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In the past, evolutionary biologists contemplating the absence of wheels in nature agreed that the explanation was not undesirability: wheels would be good for animals, just as they are for us. Animals were prevented from evolving wheels, the biologists reasoned, by the following dilemma: living cells in an animal's body are connected to the heart by blood vessels, and to the brain by nerves. Because a rotating joint is essential to a wheel, a wheel made of living cells would twist its artery vein and nerve connections at the first revolution, making living impracticable. However, there is a flaw in the argument that the evolution of wheeled animals was thwarted by the insoluble joint problem. The theory fails to explain why animals have not evolved wheels of dead tissue with no need for arteries and nerves. Countless animals, including us, bear external structures without blood supply or nerves - for example, our hair and fingernails, or the scales, claws, and ho rns of other animals. Why have rats not evolved bony wheels, similar to roller skates?Paws might be more useful than wheels in some situations, but cats' claws are retractable: why not retractable wheels?We thus arrive at the serious biological paradox flippantly termed the RRR dilemma: nature's failure to produce rats with retractable roller skates.
从前，研究自然界没有轮子的进化论生物学家都同意不能用无此需要来解释这种现象：轮子对于动物会像对于我们人类一样有好处。生物学家们推论，动物没有进化出轮子是由下述困难所致：动物身上的活细胞通过血管与心脏相连，通过神经与大脑相连。因为一个旋转的接头对轮子来说是至关重要的，由活的细胞构成的轮子在第一次转动时便会扭伤其上的动脉和神经的连结，因而不现实。
不过，动物未能进化出轮子是受阻于无法解决接头问题的说法有一个缺陷。这种理论无法解释为何动物没有进化出由死组织构成的而无需动脉和神经的轮子。包括人在内的无数动物都有一些没有血液供应和神经的体外构造，例如，我们的头发和指甲，或者鳞片、爪子和其它一些动物的角。为什么老鼠没有进化出类似于滑轮溜冰鞋的骨质的轮子呢?在某些情况下，爪子可能比轮子更有用，但猫的爪子是可以伸缩的：为什么不能有可以伸缩的轮 子呢?这样，我们便得出了一个被戏称为 RRR 的严肃的生物学悖论：大自然未能产生出有可伸缩的滑轮溜冰鞋的老鼠。
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