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，科学家在化石中第一次发现了尼安德特人DNA的碎片。此后，他们从陆续出土的多块尼安德特人的骨头中提取了基因物质，乃至完整的基因组。他们的研究收获了一个惊人的发现：今天，在除了非洲人之外的人类身上，有1%到2%的DNA来自尼安德特人。

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.

这份基因遗产，是大约5万年前尼安德特人和欧亚人的共同祖先混种繁殖的结果。近期的研究表明，尼安德特人的基因甚至影响到当今人类的健康，与包括过敏和抑郁在内的多个病状有关。

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，科学家发现基因的流动是双向的。在《自然》杂志(Nature)周三发布的一篇研究论文中，一组科学家提交的案例表明，混种繁殖使得西伯利亚的尼安德特人带有部分人类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.

这群科学家得出结论，这种交融发生在约10万年前。这一日期令人困惑，因为大量证据显示，直到5万到6万年前，如今不在非洲的人类的祖先才走出了非洲大陆。

It’s possible, then, that these Neanderthals acquired DNA from a mysterious early migration of humans.

那么，这些尼安德特人有可能是从一群早期的神秘人类迁徙群体身上获得了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.”

“我认为，现在我们已经让所有人信服，我们的观测结果是真实的，”来自美国冷泉港实验室(Cold Spring Harbor Laboratory)的遗传学家亚当·西佩尔(Adam Siepel)表示。他是这项新研究的共同作者。“但观测结果背后的解释，依然争议重重。”

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.

在大约60万年前，拥有非洲共同祖先的人类和尼安德特人分道扬镳。此后的某个时刻，尼安德特人的祖先扩散到欧洲、中东和中亚地区。

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.

迁徙途中，尼安德特人演化出矮壮而有力的独特体型，并成为狩猎大型动物的佼佼者。最后一代尼安德特人似乎在大约4万年前灭亡。

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，德国莱比锡马克斯·普朗克演化人类学研究所(Max Planck Institute for Evolutionary Anthropology)提取了尼安德特人的一个大约60%的基因组，所用样本来自克罗地亚一处洞穴中发掘出的化石。科学家发现，尼安德特人和如今的欧亚人共享一定的基因突变，但与现代非洲人并无共通之处。科学家得出结论，人类一定是在离开非洲大陆后和尼安德特人进行了混种繁殖。

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.

三年后，马克斯·普朗克的研究团队重构了一名男性尼安德特人的完整基因组。这些基因来自西伯利亚阿尔泰山脉发掘的一块距今至少5万年的趾骨。通过比较阿尔泰山脉尼安德特人的基因组和现代人类的DNA，确认了混种繁殖的存在。

Recently, the researchers harvested more DNA from European Neanderthal fossils, putting together very detailed reconstructions of a single chromosome, Chromosome 21.

近期，研究者从欧洲尼安德特人化石上收获了更多DNA信息，重构出非常详尽的单条染色体——第21号染色体。

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.

马克斯·普朗克研究所的遗传学家塞尔吉·卡斯特利亚诺(Sergi Castellano)着手比较了欧洲尼安德特人的DNA和西伯利亚尼安德特人的基因。让他感兴趣的是，尼安德特人群体相互隔离的程度，以及他们与当今人类的关联。

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.

他邀请了西佩尔博士加入。在过去数年里，西佩尔构建出强大的统计学模型来追踪DNA如何随时间而演化。于是，他和同事能够比较这些基因组，推断他们共同的历史：比如，他们的祖先是如何分化的，祖先的人口规模又有多大。

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.

西佩尔、卡斯特利亚诺及同事利用这些模型分析了尼安德特人和人类的DNA。他们的分析结果证实了之前的研究：尼安德特人的DNA片段的确出现在了现代欧亚人的身上。

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.

不过，他们还发现了“基因流动”的又一个例子，而且是个出人意料的例子：具体来说，是阿尔泰山脉的尼安德特人与今天的非洲人有一些同样的突变，而不是欧亚人。

That pattern suggests that an African lineage of humans interbred with the ancestors of the Altai Neanderthal after they split from other Neanderthals.

这意味着，有一支非洲人类，在阿尔泰山脉尼安德特人的祖先与其他尼安德特人分开之后，与其进行过混种繁殖。

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.

卡斯特利亚诺和西佩尔起初以为是自己的分析出了错，于是回头去修正。但他们却没有找到问题。

“We poked and prodded and poked and prodded, and couldn’t get it to go away,” said Dr. Siepel.

“我们这里看看，那里瞧瞧，就是没有办法让它消失，”西佩尔说。

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.

一旦科学家们接受了混种繁殖的情况真实存在，他们就开始估算它发生的时间。DNA在代际遗传的时候，会进行重组，而这就可以用来建立某种时间表。

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.

基于这种基因重组，科学家们估算出，人类与阿尔泰山脉尼安德特人的祖先在大约10万年前进行过混种繁殖——远在普遍认为的人类离开非洲的时间之前。“这一观测结果给我们的工作造成了很大的麻烦，”西佩尔说。

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.

同为这项新研究作者的多伦多大学(University of Toronto)古生物学家本斯·维奥拉(Bence Viola)表示，刚听到这些基因数据的消息时，他本人满腹狐疑。“实在是奇怪，这就是我的主要印象，”他说。

Eventually, however, Dr. Castellano persuaded him the data were strong, and Dr. Viola looked over the fossil record for possible explanations for the interbreeding.

不过，卡斯特利亚诺最终说服他相信数据是可靠的。于是维奥拉博士查看了或许可以解释这一混种繁殖的化石记录。

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.

比方说，上世纪30年代，科学家们在以色列发现了一批12万年前的化石，与现代人类有诸多相似之处。许多科学家认为，它们代表了人类走出非洲的一次失败尝试，与今天的人类没有基因联系。

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，中国研究者在一处洞穴发现了另一个有意思的线索：47颗牙齿。这些科学家估算牙齿有8到12万年历史，并提出它们属于现代人类。

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.

中国科学家的研究并未完全说服维奥拉。他认为，能与证据吻合得最好的情景是，扩散到中东地区的人类与当地的尼安德特人进行了混种繁殖。然后尼安德特人东迁到西伯利亚地区，身上带着人类的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.”

“看起来很有说服力，”没有参与这项研究的斯坦福大学(Stanford University)遗传学家乔纳森·K·普里查德(Jonathan K. Pritchard)这样评价道。“总体而言，他们提出的这套说辞比较条理分明。”

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，对古DNA的更多研究将有助于解答这项新研究提出的许多问题，并帮助科学家理解尼安德特人与人类之间亲密的相互来往的范围。

“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.”

“很快就会出现许许多多的新数据，”他说。“我认为，在接下来的几年里，会有多得多的碎片冒出来，也就有可能把它们都拼接起来。”