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.
