ANN ARBOR, Mich.Many creatures including
our fellow primates, the New World Monkeys, rely on highly specific
scent molecules called pheromones to find a suitable mate. Even
our humble mammal cousin, the mouse, was found to have 140 genes
just for pheromone receptors when its genome was completely sequenced
earlier this year.
But humans are clueless when it comes to pheromone
signals, according to University of Michigan evolutionary biologist
Jianzhi “George” Zhang. He believes color vision put
our pheromones out of business.
Our closest relatives on the primate family tree
rely on “sexual swelling” and gaudy, colorful patches
of skin to signal their reproductive fitness and fertility, Zhang
said. In fact, though humans and these apes still carry genes that
should create pheromone receptors in our noses, these genes have
mutated to the point that they are merely pseudogenesthey
don’t function any more.
Zhang has used the genes of people and primates
to get at the answer to this intriguing puzzle. Zhang (pronounced
Zong), is an assistant professor in Ecology and Evolutionary Biology
in the College of Literature, Science and Arts. Zhang’s paper
on the topic appears this week in the online Proceedings of the
National Academy of Sciences.
Zhang believes that a significant gene duplication made the difference
and that it happened sometime between 23 million years ago and the
split of the New World and Old World primates about 35 million years
An ancestor of the Old World primates (humans,
chimps, gorillas, orangutans, gibbons, baboons and guerezas) developed
a second copy of the red/green color-vision gene, which resides
on the X chromosome. Female New World monkeys have full color vision
because females have two X chromosomes that harbor both red and
green color vision genes. But males only have one X chromosome,
so New World males have only one copy of either the red or green
gene, and that leaves them color-blind. After the red/green gene
duplication in the Old World family however, even the males got
color vision too.
“Color vision made pheromones unnecessary,”
Zhang said. As a channel for sexual signaling, color vision works
better at a distance than pheromones, Zhang believes. A pheromone
attaches to a water molecule, drifts about in the air currents and
finally lands on the proper receptor in someone else’s nose.
The receiver can’t immediately be sure who sent it, where
it came from or when. But with sexual swelling, everyone in the
troop can see precisely when and where the signal is, even at a
Sexual swelling occurs in about 10 percent of
all primate species, but only in the Old World species of Africa
and Asia, which is where humans probably originated, as well.
To test their idea, Zhang’s team zeroed
in on a human gene called TRP2, which makes an ion channel that
is unique to the pheromone signaling pathway. They found that in
humans and Old World primates, this gene suffered a mutation just
over 23 million years ago that rendered it dysfunctional. But because
we could use color vision for mating, it didn’t hurt us. In
turn, the pheromone receptor genes that rely on this ion channel
fell into disuse, and in a random fashion, mutated to a dysfunctional
state because they haven’t experienced any pressure from natural
selection. Zhang calls this process “evolutionary deterioration.”