Studies have shown that over time, most humans need eight to eight and a half hours of sleep for the best health.

VOA: special.2009.08.26

And one of the reasons that's most responsible for that is that humans aren't dying at as young an age from infectious diseases as they did three or four hundred years ago.

最重要的一个原因就是,人们不会在年轻的时候死于传染性疾病,而这在三四百年前确实会发生

耶鲁公开课 - 生物医学工程探索课程节选

And,Mister Hellsten says, genetic changes are rare for frogs, humans and most organisms.

VOA: special.2010.06.29