Are senses a zero-sum game? The Economist implies it in this article that indicates that the focus on vision diminished the primate, and especially human, sense of smell. I’ve copied the full article below….
Note: Read The Emperor of Scent for an alternative account of the physiology of smell….
MORE COLOUR, LESS ODOUR
JULY 24TH 2003
Gaining colour vision, it seems, cost people much of their sense of
THERE is a theory that the human sense of smell began to atrophy when
people learned to cook. Since cooking neutralised the worst toxins in
food, it became less important to be able to sniff out evil-smelling
ingredients. But at the International Congress of Genetics, held
earlier this month in Melbourne, Australia, a group of researchers
presented evidence that it was actually the evolution of colour vision
that caused creeping desensitisation to odours.
People detect smells when particular molecules lock on to receptor
proteins embedded in the lining of the nose. The interaction between a
molecule and a receptor triggers a pulse of electrical activity that is
transmitted to the brain.
Most odoriferous molecules activate more than one type of receptor. The
brain recognises an odour by the pattern of receptors activated. Humans
have about 1,000 different sorts of odour receptor (OR), so the number
of patterns that can be generated and recognised is impressive. Even
so, the range and subtlety of the human sense of smell is poor compared
with that of other mammals, and it has been found over the past few
years that this poverty is a reflection of genetics. The genes that
encode ORs form the largest of the mammalian gene families. Yet in
humans 60% of them are actually so-called pseudo-genes. In other words
they have been rendered inactive by mutations.
To find out if humans are unusual among primates in having lost such a
high proportion of their ORs, a team of researchers led by Yoav Gilad
of the Weizmann Institute, in Israel, picked 50 human OR genes at
random. The team then found their counterparts in several species of
primate, and also in the mouse, and compared the ratio of pseudogenes
with intact, functional genes across the species.
In the mouse, around 20% turned out to be pseudogenes, whereas in
chimpanzees, gorillas and orang-utans the level was closer to 30%.
Old-world monkeys had lost more genes than new-world monkeys, which in
turn had lost substantially more than the mouse. And humans were way
out in front with a massive 60% erosion of ORs. According to Dr Gilad,
humans have accumulated disruptive mutations in OR genes four times
faster than any of the other species tested.
Moreover, the distinction between new world and old world was so clear
that, as Dr Gilad says, “It’s almost as if we can map the beginning of
the accelerated rate of accumulating OR pseudogenes from their
divergence.” But there was an anomaly. When the researchers plotted
their findings on a graph, they found that the howler monkey, a
new-world species, fell in with its old-world cousins.
Why would this sudden increase in OR loss have occurred both in the
old-world and in one lineage of new-world primates? The researchers
were struck by the fact that howler monkeys, alone among new-world
species, share with old-world primates the capacity for full colour, or
Trichromatic vision involves three pigments, called opsins, that are
sensitive to different wavelengths of light. In humans and their
old-world relatives the medium- and long-wave opsins are controlled by
separate genes on the X chromosome. But in most new-world monkeys there
is only one opsin gene on the X chromosome.
Confusingly, this gene can exist in two forms, which produce opsins
sensitive to different wavelengths. So trichromacy can occur in these
animals. But it can only happen in females, who have two X chromosomes,
one inherited from each parent. If these carry different forms of the
gene, a female’s eyes will be equipped with all three pigments. Males,
who have only one X chromosome, always lack a third pigment. So do
those females whose X chromosomes carry identical opsin genes.
The researchers believe that the emergence of separate opsin genes on
the X chromosome–and hence full colour vision–is probably connected
with the shrinkage of the OR family. The better you can see, the less
you need to smell. Since senses are costly to maintain, natural
selection will eliminate redundant ones. Most mammals communicate by
scent. Old-world primates, though, are big on visual communication,
with coloured faces and (in the case of females) coloured sexual
swellings. And people have gone a step further, creating a range of
colourful signals with the clothes they wear. Whether the additional
communication provided by language is another such selective pressure
remains to be seen.