科學家發現尼安德特人攜帶人類DNA
In 1997, scientists found the first scrap of Neanderthal DNA in a fossil. Since then, they have recovered genetic material, even entire genomes, from a number of Neanderthal bones, and their investigations have yielded a remarkable surprise: Today, 1 to 2 percent of the DNA in non-African people comes from Neanderthals.
1997年，科學家在化石中第一次發現了尼安德特人DNA的碎片。此後，他們從陸續出土的多塊尼安德特人的骨頭中提取了基因物質，乃至完整的基因組。他們的研究收穫了一個驚人的發現：今天，在除了非洲人之外的人類身上，有1%到2%的DNA來自尼安德特人。
That genetic legacy is the result of interbreeding roughly 50,000 years ago between Neanderthals and the common ancestors of Europeans and Asians. Recent studies suggest that Neanderthal genes even influence human health today, contributing to conditions from allergies to depression.
這份基因遺產，是大約5萬年前尼安德特人和歐亞人的共同祖先混種繁殖的結果。近期的研究表明，尼安德特人的基因甚至影響到當今人類的健康，與包括過敏和抑鬱在內的多個病狀有關。
Now scientists have found that the genes flowed both ways. In a study published on Wednesday in Nature, a team of scientists reports that another instance of interbreeding left Neanderthals in Siberia with chunks of human DNA.
如今，科學家發現基因的流動是雙向的。在《自然》雜誌(Nature)週三發佈的一篇研究論文中，一組科學家提交的案例表明，混種繁殖使得西伯利亞的尼安德特人帶有部分人類DNA。
This exchange, the scientists conclude, took place about 100,000 years ago. That’s a puzzling date, because a great deal of evidence indicates that the ancestors of today’s non-Africans did not expand out of Africa until 50,000 to 60,000 years ago.
這羣科學家得出結論，這種交融發生在約10萬年前。這一日期令人困惑，因爲大量證據顯示，直到5萬到6萬年前，如今不在非洲的人類的祖先才走出了非洲大陸。
It’s possible, then, that these Neanderthals acquired DNA from a mysterious early migration of humans.
那麼，這些尼安德特人有可能是從一羣早期的神祕人類遷徙羣體身上獲得了DNA。
“I think at this point we’ve convinced everybody the observation is real,” said Adam Siepel, a geneticist at Cold Spring Harbor Laboratory and a co-author of the new study. “But the story behind the observation is still very much in dispute.”
“我認爲，現在我們已經讓所有人信服，我們的觀測結果是真實的，”來自美國冷泉港實驗室(Cold Spring Harbor Laboratory)的遺傳學家亞當·西佩爾(Adam Siepel)表示。他是這項新研究的共同作者。“但觀測結果背後的解釋，依然爭議重重。”
Humans and Neanderthals split from a common ancestor in Africa about 600,000 years ago. At some point afterward, the ancestors of Neanderthals spread to Europe, the Middle East and Central Asia.
在大約60萬年前，擁有非洲共同祖先的人類和尼安德特人分道揚鑣。此後的某個時刻，尼安德特人的祖先擴散到歐洲、中東和中亞地區。
Along the way, Neanderthals took on a distinctive anatomy — a stocky, powerful build — and became impressive hunters of big game. The last Neanderthals appear to have died about 40,000 years ago.
遷徙途中，尼安德特人演化出矮壯而有力的獨特體型，併成爲狩獵大型動物的佼佼者。最後一代尼安德特人似乎在大約4萬年前滅亡。
In 2010, scientists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, recovered about 60 percent of a Neanderthal genome from fossils found in a Croatian cave. Neanderthals shared certain mutations with living Europeans and Asians, the scientists found — but not with modern Africans. They concluded that humans must have interbred with Neanderthals after leaving Africa.
2010年，德國萊比錫馬克斯·普朗克演化人類學研究所(Max Planck Institute for Evolutionary Anthropology)提取了尼安德特人的一個大約60%的基因組，所用樣本來自克羅地亞一處洞穴中發掘出的化石。科學家發現，尼安德特人和如今的歐亞人共享一定的基因突變，但與現代非洲人並無共通之處。科學家得出結論，人類一定是在離開非洲大陸後和尼安德特人進行了混種繁殖。
Three years later, the Max Planck team reconstructed the complete genome of a male Neanderthal from a toe bone dating back at least 50,000 years, which had been discovered in the Altai Mountains of Siberia. Comparing the Altai genome to modern human DNA confirmed the interbreeding.
三年後，馬克斯·普朗克的研究團隊重構了一名男性尼安德特人的完整基因組。這些基因來自西伯利亞阿爾泰山脈發掘的一塊距今至少5萬年的趾骨。通過比較阿爾泰山脈尼安德特人的基因組和現代人類的DNA，確認了混種繁殖的存在。
Recently, the researchers harvested more DNA from European Neanderthal fossils, putting together very detailed reconstructions of a single chromosome, Chromosome 21.
近期，研究者從歐洲尼安德特人化石上收穫了更多DNA信息，重構出非常詳盡的單條染色體——第21號染色體。
Sergi Castellano, a geneticist at Max Planck Institute, set out to compare this European Neanderthal DNA to the genes from Siberia. He was curious about how isolated the populations of Neanderthals were from one another, and how they were related to humans alive today.
馬克斯·普朗克研究所的遺傳學家塞爾吉·卡斯特利亞諾(Sergi Castellano)着手比較了歐洲尼安德特人的DNA和西伯利亞尼安德特人的基因。讓他感興趣的是，尼安德特人羣體相互隔離的程度，以及他們與當今人類的關聯。
He was joined by Dr. Siepel, who has developed powerful statistical models in recent years to trace how DNA changes over time. He and his colleagues are able to compare genomes and infer their common history: how their ancestors split apart, for example, and how large the populations of their ancestors were.
他邀請了西佩爾博士加入。在過去數年裏，西佩爾構建出強大的統計學模型來追蹤DNA如何隨時間而演化。於是，他和同事能夠比較這些基因組，推斷他們共同的歷史：比如，他們的祖先是如何分化的，祖先的人口規模又有多大。
Dr. Siepel, Dr. Castellano and their colleagues used these models to analyze the Neanderthal DNA and that of humans. Their analysis confirmed previous studies: Some Neanderthal DNA did indeed end up in modern Europeans and Asians.
西佩爾、卡斯特利亞諾及同事利用這些模型分析了尼安德特人和人類的DNA。他們的分析結果證實了之前的研究：尼安德特人的DNA片段的確出現在了現代歐亞人的身上。
But they also found another example of so-called gene flow, and an unexpected one at that: The Altai Neanderthals in particular shared some mutations with living Africans, but not with Europeans and Asians.
不過，他們還發現了“基因流動”的又一個例子，而且是個出人意料的例子：具體來說，是阿爾泰山脈的尼安德特人與今天的非洲人有一些同樣的突變，而不是歐亞人。
That pattern suggests that an African lineage of humans interbred with the ancestors of the Altai Neanderthal after they split from other Neanderthals.
這意味着，有一支非洲人類，在阿爾泰山脈尼安德特人的祖先與其他尼安德特人分開之後，與其進行過混種繁殖。
Dr. Castellano and Dr. Siepel thought at first there was something wrong with their analysis, so they went back to fix it. But when they looked for the problem, they could not find it.
卡斯特利亞諾和西佩爾起初以爲是自己的分析出了錯，於是回頭去修正。但他們卻沒有找到問題。
“We poked and prodded and poked and prodded, and couldn’t get it to go away,” said Dr. Siepel.
“我們這裏看看，那裏瞧瞧，就是沒有辦法讓它消失，”西佩爾說。
Once the scientists accepted that the interbreeding was real, they estimated when it happened. When DNA gets passed down through the generations, it gets shuffled into new arrangements that can be used to build a sort of timeline.
一旦科學家們接受了混種繁殖的情況真實存在，他們就開始估算它發生的時間。DNA在代際遺傳的時候，會進行重組，而這就可以用來建立某種時間表。
Based on this gene shuffling, the scientists estimated that humans and the ancestors of the Altai Neanderthals interbred about 100,000 years ago — long before people were thought to have left Africa. “This observation throws a wrench in the works,” said Dr. Siepel.
基於這種基因重組，科學家們估算出，人類與阿爾泰山脈尼安德特人的祖先在大約10萬年前進行過混種繁殖——遠在普遍認爲的人類離開非洲的時間之前。“這一觀測結果給我們的工作造成了很大的麻煩，”西佩爾說。
Bence Viola, a paleontologist at the University of Toronto and a co-author of the new study, said he was skeptical when he first learned of the genetic data. “It’s really weird, that’s my main impression,” he said.
同爲這項新研究作者的多倫多大學(University of Toronto)古生物學家本斯·維奧拉(Bence Viola)表示，剛聽到這些基因數據的消息時，他本人滿腹狐疑。“實在是奇怪，這就是我的主要印象，”他說。
Eventually, however, Dr. Castellano persuaded him the data were strong, and Dr. Viola looked over the fossil record for possible explanations for the interbreeding.
不過，卡斯特利亞諾最終說服他相信數據是可靠的。於是維奧拉博士查看了或許可以解釋這一混種繁殖的化石記錄。
In the 1930s, for example, scientists discovered 120,000-year-old fossils in Israel that looked a lot like living humans. Many scientists viewed them as a failed human migration from Africa, with no genetic connection to humanity today.
比方說，上世紀30年代，科學家們在以色列發現了一批12萬年前的化石，與現代人類有諸多相似之處。許多科學家認爲，它們代表了人類走出非洲的一次失敗嘗試，與今天的人類沒有基因聯繫。
In October, Chinese researchers discovered another intriguing clue in a cave: 47 teeth that they estimate are between 80,000 and 120,000 years old. The scientists argue that the teeth belonged to modern humans.
去年10月，中國研究者在一處洞穴發現了另一個有意思的線索：47顆牙齒。這些科學家估算牙齒有8到12萬年曆史，並提出它們屬於現代人類。
Dr. Viola is not completely convinced by the Chinese study. Instead, he thinks the scenario that best fits the evidence is that humans expanding into the Middle East encountered and interbred with Neanderthals there. The Neanderthals then migrated east to Siberia, taking human DNA with them.
中國科學家的研究並未完全說服維奧拉。他認爲，能與證據吻合得最好的情景是，擴散到中東地區的人類與當地的尼安德特人進行了混種繁殖。然後尼安德特人東遷到西伯利亞地區，身上帶着人類的DNA。
“It seems pretty solid,” said Jonathan K. Pritchard, a geneticist at Stanford University who was not involved in the study. “Overall, the story they’re telling is pretty coherent.”
“看起來很有說服力，”沒有參與這項研究的斯坦福大學(Stanford University)遺傳學家喬納森·K·普里查德(Jonathan K. Pritchard)這樣評價道。“總體而言，他們提出的這套說辭比較條理分明。”
Dr. Pritchard said that more examinations of ancient DNA would help resolve the many questions raised by the new study and help scientists understand the scope of the intimate back-and-forth between Neanderthals and humans.
普里查德博士表示，對古DNA的更多研究將有助於解答這項新研究提出的許多問題，並幫助科學家理解尼安德特人與人類之間親密的相互來往的範圍。
“There’s going to be a lot more data really soon,” he said. “I would expect in the next few years we’ll have much, much more of the jigsaw puzzle, and it will be possible to piece this all together.”
“很快就會出現許許多多的新數據，”他說。“我認爲，在接下來的幾年裏，會有多得多的碎片冒出來，也就有可能把它們都拼接起來。”
