We humans are conspicuous among the 5,000 or so mammal species in that we are effectively naked. Just consider what your pet dog or cat (or, for that matter, a polar bear) would look like, and how it might feel, if its furry coat were shorn.
| Scientists have suggested three main explanations for why humans lack fur. All revolve around the idea that it may have been advantageous for our evolving lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.
The aquatic-ape hypothesis suggests that six million to eight million years ago apelike ancestors of modern humans had a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in providing us with an excuse for being overweight—paleontological evidence for an aquatic phase of human existence has proven elusive.
The second theory is that we lost our fur in order to control our body temperature when we adapted to life on the hot savannah. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. The body-cooling idea seems sensible, but even though lacking fur might have made it easier for us to lose heat during the day, we also would have lost more heat at night, when we needed to retain it.
Recently, a colleague and I suggested that ancestors to modern humans became naked as a means to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other "ectoparasites." These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.
|
| Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling "overcooked sausages with buck teeth," also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.
Once hairlessness had evolved this way, it may have become subject to sexual selection—being a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock's tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction. |
译文:
揭秘人体—高级动物不长毛？
在5000多种哺乳动物中，只有人类身体明显是光的，与其他动物大不相同．试想一下，假如把你的宠物狗或猫（甚至于联想到一只北极熊）的毛剪掉，它们的样子怎样？摸上去的感觉又怎样？
科学家针对人类没有长皮毛已经给出了三点重要解释．所有的解释都围绕着这样一个观点:尽管人类在600万年前与现存最相近的动物黑猩猩拥有共同的祖先,但体毛变得越来越少,这对我们的血统进化是有益的.
水栖猿假说表明6百万或8百万年前现代人类祖先类人猿靠在浅水里寻找食物为生过着半水生生活.皮毛不能在水里有效地隔热,因此这个理论声称我们象其他水生哺乳动物一样,身体脂肪较高,因此随着进化失掉皮毛。.按照这个解释推想人类身体超重是有益的--人类生存的水生阶段的古生物学证据证明这个观点不可靠.
第二个解释为人类失去体毛是为了控制体温以便适应炎热的草原生活.我们类人猿祖先大部分时间是在凉爽的森林里度过的,但是对于身体多毛直立行走的原始人在阳光照射下行走就会感到过热.保持身体凉爽的想法似乎是合乎清理的,但试想他们失去体毛在白天可以让身体很快散失热量,那么当他们在晚上需要保持热量时,岂不也同样失去了更多的热量?
最近我和我的一个同事提出这样一个观点：从祖先到现代人类身体不长毛是为了减少成群寄居于皮毛的体外寄生虫的流行机会.多毛的外皮为那些象壁虱、虱子、咬蝇等害虫和其他体外寄生虫提供了舒适、安全的居所。这些生物不但会刺激、烦扰人类还会携带病毒、细菌和传播基于原生动物的疾病如：疟疾、嗜睡、西尼罗河病和莱姆关节炎等，这些均能使人类引起慢性医学疾病，甚至死亡。人类褪掉皮毛从而减少了寄生虫的数量，同时采取生火、建造房屋、逢制衣服的方法避免了在晚上及寒冷的气候中受冻 。
人类感染虱子(仅局限于身体多毛的地方)似乎支持寄生虫假说.不长毛的鼹鼠,被描述为类似"长着雄鹿牙的煮得过了头的香肠"的动物也似乎支持这个假说.他们群居在地下,寄生虫很容易传播.但是他们身体相互取暖加上地下地方狭小致使这些动物不会在寒冷的天气散失热量,才使他们身体不长毛成为可能.一旦用这种方式进化脱毛,可能要受性别选择控制--成为吸引异性的特征.光滑、洁净的皮肤是健康的标志，（像孔雀的尾部一样） 从而我们能够解释为什么妇女要比男人体毛少，而且她们热中于去除体毛。尽管头上可能看出长虱子，但头发可以免于太阳照射，并在冷天中御寒。人类的阴毛起着增强分泌信息素的作用或者说是为了散发气味吸引异性。