more brain / gene stuff

[Colin McHenry <cmchenry@westserv.net.au>]

Some more reports on this issue. I know it's an old thread.. but it 
continues to have palaeontological application.  Just last night, on 
the  ABC special on Thylacoleo, someone was trying to use the degree of 
folding of the cortex to extrapolate behavioural capabilities (with no 
mention of body size).  Anyway:

http://www.guardian.co.uk/science/story/0,,1851900,00.html

Article is below.

Colin

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The brain genes that gave man a head start on chimpanzees

James Randerson, science correspondent
Thursday August 17, 2006
The Guardian

Scientists have identified perhaps the most crucial genetic region that 
makes us human. By comparing human DNA with that of chimpanzees and 
other animals they have found the region of the genome subjected to the 
strongest natural selection since we shared a common ancestor with chimps.

The 108-letter stretch of DNA contains two genes that appear to control 
brain development. The researchers speculate that the blistering pace of 
evolution indicates that they may have been crucial in the rapid 
increase in brain size and complexity that occurred in the human 
lineage. Our brains are three times larger than our closest relatives, 
the chimps.

"It's evolving incredibly rapidly," said Katherine Pollard at the 
University of California in Davis. "It's really an extreme case." Most 
of the 15m or so differences between the chimp genome and our own are 
random, inconsequential changes that make no difference to our 
appearance or abilities. To sort the interesting changes from the less 
influential ones, Professor Pollard looked further down the evolutionary 
tree to find regions of DNA that really are useful. She and her 
colleagues first looked for bits of the genome that are nearly identical 
in the mouse, rat and chimpanzee. These shared a common ancestor around 
80m years ago, so the scientists reasoned that any DNA region that had 
not changed much in this time must be crucial for survival and that 
changes in its sequence would lead to problems rapidly weeded out by 
natural selection.

They then trawled these conserved regions for instances where the human 
equivalent had changed a lot. The beauty of the technique, according to 
experts, is that they did not have to know what the DNA was actually 
doing. Top of the list is a 108-letter sequence called HAR1 (Human 
Accelerated Region 1) which contains two genes. This region differs by 
just two changes between chimps and chickens, which shared a common 
ancestor around 310m years ago. But since humans and chimps split 5m 
years ago there have been 18 changes.

"There has been tremendous pressure for millions of years to keep the 
sequence as it was. Then something happened in our lineage," said Pierre 
Vanderhaueghen at the Free University of Brussels in Belgium.

He was able to get clues by adding colour labelled chemicals that would 
stick to the RNA product produced by the genes to slices of brain tissue 
from human foetuses. The brains were obtained from aborted or miscarried 
foetuses and were used with the consent of the mothers.

His results showed that one of the genes is expressed strongly in the 
developing neocortex during weeks seven to nine of pregnancy. "It's a 
very exciting finding because it is expressed in cells that have a 
fundamental role in the design and development of the mammalian cortex," 
said co-author David Haussler, also at the University of California.



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Colin McHenry
School of Environmental and Life Sciences (Geology)
University of Newcastle
Callaghan NSW 2308
Australia
Tel: +61 2 4921 5404
Fax: + 61 2 4921 6925

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