Wednesday, 27 September, 2000, 04:32 GMT 05:32 UK The smell of success Humans have genes for hundreds of olfactory receptors Scientists believe our sense of smell played a crucial role in evolution, helping our Stone Age ancestors to hunt, avoid poisonous food and even select a mate. By comparing tiny variations in the DNA sequences of chimps and humans, researchers in Israeli have concluded that changes in about 1,000 smell receptor genes contributed to the rise of the human race. The new study suggests that although we no longer need our sophisticated sense of smell to survive in the modern world, the sense still plays an important role, perhaps in sexual attraction. The authors believe smell receptors are one of the few examples of adaptive molecular evolution in humans - mutations in DNA that gave our ancestors an advantage over other early humans. The team, based at the Weizmann Institute of Science, Rehovot, Israel, compared segments of human and chimp DNA. They focused on a region, located on human chromosome 17, which contains about 1,000 smell receptor genes used to detect different odours. In humans, about half of these are functional genes. The others, known as pseudogenes, are silent, and are no longer used. Chimp cousins A comparison of the DNA sequences of humans with those of chimps gives clues to how and why our keen sense of smell evolved. The researchers found that the olfactory genes we use today had evolved through the mechanism of positive or advantageous selection. "Imagine everyone has the same gene and thus the same ability and then one human is born with a mutation in that gene and it changes its ability for the better," said Yoav Gilad, who carried out much of the research. "It's got a better sense of smell now. It can smell something that only he can smell, others cannot. "Let's say this smell is a smell of a poisonous plant. He knows by a smell that it is bad for him, while others might try it. It gives him an advantage. We call that a greater fitness. "Sooner than later, everybody will have this mutation, this new variant of gene, because this is an advantageous mutation." Chemical attraction The work raises an intriguing question: why maintain a sharp sense of smell when it is no longer needed? One theory is that smell, even today, plays a role in sexual attraction. Commenting on the study, Dr Mark Seielstad, of the Harvard School of Public Health, Boston, Massachusetts, US, said there were data to suggest that our choice of partner was influenced by body chemicals. "These days it is difficult to imagine a circumstance where our own survival has hinged on the ability to sense a particular odourant," said Dr Seielstad, "But evidence indicating that our choice of mates may be influenced by the sense of smell continues, surprisingly, to accumulate." The research is published in the journal Nature Genetics.