漂亮衣服会导致嗅觉退化？

二师兄UID：118950

译自 《经济学人》 2003.7.24

长期以来人们认为是cook导致了人类嗅觉的退化，因为cook把那些不好的气味都中和了，所以人类变得没有必要嗅出那些evil smelling的东东。
但是最近在Melbourne召开的国际基因大会上科学家们提出了不同的观点。他们认为是视觉的进化导致了嗅觉灵敏度的急剧下降。

人们是通过鼻子内侧的一些Odour Receptor (OR)接触分子来感知气味的。人类有1000多种ORs，但是跟动物比起来还是少的可怜。人类的嗅觉的贫乏实际上是自身基因种类数量的反映，人类的嗅觉基因比较少，而且有60%是所谓的伪基因（pseudo-genes），也就是说是退化了的。

为什么在灵长类中人类丢掉那么多OR呢？以色列魏茨曼大学的Yoav Gilad随机选择了50种人类的OR基因，并和其他的灵长类动物以及老鼠等的对应基因进行了比较。

老鼠只有20%左右的（OR）基因退化成伪基因，而黑猩猩、大猩猩和猩猩则接近30%。古代（old-world）的猴子比现代的(new-world)猴子失去了更多的OR基因。Yoav Gilad说人类有四次急剧的进化，所以一下子失去了60%。

此外新旧世界的猴子为什么差别这么大呢？看起来在开始部分似乎也可以用进化加剧来解释（？？？）。但是新猴子中一种叫做吼猴的异类和他那些老猴子表哥比较像。

为什么老猴子和新猴子的一支OR丢失的这么快呢？科学家们注意到吼猴和那些老猴子都有分辨全色彩的能力，也就是"三原色视觉"。


"三原色视觉"涉及到三种色素，叫做opsin(视蛋白)，能够感知不同波长的光。人类和那些老猴子的中长波视蛋白是由x染色体上不同的基因控制的。但大多数新猴子的x染色体中只有一种视蛋白。

令人迷惑的是，（在老猴子当中）这种感知不同波长的光的基因可以以两种形式存在。所以这些动物也可以感知三原色，但是只有雌性可以，因为她们有两个x染色体。如果这两个染色体带有了不同形式的基因，那么这个雌性动物的眼睛就可以看到三原色。只有一个x染色体的雄性（老猴子）总是缺乏第三种色素，那些两个x染色体带有相同基因的雌性老猴子也是如此。

研究者相信，x染色体内不同视蛋白基因的偶发事件（应该是指基因有时候相同，有时候不同）-也就是全色彩视觉-可能与OR家族的退化有关。你看得更好，就较少需要嗅。保持感觉是很昂贵的事情（so does it in relationship, I think）自然选择减少了冗余的感觉功能。大多数哺乳动物通过嗅觉交流，但是老世界的猴子则在视觉交流方面很在行，例如色彩丰富的脸，凹凸有致的性感身材（应该是指丰乳肥臀）。人类则在这条道路上走的更远：他们通过穿的衣服创造了更为丰富的色彩信号。是不是语言方面的交流也会导致嗅觉退化呢？走着瞧吧。

Smell, vision and genes

More colour, less odour

Jul 24th 2003 | MELBOURNE
From The Economist print edition


Gaining colour vision, it seems, cost people much of their sense of smell


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.

二师兄UID：118950

我忽然大概领悟到了这句话的意思了：

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.


此外，新猴子和旧猴子之间的分别是如此清晰，Dr Gilad说：看起来我们几乎可以把OR伪基因增加（的速率开始不断上升？？？）的原因归结于新旧猴子之间的差别（即：猴子的进化导致了伪基因的增加/嗅觉的退化）。但是，事实又不尽如此：一种叫“喊叫猴”的新猴子却和老猴子比较像。这又说明了什么呢？科学家们抓住这个矛盾，深入研究下去，终于揭开了真相，啊！bluhabluha~~~~

从这里也可以推断出所谓的 old-world monkey/ new-world monkey不是指旧大陆/新大陆的猴子，而是指古代/现代的猴子。

呵呵，高兴。