WE HAVE less in common with our nearest animal relative than we thought, at least if our DNA is anything to go by. New comparisons of human and chimpanzee DNA show there are more differences than we realised.
For years, one very special number has helped shape both our sense of self and our sense of kinship with our closest relative, Pan troglodytes. We鈥檙e told that we share 98.5 per cent of our DNA with chimps, a figure touted so widely it has almost become a mantra. Now it seems that number is wrong. We actually share less than 95 per cent of our genetic material, so the difference is three times as great as was thought.
The new figure came to light when Roy Britten of the California Institute of Technology became suspicious about the reliability of the 98.5 per cent figure. It was originally derived from a technique that Britten himself developed decades ago at Caltech with his colleague Dave Kohne. By measuring the temperature at which corresponding stretches of DNA from two species comes apart, you can work out how different they are.
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But the technique only picks up one type of variation, called a single base substitution. This is when a single 鈥渓etter鈥� of the genetic code differs in corresponding strands of DNA from the two species.
But there are two other major types of variation which the previous analyses ignored. Insertions add a whole section of DNA to one species but not to the corresponding strand of the other. Likewise, deletions mean that a piece of DNA has been lost from one species but not from the other. Together, they鈥檙e termed 鈥渋ndels鈥�.
So Britten looked for the true variation between the two species by analysing five stretches of chimp DNA just published on the Internet by teams from the Baylor College of Medicine in Houston, Texas, and from the University of Oklahoma.
When he compared these to corresponding pieces of human DNA, he found that single base substitutions accounted for a difference of 1.4 per cent, very close to the expected figure. But he also found that the DNA of both species was littered with indels that add around 4 per cent to the genetic differences between us and chimps (Proceedings of the National Academy of Sciences, DOI/10.1073/pnas.172510699).
The result is only based on about 1 million DNA bases out of the 3 billion which make up the human and chimp genomes, says Britten. 鈥淚t鈥檚 just a glance.鈥�
But the differences appear to be evenly distributed across 鈥渏unk鈥� regions of DNA and regions that contain genes.