THE genetic blueprint of the chimpanzee, humanity’s closest cousin, has been mapped in its entirety by scientists, narrowing the search for the genes that make us human.
Comparison of the chimp genome with its human equivalent has already revealed small but striking differences that could explain distinctly human traits such as language, and further analysis promises critical insights into the health and evolution of our species.
The two most sophisticated great apes share almost 99 per cent of their functional genes, and even when differences in less significant DNA are taken into account they remain 96 per cent identical.
Of the three billion genetic letters or “base pairs” in which both genomes are written, just 40 million — fewer than 4 per cent — are not perfectly identical in humans and chimps.
Robert Waterston, of the University of Washington in Seattle, one of the leaders of the international research team, said: “Within those 40 million differences are clearly the genetic bases of what makes us human.
“As our closest living evolutionary relatives, chimpanzees are especially suited to teach us about ourselves. We still do not have in our hands the answer to a most fundamental question: what makes us human? “But this genomic comparison dramatically narrows the search for the key biological differences between the species.”
Tarjei Mikkelsen, of the Broad Institute in Boston, said: “We are all interested in the question of just what makes us human, and now in a sense we can answer that question.
“Any distinctly human trait caused by DNA is caused by one or more of these 40 million genetic changes.”
About 53 human genes that are completely or partially absent in chimpanzees have so far been identified, including several that seem to affect medical characteristics such as susceptibility to Alzheimer’s disease and diabetes.
There appear to be too few of these major genetic changes, however, to explain the differences between humans and chimps, such as brain size and sophistication, language and an upright, bipedal gait.
Early comparisons of the two genomes suggest that many distinctions emerge instead from subtle changes in the way that similar genes are switched on and off in each species.
The DNA used to map the chimpanzee genome, which is published today in the journal Nature, came from the blood of an adult male called Clint, who was kept in captivity at the Yerkes National Primate Research Centre in Atlanta, Georgia. Clint died last year of heart failure at the relatively young age for a chimp of 24, but two colonies of his cells have been preserved for future study.
A first draft of the genome was published in December 2003, and the finished version is now also complete.
The chimpanzee is only the fourth mammal to have its genome sequence completed, after humans, rats and mice, though a draft is available for the dog. Of these species, humans and chimps are by far the most similar. The differences between them are ten times fewer than those between mice and rats, and sixty times fewer than those between humans and mice.
Any two humans, however, are ten times more similar genetically than any person is to a chimpanzee. The degree of genetic proximity between people and chimps has led scientists to call for tougher worldwide restrictions on their use in research. Vivisection on any of the great apes is already banned in Britain.
One of the primary benefits of the completed chimp genome will be in medical research, where comparisons with the human version are already providing important clues and leads.
“Just as we use genetic differences between humans to probe the nature of disease, to figure out the processes and even to come up with new therapies, we can use the genetic differences between humans and chimpanzees to understand how differences arise and the biology behind this,” Professor Waterston said.
Three genes that are involved in inflammation in humans, for example, are missing in chimps, while humans lack a gene called caspase-12 that appears to protect chimps against Alzheimer’s disease. The FOXP2 gene, which is present in humans but not in chimpanzees, is implicated in the development of language.
LaDeana Hillier, of Washington University in St Louis, said: “This represents the tip of the iceberg when it comes to exploring the genomic roots of our biological differences.”
DECODING THE LIVING WORLD
- The genomes of more than 180 organisms have been sequenced since 1995
- The genomes of mice and rats are 88 per cent similar to that of humans and have versions of nearly every human gene, hence the animals’ extensive use in medical research
- The dog genome is also 75 per cent similar to that of humans. One of the dogs sequenced in the dog genome project was a poodle named Shadow. Shadow belongs to Craig Venter, whose own genome was sequenced during the human genome project
- The pufferfish genome was sequenced in 2004. It shares many genes with humans, but is eight times smaller. During the course of the pufferfish genome project 961 new human genes were discovered
- Other organisms whose genomes have been sequenced are the chicken, sequenced in 2004, with a 60 per cent similarity to that of humans, the fruit fly (50 per cent) and the daffodil (35 per cent)
