(本文選自《經濟學人》20190202期)
背景介紹：
大熊貓是中國的國寶，也是一種稀有動物，目前全球僅有不到3000隻。眾所周知，大熊貓通常只吃竹子，那麼它是從古至今一直以竹子為食呢，還是因為某種原因在進化的過程中才形成這種獨特的飲食習慣，本文將探討大熊貓以竹子為食的原因。
Panda evolution
熊貓的演化
The giant panda’s unusual diet may be the result of human pressure
人類的壓迫可能導致了大熊貓不同尋常的飲食習慣
Civilisation’s rise drove it into China’s bamboo forests
文明的崛起把大熊貓趕進了中國的竹林
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.
大熊貓深受自然資源保護者的喜愛。它是世界自然基金會（前身為世界野生動物基金會）的吉祥物，有著引人注目的黑白毛髮，是世界上最具辨識度的大型動物之一。
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 an obligate herbivore.
從物種的演化來看，大熊貓也是一種奇怪的動物。它是熊科動物的一種，因而根據它們通常的飲食習慣，大熊貓應該是屬食肉目的一種哺乳動物。但它卻是一種專性食草動物。
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.
大熊貓是稀有動物。據樂觀估計，全球大熊貓的數量大約為2000-3000隻，這還包括那些生活在中國中部地區幾片殘存的竹林中的野生大熊貓。悲觀主義者認為這個數據有所偏高。
It is ironic, then, that this icon of the natural world might actually be an accidental consequence of human activity.
具有諷刺意味的是，大熊貓這個自然界的明星很有可能是人類活動所導致的一個意外產物。
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.
不過，這只是北京動物研究所的魏輔文最近在《當代生物》雜誌上發表的一篇文章對這一結果所做的一種合理的解釋。
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.
大熊貓不僅僅是食草動物，還是單食主義者，它們除了吃竹子外，幾乎不吃其他任何東西。魏輔文想知道這種向單一飲食轉變的過程是何時發生的。答案是，比所有人預想的時間都要早得多。
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.
過去對大熊貓飲食變化的估計依賴於對它們頭骨和基因的研究。400萬年前的顎骨化石顯示，現代大熊貓的祖先已經開始食用很多難嚼的蔬菜了。
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.
對Tas1r1基因的分析也得出了相同的結論，這是一種負責「鮮味」味覺感受的基因，可以讓大熊貓察覺到穀氨酸（肉類食物中常見的成分）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.
研究發現，大約420萬年前，對這一基因有利的選擇壓力開始逐漸降低。傳統理論認為，大約200萬年前，大熊貓就已經完成了全竹飲食的轉變。
Dr Wei has, however, brought a third line of evidence to bear. This is the isotopic composition of the animals』 bones and teeth.
而魏輔文又提出了第三條證據，依據的是大熊貓骨骼和牙齒的同位素組成。
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.
魏輔文對12隻古代大熊貓（可以追溯到1.1萬至5000年前）骨骼中的碳同位素和氮同位素進行了研究，並將它們與現代大熊貓骨骼中兩種元素的同位素進行了對比。
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.
與現代大熊貓相比，古代大熊貓的骨骼中15N/14N值和13C/12C值的變化範圍更大。這表明它們的飲食範圍更廣。
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.
從牙齒化石中收集的氧同位素也證明了這一點。古代大熊貓的18O/16O值變化較大，這表明它們生活在比現代大熊貓更多樣化的環境中。
The question is, what made them change?
問題是，是什麼讓他們改變了（飲食和居住環境）？
Dr Wei does not speculate. But there is one obvious possible culprit: the spread of Homo sapiens.
雖然魏輔文沒有做出任何推測，但有一個顯而易見的罪魁禍首:人類的擴張。
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.
不斷增長的人口迫使大熊貓的祖先們轉移到了邊緣地區，在那裡除了竹子幾乎沒什麼可以吃的。如果竹子是唯一可吃的食物，那麼那些喜歡吃竹子的大熊貓將在進化上處於優勢。
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.
以竹子為食的現代大熊貓是受到了人類壓迫的動物的代表，可能正是由於人類的壓迫，它們才變成了現在的樣子。
(紅色標註詞為重難點詞彙)
本文翻譯：Vinnie
校核：Vinnie
編輯：Vinnie
通過對大熊貓頭骨和基因的研究，動物學家發現原來大熊貓以前並不是一個「素食主義者」，而是在進化的過程中形成了這種特殊的飲食習慣。北京動物研究所的魏輔文最近通過對大熊貓骨骼和牙齒同位素組成的分析也證實了這一點。《經濟學人》認為是人類的壓迫導致大熊貓不得不遷往竹林，最終演化為以竹子為食的飲食習慣。對此，你有什麼看法，歡迎在留言區寫下你的觀點。
mascot ['mæskɑt] n. 吉祥物；福神
recognisable ['rekəgnaɪzəbl] adj. 可辨認的；可認識的；可承認的
obligate ['ɑblɪ,ɡeɪt] vt. 使負義務；強使，強迫；對…施以恩惠 adj. 有責任的，有義務的；必需的
fragment ['fræɡmənt] n. 碎片；片段或不完整部分 vt. 使成碎片 vi. 破碎或裂開
culprit ['kʌlprɪt] n. 犯人，罪犯；被控犯罪的人
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