetic level, although the chimpanzee genome has not yet been decoded, scientists have long known from sampling bits of it that chimp DNA is 98.7 percent identical with human DNA.

Paabo's result seems to offer a rescue from this predicament. Even though human and chimp genes are very similar, those used to operate brain cells show very different patterns of activity, the study found.

With another kind of test, they compared gene activity levels in the blood, liver and brain and found that in the first two tissues humans and chimps were quite similar.

But the gene activity levels in the human brain were strikingly different from those in the chimp, suggesting that the rate of evolution of human brain genes has been five times faster than that of the chimp's brain genes.

The activity of genes can be measured by gene chips, which contain versions of up to 18,000 human genes anchored on a glass slide to different sites in a grid-shaped array.

Fluid extracted from living cells is washed over the array, and the square containing each gene lights up if a copy of that gene is present in the fluid. The more copies, the more intense the signal.

Because human and chimp genes are so similar, a gene chip studded with human genes can detect the activity of genes in a chimpanzee neuron reasonably well. Chimp gene chips are not yet available.

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