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.

    從前，研究自然界沒(méi)有輪子的進(jìn)化論生物學(xué)家都同意不能用無(wú)此需要來(lái)解釋這種現(xiàn)象：輪子對(duì)于動(dòng)物會(huì)像對(duì)于我們?nèi)祟?lèi)一樣有好處。生物學(xué)家們推論，動(dòng)物沒(méi)有進(jìn)化出輪子是由下述困難所致：動(dòng)物身上的活細(xì)胞通過(guò)血管與心臟相連，通過(guò)神經(jīng)與大腦相連。因?yàn)橐粋�(gè)旋轉(zhuǎn)的接頭對(duì)輪子來(lái)說(shuō)是至關(guān)重要的，由活的細(xì)胞構(gòu)成的輪子在第一次轉(zhuǎn)動(dòng)時(shí)便會(huì)扭傷其上的動(dòng)脈和神經(jīng)的連結(jié)，因而不現(xiàn)實(shí)。

    不過(guò)，動(dòng)物未能進(jìn)化出輪子是受阻于無(wú)法解決接頭問(wèn)題的說(shuō)法有一個(gè)缺陷。這種理論無(wú)法解釋為何動(dòng)物沒(méi)有進(jìn)化出由死組織構(gòu)成的而無(wú)需動(dòng)脈和神經(jīng)的輪子。包括人在內(nèi)的無(wú)數(shù)動(dòng)物都有一些沒(méi)有血液供應(yīng)和神經(jīng)的體外構(gòu)造，例如，我們的頭發(fā)和指甲，或者鱗片、爪子和其它一些動(dòng)物的角。為什么老鼠沒(méi)有進(jìn)化出類(lèi)似于滑輪溜冰鞋的骨質(zhì)的輪子呢?在某些情況下，爪子可能比輪子更有用，但貓的爪子是可以伸縮的：為什么不能有可以伸縮的輪 子呢?這樣，我們便得出了一個(gè)被戲稱(chēng)為 RRR 的嚴(yán)肅的生物學(xué)悖論：大自然未能產(chǎn)生出有可伸縮的滑輪溜冰鞋的老鼠。