• printer friendly
• mail this article
• iPod format
• digg

A breakthrough study conducted by UCLA scientists was published last week, leading to further understanding of evolution by comparing the brains of humans and chimpanzees.

The study found that evolution occurs not only on the level of individual genes, but also in the interaction between genes, called a network.

Scientists say 6 million years ago chimpanzees and humans diverged from a common ancestor and evolved into different species, sharing around 95 percent of their genes.

“One of the issues is that the genomes are extremely similar, but how can (the species) be so different? It must have to do with the expression patterns, when and where the genes are turned off and on,” Dr. Daniel Geschwind, lead researcher and professor of human genetics at the David Geffen School of Medicine, said.

This study used a new approach to understand genetic differences, analyzing genes that work together to express traits, rather than looking at individual genes that are turned on or off.

Using the new analytical approach developed by UCLA associate professor of biostatistics and human genetics Steve Horvath, the scientists identified networks of genes that correspond to specific brain regions, and compared them to those of a chimp.

Individual genes function within systems of related genes, not in isolation, said the publication’s first author Michael Oldham, a UCLA genetics researcher.

Comparing the gene networks of humans to those of chimpanzees showed that the same genes had different locations in the two species’ networks. A gene’s position in a network is important from a functional standpoint.

“If a gene is central in a network, it is more important than those that are peripheral,” said Geschwind.

Genes that are central in a network are more likely to undergo higher pressure for positive natural selection.

“Some of the things that are different between humans and chimpanzees are likely to be under selection, indicated by the position of the gene in the network ... This is a unified way of thinking about evolution,” Geschwind said.

Often researchers found genes that were peripheral in chimpanzee networks were central in human networks, he added.

The scientists found that there was a great difference in the networks for energy metabolism in the brains of humans and chimpanzees.

“Many energy genes in the human cortex had more connections than those in chimpanzees,” which supports the idea that human brains require more energy than chimpanzee brains, Oldham said.

“There is converging evidence that something important happened to energy metabolism (during evolution). It would make sense that along with a growth of size and more sophisticated tasks, (the brain also developed) a more sophisticated use of energy,” he said.

Horvath added the brains of the species can be seen as two engines. A 12-cylinder car engine would use more energy than a 6-cylinder engine because it has better performance and is more powerful.

The differences arose as humans and chimpanzees evolved from their common ancestor, when certain genes became central in networks and others became peripheral.

The scientists observed that the level of rearrangement of genes varied based on the particular part of the brain. The networks that were most dissimilar were found in the more highly evolved parts of the brain, reflecting evolutionary lineage, Horvath said. The cerebral cortex, where most variations in networks between the two species were found, is the brain’s most highly evolved region and is three times larger in humans than in chimps.