Science&Health: Study looks at human evolution

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.