Gene controlling brain-size linked to human evolution:Study

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Washington, Dec 26 (UNI) Scientists have identified a gene that controls the size of the brain, a finding that will give an insight into what it means to be human.

Humans have extraordinarily large and complex brains, even when compared with our close relatives, the research said. The team in Edinburgh found that when the newly identified gene was faulty, the brain and body shrunk as a result.

Andrew Jackson of the Medical Research Counci, MRC, Human Genetics Unit, Edinburgh and colleagues at the MRC Genome Damage And Stablity Centre studied families who have members with Seckel syndrome, which retards growth in the womb, leading to short stature and a markedly reduced brain size (microcephaly).

They reported in the journal Nature Genetics that small brain size is linked to faults in a gene called PCNT and found that this gene works with another gene linked to the condition, called ATR, which is involved in DNA repair.

The PCNT gene is responsible for the manufacture in the body of a protein called pericentrin, a component of the centrosome, an organ in cells (organelle) that is essential for the process of cell division that underpins growth, Dr Jackson said.

''These results helps us understand how these microcephaly genes work, bringing together centrosomal proteins and DNA damage signalling'' Telegraph quoted Dr Jackson as saying.

Other genes involved in the working of the centrosome have previously been found to be mutated in different forms of microcephaly, suggesting that it is essential for determining brain size, he added.

When comparing different species, brain size does present a correlation with intelligence. For example the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50.

Among people, modern studies using brain scans have shown that brain size shows a rough correlation with IQ among adults of the same sex. The brain is a metabolically expensive organ, and consumes about 25 per cent of the body's energy.

Another evolutionary constraint is fitting the head through the birth canal. Thus, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains.

But in the ancestors of humans, having bigger and more complex brains appeared to have carried a particularly large advantage, much more so than for other mammals.

These traits allowed individuals with ''better brains'' to leave behind more descendants. As a result, genetic changes that produced bigger and more complex brains spread in the population very quickly.

This led ultimately to a dramatic ''speeding up'' of evolution in genes controlling brain size and complexity.


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