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July 24, 2003

How humans lost a sense (sort of)

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....

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.

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.

Posted by razib at 10:29 AM

What I find most interesting is that in the ~6 million years since we diverged from the rest of the great apes, we've apparently managed to lose half the functional OR genes that we once had, and this in spite of the fact that chimps seem to have better (in every sense of the term) color vision than we do.

An interesting tangent on this issue - I find it striking how few people are aware that as a species, we have been growing less physically robust over the last 40,000 years for which there is enough evidence to say one way or another. I'm not talking so much about height, which seems to have stayed pretty constant for about 1.8 million years, but about skeletal structure and brain size.

The Cro-Magnons were noticeably stronger and bigger-brained than modern Europeans - and the same pattern holds true in other geographical regions - while the Cro-Magnons in their turn had smaller brains and frailer bodies than the Neandertals they pushed aside. Nevertheless, the technological trend has been upwards rather than downwards; how do advocates of craniometry as an indicator of intelligence explain this apparent contradiction, unless one assumes that most of them aren't even aware of the trend?

Posted by: Juvenal at July 24, 2003 11:43 AM

how do advocates of craniometry as an indicator of intelligence explain this apparent contradiction

The contradiction of topical fidelity?

Posted by: Jason Malloy at July 24, 2003 11:53 AM

i believe that humans have been shrinking since the end of the last ice age until the 18th century....

also, i thought erectus was taller than homo sapiens.

Posted by: razib at July 24, 2003 11:59 AM

If you look at a movie from the 1950s and one produced recently, the humans in the recent movie look much bigger and more muscular.

Posted by: Gordon Gekko at July 24, 2003 12:34 PM

"The contradiction of topical fidelity?"

If you don't find what I have to say interesting, why not ignore it? Or are you simply unable to bear the thought that others might have opinions you find boring? You seem to have a tendency to snarkiness that serves you poorly. Why don't you try containing your urge to snipe for a change?

"i believe that humans have been shrinking since the end of the last ice age until the 18th century....

also, i thought erectus was taller than homo sapiens."

The change in human stature in that time frame, as far as I can tell, was due almost entirely to the shift from hunter-gatherer lifestyles to agriculture. Farmers may have outbred hunter-gatherers, but their quality of diet, and general quality of life, tended to be poorer.

With respect to erectus - I'd say it depends on what modern human populations we look at. If we exclude groups that have adapted to cold habitats by becoming shorter and squatter, height seems to have remained essentially unchanged between erectus and sapiens.

Posted by: Juvenal at July 24, 2003 12:37 PM

Domesticated sheep have much smaller brains than wild sheep. I'm sure the same pattern occurs with 'civilisation', since everything is provided for; and wrong decisions aren't life-threatening.

Posted by: fredrik at July 24, 2003 01:42 PM

If you don't find what I have to say interesting . . .

To the contrary, I do find you interesting. I just find you at your worst when it comes to the issues that vex you (read: issues that suggest racial difference). In this case your use of animal head-size to bizzarely suggest that head-size variation within humans isn't correlated with intelligence. The "contradiction" is, of course, phony.

Sorry, the issue just seemed rather interjected.

Posted by: Jason Malloy at July 24, 2003 01:52 PM

Domesticated sheep have much smaller brains than wild sheep. I'm sure the same pattern occurs with 'civilisation', since everything is provided for; and wrong decisions aren't life-threatening.

you sound like konrad lorenz who held much the same sort of opinions (and to some extent, w.d. hamilton too).

also, on erectus, i'll look it up at some point, but i believe that the average erectus was around 6 footish, which is above the human mean, and in fact, only the tallest of human populations (the nilotic peoples) top this. but i could be wrong....

Posted by: razib at July 24, 2003 02:20 PM

" I just find you at your worst when it comes to the issues that vex you (read: issues that suggest racial difference)"

If I fail to back my arguments with facts or evidence, then by all means have a go at me. Otherwise, I fail to see the problem in exposing one's views to criticism. If I am wrong, you're more than free to say so, and I will be shown up for the fool that I am.

"The "contradiction" is, of course, phony."

But why is it "phony"? It certainly isn't obvious to me, which is why I asked the question. If head size has been shrinking as we have grown more advanced, and if it does tie in with intelligence, doesn't it suggest that we are simultaneously growing less innately intelligent and more knowledgeable? That just strikes me as plain weird.

"Sorry, the issue just seemed rather interjected."

It seemed like a natural segue to me, in that as a species we seem to be going physically downhill in many dimensions, even while we thrive in numbers; not just in our sense of smell, but in the rate of myopia, color-blindness and a whole bunch of other illnesses. In the developed world, natural selection has almost totally ceased to act during childhood, and millions of people who would have no chance of survival in a more challenging environment now grow up to bear families. If you're interested in genetics and the human condition, shouldn't this broader phenomenon be of interest to you?

Posted by: Juvenal at July 24, 2003 02:26 PM

I read a Scientific American article a while back disucssing studies of the brain structure of a certain type of mole. It indicates that there may be some "competition" for brain space among nerves for the various senses. Perhaps the same process selected nerves for color vision rather than for a powerful sense of smell.

The link is :Scientific American: Star-Nosed Mole's Nerves Compete for Brain Space

Posted by: cks at July 24, 2003 02:29 PM

Technological output is a function of individual ability and social organization. So, even if ability decreased during man's evolutionary history (and there is no data for this thesis), that is compatible with increased technological output.

Posted by: Dienekes at July 24, 2003 06:51 PM

I agree with Juvenal, we haven't discussed much about dysgenics or the what traits the environment actually selects for most of the time.
My two cents: Ambitious/intelligent women go to college, then business, grad school, wherever. They have fewer children. Are people getting less ambitious as a result? Are only women getting less ambitious, because successful men have more kids than less successful men?

Posted by: rob at July 25, 2003 06:57 AM

See my post of over a month ago Evolution Of Color Eyesight Led To Loss Of Pheromone Response about the TRP2 gene.

Posted by: Randall Parker at July 28, 2003 02:08 PM