Prehistoric humans never threw spears at Tyrannosaurus rex but really did hunt mammoths and mastodons.

史前人类从未向雷克斯暴龙扔过长矛，但确实猎杀过猛犸象和乳齿象。

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It will never be known how and when this numeration ability developed, but it is certain that numeration was well developed by the time humans had formed even semi-permanent settlements.

我们永远也不会知道这种计算能力是何时、如何发展起来的，但可以肯定的是，在人类形成半永久定居点的时候，这种计数法已经发展得很好了。

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But we never know how well it's going to hold up with humans.

但我们从来不知道，这项结果用在人类身上，好使不好使。

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Initial tests looking for "markers" on the surface of the bacterium showed it to be the O104 strain, which the World Health Organization said was rare, had been seen in humans before, but never in an outbreak.

BBC: Q&A: E. coli outbreak

While it will probably take at least another 20 years before Zubrin -- or anyone else for that matter -- makes it to the Martian surface, NASA and other space agencies are already drawing up plans for a voyage that will present astronauts not only with physical but also psychological challenges never faced by humans before.

CNN: Mars on the brain? Red Planet pioneers to face cosmic mind trip

Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, told the BBC that the idea had been discussed repeatedly but "experiments have never come close to suggesting it might work in humans".

BBC: Stem cell op may 'restore sperm'