2018年6月大学英语六级听力常考时文：Evolution and Wheels
1.What connects the living cells to the heart in an animal's body? ...2.What is crucial to a wheel? ...
3.What is the feature of cats' claws? ...
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.
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