Research by an international team, examined the sequence of one of 25 genes that encode bitter taste receptors in the tongue, and reviewed the occurrence of the gene in 60 human populations across the world.

Apparently by reconstructing the history of the gene, they found evidence to suggest that humans' ability to taste bitterness developed through evolutionary selection.

Lead author Nicole Soranzo, of the University College London, says that being able to detect signatures of selection for a bitter taste receptor, suggests that sensory detection of dangerous foods played an important role at certain times during the course of our evolution.

The research is published in the July 26 issue of Current Biology.

The researchers say they doubt the stress gene itself decides how long an animal lives, but instead reflects some as-yet unknown trait.

According to most scientists, the life span of living creatures is affected by a combination of genetic, environmental and chance factors, and studies done in twins suggests that genes are only about 15 percent to 30 percent responsible for how long an otherwise healthy person will live.

Rea says this study begins to address the question of why genetically identical organisms raised in identical environments still age at different rates.

In future, he says, it might be possible to test people for stress compounds such as hsp-16.2 and predict how long they will live, and even be able to adjust each stress-response system and set them for maximum longevity, which is believed to be about 120 years.

The report is published in this week's issue of the journal Nature Genetics.