[monkeywire] Unraveling where chimp and human brains diverge

Josh Greenman josh.greenman at gmail.com
Mon Nov 13 18:49:21 EST 2006


Unraveling where chimp and human brains diverge

Six million years ago, chimpanzees and humans diverged from a common
ancestor and evolved into unique species. Now UCLA scientists have
identified a new way to pinpoint the genes that separate us from our
closest living relative – and make us uniquely human. The Proceedings
of the National Academy of Sciences reports the study in its Nov. 13
online edition.

"We share more than 95 percent of our genetic blueprint with chimps,"
explained Dr. Daniel Geschwind, principal investigator and Gordon and
Virginia MacDonald Distinguished Professor of Human Genetics at the
David Geffen School of Medicine. "What sets us apart from chimps are
our brains: homo sapiens means 'the knowing man.'

"During evolution, changes in some genes altered how the human brain
functions," he added. "Our research has identified an entirely new way
to identify those genes in the small portion of our DNA that differs
from the chimpanzee's."

By evaluating the correlated activity of thousands of genes, the UCLA
team identified not just individual genes, but entire networks of
interconnected genes whose expression patterns within the brains of
humans varied from those in the chimpanzee.

"Genes don't operate in isolation – each functions within a system of
related genes," said first author Michael Oldham, UCLA genetics
researcher. "If we examined each gene individually, it would be
similar to reading every fifth word in a paragraph – you don't get to
see how each word relates to the other. So instead we used a systems
biology approach to study each gene within its context."

The scientists identified networks of genes that correspond to
specific brain regions. When they compared these networks between
humans and chimps, they found that the gene networks differed the most
widely in the cerebral cortex -- the brain's most highly evolved
region, which is three times larger in humans than chimps.

Secondly, the researchers discovered that many of the genes that play
a central role in cerebral cortex networks in humans, but not in the
chimpanzee, also show significant changes at the DNA level.

"When we see alterations in a gene network that correspond to
functional changes in the genome, it implies that these differences
are very meaningful," said Oldham. "This finding supports the theory
that variations in the DNA sequence contributed to human evolution."

Relying on a new analytical approach developed by corresponding author
Steve Horvath, UCLA associate professor of human genetics and
biostatistics, the UCLA team used data from DNA microarrays – vast
collections of tiny DNA spots -- to map the activity of virtually
every gene in the genome simultaneously. By comparing gene activity in
different areas of the brain, the team identified gene networks that
correlated to specific brain regions. Then they compared the strength
of these correlations between humans and chimps.

Many of the human-specific gene networks identified by the scientists
related to learning, brain cell activity and energy metabolism.

"If you view the brain as the body's engine, our findings suggest that
the human brain fires like a 12-cylinder engine, while the chimp brain
works more like a 6-cylinder engine," explained Geschwind. "It's
possible that our genes adapted to allow our brains to increase in
size, operate at different speeds, metabolize energy faster and
enhance connections between brain cells across different brain

Future UCLA studies will focus on linking the expression of
evolutionary genes to specific regions of the brain, such as those
that regulate language, speech and other uniquely human abilities.

>From UCLA

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