【英语时差8，16】直立行走

 Among all the primates¹, we humans are the only ones that stand on two legs.   You may have seen other animals stand on two legs briefly², such as a cat or a dog. But they always return to their original posture³. Even apes, who have arms and hands much like humans, still put them down to walk.

How did it come about that we do every day what nobody else does? Well, one theory says that standing⁴ up is the result of reaching up.

Think of it this way. You may have seen a cat go up on its hind⁵ legs in order to pull food down off a table. Suppose those same conditions remained in place for a hundred thousand generations. The only food was always on tables, and only accessible to cats who could reach for it. In time, the cats who couldn't manage that trick would die out. The ones who could manage it would survive and reproduce. A cycle begins.

It is thought that this kind of scenario⁶ may have led the predecessors⁷ to human beings to go from being four-footed to being upright creatures. In the forests and savannas⁸ where our species got its start, being able to reach up and pluck fruit trees was a good thing. We see chimpanzees do this even today, standing temporarily on two feet while grabbing food with their hands.

Given a few million years, these conditions may have given rise to an animal that can stand up all the time, not just briefly. That animal is us.

So, what comes next? Next time. 

1 primates   

primate的复数

参考例句：

• Primates are alert, inquisitive animals. 灵长目动物是机灵、好奇的动物。
• Consciousness or cerebration has been said to have emerged in the evolution of higher primates. 据说意识或思考在较高级灵长类的进化中已出现。

2 briefly   

adv.简单地，简短地

参考例句：

• I want to touch briefly on another aspect of the problem.我想简单地谈一下这个问题的另一方面。
• He was kidnapped and briefly detained by a terrorist group.他被一个恐怖组织绑架并短暂拘禁。

3 posture   

n.姿势，姿态，心态，态度；v.作出某种姿势

参考例句：

• The government adopted an uncompromising posture on the issue of independence.政府在独立这一问题上采取了毫不妥协的态度。
• He tore off his coat and assumed a fighting posture.他脱掉上衣，摆出一副打架的架势。

4 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

5 hind   

adj.后面的，后部的

参考例句：

• The animal is able to stand up on its hind limbs.这种动物能够用后肢站立。
• Don't hind her in her studies.不要在学业上扯她后腿。

6 scenario   

n.剧本，脚本；概要

参考例句：

• But the birth scenario is not completely accurate.然而分娩脚本并非完全准确的。
• This is a totally different scenario.这是完全不同的剧本。

7 predecessors   

n.前任( predecessor的名词复数 )；前辈；（被取代的）原有事物；前身

参考例句：

• The new government set about dismantling their predecessors' legislation. 新政府正着手废除其前任所制定的法律。 来自《简明英汉词典》
• Will new plan be any more acceptable than its predecessors? 新计划比原先的计划更能令人满意吗？ 来自《简明英汉词典》

8 savannas   

n.（美国东南部的）无树平原( savanna的名词复数 )；（亚）热带的稀树大草原

参考例句：

• Therefore the jungles and the savannas know of no juvenile delinquency! 因此，丛林和荒原里没有“少年犯罪。” 来自辞典例句
• My soul flits away into the virgin forests and to the savannas. 我的灵魂飞向森林中的处女地和广漠的平原。 来自互联网