(本文选自《经济学人》20190202期)

背景介绍：

       大熊猫是中国的国宝，也是一种稀有动物，目前全球仅有不到3000只。众所周知，大熊猫通常只吃竹子，那么它是从古至今一直以竹子为食呢，还是因为某种原因在进化的过程中才形成这种独特的饮食习惯，本文将探讨大熊猫以竹子为食的原因。

Panda evolution

熊猫的演化

The giant panda’s unusual diet may be the result of human pressure

人类的压迫可能导致了大熊猫不同寻常的饮食习惯

Civilisation’s rise drove it into China’s bamboo forests

文明的崛起把大熊猫赶进了中国的竹林

The giant panda is beloved of conservationists. It is the mascot of the WWF (World Wide Fund for Nature, formerly the World Wildlife Fund) and, with its striking black-and-white pelage, is one of the most recognisable large animals in the world. 

大熊猫深受自然资源保护者的喜爱。它是世界自然基金会（前身为世界野生动物基金会）的吉祥物，有着引人注目的黑白毛发，是世界上最具辨识度的大型动物之一。

It is also evolutionarily weird. It is a type of bear, and therefore a member of the order of mammals known, after their usual dietary habits, as the Carnivora. But it is anobligate herbivore.

从物种的演化来看，大熊猫也是一种奇怪的动物。它是熊科动物的一种，因而根据它们通常的饮食习惯，大熊猫应该是属食肉目的一种哺乳动物。但它却是一种专性食草动物。

And it is rare. Optimistic estimates put the global panda population at between 2,000 and 3,000 individuals—with all those not living in zoos occupying a few fragments of bamboo forest in central China. Pessimists reckon those numbers are on the high side. 

大熊猫是稀有动物。据乐观估计，全球大熊猫的数量大约为2000-3000只，这还包括那些生活在中国中部地区几片残存的竹林中的野生大熊猫。悲观主义者认为这个数据有所偏高。

It is ironic, then, that this icon of the natural world might actually be an accidental consequence of human activity. 

具有讽刺意味的是，大熊猫这个自然界的明星很有可能是人类活动所导致的一个意外产物。

Yet this is a plausible interpretation of results just published in a paper in Current Biology, by Wei Fuwen of the Institute of Zoology, in Beijing.

不过，这只是北京动物研究所的魏辅文最近在《当代生物》杂志上发表的一篇文章对这一结果所做的一种合理的解释。

Pandas are not merely herbivores, they are monovores—eating bamboo to the exclusion of almost anything else. Dr Wei wondered when this transition to monovory happened. The answer was, far more recently than anyone had expected.

大熊猫不仅仅是食草动物，还是单食主义者，它们除了吃竹子外，几乎不吃其他任何东西。魏辅文想知道这种向单一饮食转变的过程是何时发生的。答案是，比所有人预想的时间都要早得多。

Past estimates of changes in pandas’ diets have depended on studies of their skulls and genes. The jaws of 4m-year-old fossils suggest that the ancestors of modern pandas were already by then eating a lot of tough vegetable matter. 

过去对大熊猫饮食变化的估计依赖于对它们头骨和基因的研究。400万年前的颚骨化石显示，现代大熊猫的祖先已经开始食用很多难嚼的蔬菜了。

Analysis of a gene called Tas1r1, responsible for the taste sensation called “umami”, which detects glutamic acid, a common component of meat, tells a similar story. 

对Tas1r1基因的分析也得出了相同的结论，这是一种负责“鲜味”味觉感受的基因，可以让大熊猫察觉到谷氨酸（肉类食物中常见的成分）的存在。

It indicates that selective pressures in favour of this gene started to relax about 4.2m years ago. By around 2m years ago, conventional theory has it, pandas had completed the transition to an all-bamboo diet. 

研究发现，大约420万年前，对这一基因有利的选择压力开始逐渐降低。传统理论认为，大约200万年前，大熊猫就已经完成了全竹饮食的转变。

Dr Wei has, however, brought a third line of evidence to bear. This is the isotopic composition of the animals’ bones and teeth.

而魏辅文又提出了第三条证据，依据的是大熊猫骨骼和牙齿的同位素组成。

Dr Wei studied carbon and nitrogen isotopes in the bones of a dozen ancestral pandas, dating from between 11,000 and 5,000 years ago, and compared them with those of modern pandas. 

魏辅文对12只古代大熊猫（可以追溯到1.1万至5000年前）骨骼中的碳同位素和氮同位素进行了研究，并将它们与现代大熊猫骨骼中两种元素的同位素进行了对比。

The ancient animals had a wider range of 15N/14N and 13C/12C ratios in their bones than did the modern ones. That suggests they had broader diets. 

与现代大熊猫相比，古代大熊猫的骨骼中15N/14N值和13C/12C值的变化范围更大。这表明它们的饮食范围更广。

Oxygen isotopes collected from fossil teeth told a similar story. The ancient pandas had more variable 18O/16O values, suggesting that they lived in more varied environments than do their modern kin.

从牙齿化石中收集的氧同位素也证明了这一点。古代大熊猫的18O/16O值变化较大，这表明它们生活在比现代大熊猫更多样化的环境中。

The question is, what made them change?

问题是，是什么让他们改变了（饮食和居住环境）？

Dr Wei does not speculate. But there is one obvious possible culprit: the spread of Homo sapiens. 

虽然魏辅文没有做出任何推测，但有一个显而易见的罪魁祸首:人类的扩张。

Growing human populations could easily have displaced the ancestors of modern pandas to fringe areas where there was little to eat but bamboo. And if bamboo is all there is to eat, then those that prefer to eat it will be at an evolutionary advantage. 

不断增长的人口迫使大熊猫的祖先们转移到了边缘地区，在那里除了竹子几乎没什么可以吃的。如果竹子是唯一可吃的食物，那么那些喜欢吃竹子的大熊猫将在进化上处于优势。

The modern, bamboo-eating panda—symbol of animals under pressure from man—may thus have been made the way it is by precisely such human pressures.

以竹子为食的现代大熊猫是受到了人类压迫的动物的代表，可能正是由于人类的压迫，它们才变成了现在的样子。

(红色标注词为重难点词汇) 

本文翻译：Vinnie

校核：Vinnie

编辑：Vinnie

小编说

通过对大熊猫头骨和基因的研究，动物学家发现原来大熊猫以前并不是一个“素食主义者”，而是在进化的过程中形成了这种特殊的饮食习惯。北京动物研究所的魏辅文最近通过对大熊猫骨骼和牙齿同位素组成的分析也证实了这一点。《经济学人》认为是人类的压迫导致大熊猫不得不迁往竹林，最终演化为以竹子为食的饮食习惯。对此，你有什么看法，欢迎在留言区写下你的观点。

重难点词汇：

mascot ['mæskɑt] n. 吉祥物；福神

recognisable ['rekəgnaɪzəbl] adj. 可辨认的；可认识的；可承认的

obligate ['ɑblɪ,ɡeɪt] vt. 使负义务；强使，强迫；对…施以恩惠 adj. 有责任的，有义务的；必需的

fragment ['fræɡmənt] n. 碎片；片段或不完整部分 vt. 使成碎片 vi. 破碎或裂开

culprit ['kʌlprɪt] n. 犯人，罪犯；被控犯罪的人