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Monkey genome springs surprise for human origins
21 April 2007
From New Scientist Print Edition.
Bob Holmes

What makes us human? We are closer to an answer - at least as far as our genes are concerned - with the sequencing of the genome of the rhesus macaque. Joining humans and chimpanzees, this monkey is the third primate to have its genome fully sequenced.

Of the macaque's nearly 3 billion DNA base pairs, 93.5 per cent are identical to those in the human genome. This is not unexpected for a species whose lineage diverged from our own about 25 million years ago. The human and chimp genomes, which diverged just 6 million years ago, are about 98 per cent identical.

The newly sequenced genome holds invaluable information for teasing out what mutations have made us the way we are. When biologists found differences between the human and chimp versions of a gene, they had no way of knowing which had come first. Now they can look to the macaque genome to identify the ancestral version. "We can take an unbiased look at what the genome is telling us needed to change," says David Haussler of the University of California, Santa Cruz.

A female macaque living at a research centre in San Antonio, Texas, provided the genome that was sequenced by an international consortium of more than 170 scientists at 35 institutions. The feat has already led to the discovery of some 200 genes that appear to have evolved in either the macaque or human/chimp lineages since the two diverged. Since these genes represent the levers through which evolution has caused humans to differ from macaques, researchers will be looking at them closely to help understand our more recent evolution. Those identified so far include genes for hair structure, immune response, intercellular communication and sperm-egg fusion - but not, apparently, for brain size (Science, vol 316, p 222).

One puzzling discovery is that several mutations that cause genetic diseases in humans - such as phenylketonuria and Sanfilippo syndrome, which lead to mental retardation - are the normal form in macaques and, presumably, our own ancestors. "How can genes that seem to be fine in one species give disease in another closely related one?" asks Richard Gibbs, a geneticist at Baylor College of Medicine in Houston, Texas, who led the consortium.

It is in delicately balanced genes like these that we may find some of the key steps in the evolution of modern humans. Some cases of mental retardation, Gibbs speculates, may even result from mutations that restore a key gene to its ancestral condition.

From issue 2600 of New Scientist magazine, 21 April 2007, page 15

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