Mysterious Ancient Human Found on the ‘Roof of the World’“世界屋脊”上发现神秘古人类

马娅·韦–哈斯 译/左连凯 审订/木子

The mouth of Baishiya Karst Cave nestles near the base of a towering crag at the edge of the Tibetan Plateau. Strands of colorful prayer flags crisscross the pale face of the hollow. Within the cave’s cool confines in 1980， a local monk happened on something unexpected： a jaw with two huge teeth that， while human， was definitely not like that of humans today.

A study published in Nature reveals just how much this ancient jaw has to say. A detailed analysis of its physical features as well as proteins extracted from the fossil suggest that the mandible， dated to 160，000 years ago， comes from the enigmatic human population known as the Denisovans—a sister group to the Neanderthals previously identified from scant remains found in a single cave in Siberia’s Altai Mountains.

“I just couldn’t believe that at the moment [my colleagues] told me，” says study co-author Dongju Zhang of Lanzhou University. “I was really excited.” Zhang’s excitement is justified： the jawbone， known as the Xiahe mandible after the county where it was found， fills a yawning gap in our understanding of this mysterious ancient. While the previous Denisovan fragments come only from the eponymous Denisova Cave in Siberia， people living all across Asia and Australasia today carry Denisovan DNA in their genomes. The discovery of the Xiahe jawbone at a locale over 1，400 miles away from this Siberian cave confirms Denisovans ventured much further across the continent.

It’s thought that after the Denisovans’ ancestors split from their Neanderthal relatives at least 400，000 years ago， they headed east into Asia， while early Neanderthals spread through Europe and western Asia. Modern humans first left Africa some 200，000 years ago， first as a trickle and then in waves. Eventually they encountered and interbred with Neanderthals in the Middle East. Those who trekked east into Asia likewise mated with the resident Denisovans， who left genetic fingerprints still present in Asians today.

One such Denisovan fingerprint helps modern Sherpas and Tibetans adapt to the thin air on the roof of the world. But until now， evidence of Denisovans was found only at a relatively low altitude： Denisova Cave is just 2，300 feet above sea level. The Xiahe jaw， found at roughly 10，760 feet， is the first physical evidence that Denisovans ventured up into the same altitudes as living humans carrying the genetic adaptation to low oxygen environments. And at 160，000 years old， the jawbone is four times the age of the earliest evidence of human activity in the Tibetan Plateau’s challenging climes， underscoring the remarkable resilience of our ancient relatives.

While the jaw was discovered back in the 80s， researchers only began studying it three decades later. In 2010， Lanzhou University’s Zhang， her newly minted Ph.D. in hand， turned to the strange hominin remains on the urging of her graduate advisor Fahu Chen， who led the recent study， and colleague Guangrong Dong also of Lanzhou University.

Excavations in the cave subsequently revealed large animal bones with cut marks and stone tools. Research on these artifacts is ongoing， Zhang says， so she can’t yet say whether the Denisovan group represented by the fossil actually made the tools or left their marks in the faunal remains. “It takes us step by step a long time，” Zhang says of the project.

Analysis of the jaw itself turned up even more surprises. Its morphology suggests it is neither Homo erectus nor Homo sapiens， whose remains are widespread across mainland Asia. The shape of its row of teeth， for instance， was not elongated， as they are in H erectus. And the jaw lacks a chin—a unique trait of modern humans. Most telling of all was the sheer size of the teeth， which are similar to those from Denisova Cave bearing Denisovan DNA.

For confirmation， the researchers first attempted to extract DNA from the fossil jaw. When the analysis revealed that the ancient DNA had degraded， they turned to a more durable， if less sensitive， molecular tool： the proteins built from DNA codes. The researchers extracted proteins from both the jawbone and the tooth dentine. They then used an enzyme to cut the proteins into short strands to identify the amino acid building blocks encoded by DNA. Overall， the dentine proteins appeared much closer to the Altai Denisovans than to Neanderthals or modern humans， says Frido Welker of the University of Copenhagen， who specializes in ancient hominin proteins and led this phase of the work.

There are， however， limits to what can be said from proteins. Denisovans had a stunning amount of diversity. A study published earlier this year suggested that what we call Denisovans might actually be three distinct genetic lines， one of which is nearly as different from other Denisovans as they are from Neanderthals. But the similarity of proteins across groups and through generations makes it difficult to pinpoint precisely how similar the owner of the jaw is to these three Denisovan lines—or if it was from yet another sister group.

The tantalizing connection the jaw makes between what was once thought of as a low altitude group of humans and their mysterious role in modern high-altitude adaptations also remains fuzzy， explains Emilia Huerta-Sanchez， a population geneticist at Brown University and lead author of the 2014 Nature study that first identified this genetic link. “I agree with the authors that it could be that this hominin group was high altitude adapted，” she says. “But I don’t think we know for certain.”

Huerta-Sanchez explains that the genetic tweaks thought to help modern Tibetans thrive in low-oxygen environments are not part of a sequence that codes for proteins， but instead control how much of a particular protein is made. While the jaw was found where oxygen levels are low， without the DNA itself， scientists can’t be sure the jaw’s owner carried the adaptation to survive in that thin air.

While there are still many unknowns， scientists are excited about what other clues the jaw may hold for understanding human evolution in Asia. The fossil could for instance be used to identify other Denisovans from the growing pool of hominin fossils in Asia that don’t neatly fit into the known branches of our increasingly bushy family tree， says María  Martinón-Torres， who has extensively studied the Asian fossil record. For example， a three-rooted molar in the jaw is similar to that of a tooth in a previously described mandible known as Penghu 1， hinting that it， too， may be a Denisovan.

白石崖溶洞的入口靠近青藏高原边缘高耸的悬崖底部。一串串五色经幡交错于白色洞壁上。1980年，一位当地僧侣在这个凉快的岩洞里，偶然发现了一件意想不到的物品：长着两颗大牙的颌骨，虽属人类但肯定不像现代人那样。

发表在《自然》杂志上的研究揭示了这块古颌骨的深刻含义。对其外形特征和从化石中提取的蛋白质的详细分析表明，这块16万年前的下颌骨源于神秘的丹尼索瓦人。此前从西伯利亚阿尔泰山脉的一个岩洞里发现了珍稀遗骨，经鉴定，确认其为尼安德特人，是丹尼索瓦人的姐妹群。

“我同事講这事的时候，我简直不敢相信。”本研究报告共同作者、兰州大学副教授张东菊说，“我真的很激动。”张激动得有道理：该下颌骨因其出土地夏河县而被叫做夏河颌骨，它填补了我们认识这种神秘古人类的巨大空白。虽然早前只在因丹尼索瓦人而得名的西伯利亚岩洞里发现了其骨骼碎片，但当今生活在亚洲和澳大拉西亚各地人们的基因组都携带着丹尼索瓦人的DNA。在相距上述西伯利亚岩洞1400英里远的地方发现夏河颌骨，这证实了丹尼索瓦人在这片大陆上进一步冒险远涉。

有人认为，至少40万年前，丹尼索瓦人的祖先与其亲戚尼安德特人分开以后，向东进入亚洲，而早期尼安德特人分散至欧洲和西亚。大约20万年前，现代人首次走出非洲，起初三三两两，后来成群结队。他们最终在中东与尼安德特人相遇并生息繁衍。同样，向东跋涉至亚洲的现代人与定居的丹尼索瓦人共同繁衍，从而留下仍然存在于当代亚洲人身上的丹尼索瓦人基因指纹。

其中一个丹尼索瓦人基因指纹帮助现代夏尔巴人和西藏人适应世界屋脊上的稀薄空气。但直到现在，只在较低海拔处发现了有关丹尼索瓦人的证据：丹尼索瓦洞的海拔仅2300英尺。在海拔约10760英尺处发现的夏河颌骨，首次实际证明携带低氧适应遗传基因的丹尼索瓦人登上了这一海拔。该颌骨具有16万年历史，比人类最早在青藏高原严峻气候下活动的历史高出四倍，这进一步说明我们的远古亲戚具有非凡的适应力。

虽然该颌骨发现于1980年代，但有关研究仅始于30年后。2010年，兰州大学的张东菊刚拿到博士学位，就在导师陈发虎（该研究项目负责人）及同事董光荣的鞭策下，投身研究这一陌生的原始人遗骨。

后来，在溶洞发掘出带有切削痕迹的大块动物骨头和石制工具。张说，关于这些手工制品的研究正在进行，所以她还无法确定，该化石所代表的丹尼索瓦人群是否真的会制造工具，也不能确定动物遗骸上的痕迹是他们留下的。“我们要逐步进行长期的研究。”张在谈到该项目时说道。

对夏河颌骨本身的分析甚至更令人吃惊。其结构形态说明它既不属于直立人也不属于智人，二者的遗骸广泛分布在亚洲大陆各地。例如，其牙齿排列的形状不是直立人那样的长条型，并且该颌骨没有下巴，而下巴是现代人独一无二的特征。最有说服力的是其巨大的牙齿，与出自丹尼索瓦洞、携带丹尼索瓦人DNA的那些牙齿尺寸相似。

为了确认，研究人员首先尝试提取化石颌骨的DNA。但分析表明，古老的DNA已降解，他们转而利用虽不敏感却更持久的分子：由DNA代码组成的蛋白质。他们从颌骨和牙质中提取蛋白质，然后用酶将蛋白质切割成短串，以找到由DNA编码的氨基酸成分。哥本哈根大学的弗里多·韦尔克专门从事古人类蛋白质的研究并领导了这一阶段的工作，他认为，总体来看，与尼安德特人或现代人相比，该颌骨的牙质蛋白似乎更接近阿尔泰地区的丹尼索瓦人。

但是，蛋白质所包含的信息是有限的。丹尼索瓦人拥有惊人的多样性。今年早些时候发表的一项研究表明，所谓的丹尼索瓦人其实可能有三个不同的遗传谱系，其中一个谱系几乎不同于其他丹尼索瓦人，就像他们不同于尼安德特人一样。但种群之间和代际之间的蛋白质相似性，使得很难精确说明该颌骨的主人与这三个谱系的丹尼索瓦人有多相似，抑或它是否属于另一姐妹群。

布朗大学的群体遗传学家埃米莉娅·休尔塔–桑切斯解释说，该颌骨揭示出，曾被视为生活于低海拔的人群与其现代高海拔适应基因的未知影响之间存在联系，但这一似有若无的联系仍不清晰。她带队首次发现这种遗传联系，并于2014年将研究成果发表在《自然》杂志上。“我和其他作者都认为，很可能是这个原始人群体具有高海拔适应基因。”她说，“但我们不很确定。”

休尔塔-桑切斯解释说，被认为有助于现代西藏人在低氧环境下繁衍生息的基因变化，不存在于编码蛋白质的序列内，但会控制产生特定蛋白质的数量。虽然发现颌骨的地方氧气稀薄，但没有DNA，科学家无法确定，颌骨的主人是否具有在稀薄空气里生存的适应基因。

虽然仍有很多未解之处，但该颌骨对认识亚洲人的进化还有什么启示令科学家激动不已。例如，由于人类谱系不断扩大，越来越多的亚洲原始人化石不能与谱系的已知分支完全对应，该化石便可用来鉴别其中的丹尼索瓦人，深入研究亚洲人化石记录的玛丽亚·马丁农-托雷斯说道。举例来说，该颌骨上的一颗三根臼齿与以前发现的叫做“澎湖1”的颌骨上的牙齿相似，说明“澎湖1”可能属于丹尼索瓦人。