By BBC News Online Science Editor Dr David Whitehouse

Our distant ancestors may have had a sixth sense that modern humans have lost because of a genetic mutation.

Some researchers believe that the vestige of an organ that we all have in our noses was once responsible for detecting chemical signals given off by other humans. Some even think that it still influences our behaviour.

Located just behind our nostrils are two tiny pits called the vomeronasal organ (VNO). The organ contains nerves that respond to chemicals called pheromones that are secreted by many animals. Whether humans do so as well is a matter of conjecture.

In many creatures, pheromones trigger a variety of instinctive behaviours such as aggression and mating.

Mouse gene

Professor Catherine Dulac of the Harvard Medical School and researchers have isolated in mice a gene that she believes plays a major role in the detection of pheromones.

Humans have the gene as well but in a mutated form that may make it useless for detecting pheromones. This suggests we may once have had the ability to pick up the delicate chemical language of pheromones but have now lost it because the VNO cannot develop and function properly.

The researchers are currently making a careful search for other human genes that we may use to detect pheromones other than the one we share with mice. Rats and mice have well-developed VNO's containing millions of nerve cells. The human VNO is different - it may work in the same way or it may not.

Pheromones from insects and rodents are known but so far nobody has been able to find one from humans, despite the scent products that can be bought with names like Desire.

Menstrual cycles

There is some evidence that pheromones are at work in humans. Some research suggests that the female menstrual cycle can be advanced or retarded by sniffing the scent from other females captured via underarm pads. However, some scientists believe that the VNO is not responsible for detecting these scents. Instead, they are picked up by the so-called main olfactory system which runs our general sense of smell.

Some scientists have speculated that the signals from the main olfactory system, situated further up the nose than the VNO, go to higher regions of the brain where scents can be associated with memories. Signals from the VNO, however, if it works, may be routed directly into brain regions responsible for more unconscious and instinctive behaviour.

The team at the Harvard Medical School and the Massachusetts General Hospital have isolated a molecule, called TRP2, that acts as a trigger for pheromone reception on the VNO. It works in mice and rats but apparently humans do not have either the same molecule or the nerve connections found in rodent that are sensitive to pheromones.

Professor Dulac believes that talk of a sixth sense is nonsense but that pheromones may play an as yet unappreciated role in human behaviour.