Did humans lose a sixth sense?
Sensory nerve cells from the vomeronasal organ
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.
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.
Professor Dulac: Pheromones may play an as yet unappreciated role in human
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.
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.
Frogs are stimulated by pheromones
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.