Mysterious Ancient Human Found on the ‘Roof of the World’“世界屋脊”上發現神秘古人類

馬婭·韋–哈斯 譯/左連凱 審訂/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邊緣高聳的懸崖底部。一串串五色經幡交錯于白色洞壁上。1980年，一位當地僧侶在這個涼快的巖洞里，偶然發現了一件意想不到的物品：長著兩顆大牙的頜骨，雖屬人類但肯定不像現代人那樣。

發表在《自然》雜志上的研究揭示了這塊古頜骨的深刻含義。對其外形特征和從化石中提取的蛋白質的詳細分析表明，這塊16萬年前的下頜骨源于神秘的丹尼索瓦人。此前從西伯利亞阿爾泰山脈的一個巖洞里發現了珍稀遺骨，經鑒定，確認其為尼安德特人，是丹尼索瓦人的姐妹群。

“我同事講這事的時候，我簡直不敢相信。”本研究報告共同作者、蘭州大學副教授張東菊說，“我真的很激動。”張激動得有道理：該下頜骨因其出土地夏河縣而被叫做夏河頜骨，它填補了我們認識這種神秘古人類的巨大空白。雖然早前只在因丹尼索瓦人而得名的西伯利亞巖洞里發現了其骨骼碎片，但當今生活在亞洲和澳大拉西亞各地人們的基因組都攜帶著丹尼索瓦人的DNA。在相距上述西伯利亞巖洞1400英里遠的地方發現夏河頜骨，這證實了丹尼索瓦人在這片大陸上進一步冒險遠涉。

有人認為，至少40萬年前，丹尼索瓦人的祖先與其親戚尼安德特人分開以后，向東進入亞洲，而早期尼安德特人分散至歐洲和西亞。大約20萬年前，現代人首次走出非洲，起初三三兩兩，后來成群結隊。他們最終在中東與尼安德特人相遇并生息繁衍。同樣，向東跋涉至亞洲的現代人與定居的丹尼索瓦人共同繁衍，從而留下仍然存在于當代亞洲人身上的丹尼索瓦人基因指紋。

其中一個丹尼索瓦人基因指紋幫助現代夏爾巴人和西藏人適應世界屋脊上的稀薄空氣。但直到現在，只在較低海拔處發現了有關丹尼索瓦人的證據：丹尼索瓦洞的海拔僅2300英尺。在海拔約10760英尺處發現的夏河頜骨，首次實際證明攜帶低氧適應遺傳基因的丹尼索瓦人登上了這一海拔。該頜骨具有16萬年歷史，比人類最早在青藏高原嚴峻氣候下活動的歷史高出四倍，這進一步說明我們的遠古親戚具有非凡的適應力。

雖然該頜骨發現于1980年代，但有關研究僅始于30年后。2010年，蘭州大學的張東菊剛拿到博士學位，就在導師陳發虎（該研究項目負責人）及同事董光榮的鞭策下，投身研究這一陌生的原始人遺骨。

后來，在溶洞發掘出帶有切削痕跡的大塊動物骨頭和石制工具。張說，關于這些手工制品的研究正在進行，所以她還無法確定，該化石所代表的丹尼索瓦人群是否真的會制造工具，也不能確定動物遺骸上的痕跡是他們留下的。“我們要逐步進行長期的研究。”張在談到該項目時說道。

對夏河頜骨本身的分析甚至更令人吃驚。其結構形態說明它既不屬于直立人也不屬于智人，二者的遺骸廣泛分布在亞洲大陸各地。例如，其牙齒排列的形狀不是直立人那樣的長條型，并且該頜骨沒有下巴，而下巴是現代人獨一無二的特征。最有說服力的是其巨大的牙齒，與出自丹尼索瓦洞、攜帶丹尼索瓦人DNA的那些牙齒尺寸相似。

為了確認，研究人員首先嘗試提取化石頜骨的DNA。但分析表明，古老的DNA已降解，他們轉而利用雖不敏感卻更持久的分子：由DNA代碼組成的蛋白質。他們從頜骨和牙質中提取蛋白質，然后用酶將蛋白質切割成短串，以找到由DNA編碼的氨基酸成分。哥本哈根大學的弗里多·韋爾克專門從事古人類蛋白質的研究并領導了這一階段的工作，他認為，總體來看，與尼安德特人或現代人相比，該頜骨的牙質蛋白似乎更接近阿爾泰地區的丹尼索瓦人。

但是，蛋白質所包含的信息是有限的。丹尼索瓦人擁有驚人的多樣性。今年早些時候發表的一項研究表明，所謂的丹尼索瓦人其實可能有三個不同的遺傳譜系，其中一個譜系幾乎不同于其他丹尼索瓦人，就像他們不同于尼安德特人一樣。但種群之間和代際之間的蛋白質相似性，使得很難精確說明該頜骨的主人與這三個譜系的丹尼索瓦人有多相似，抑或它是否屬于另一姐妹群。

布朗大學的群體遺傳學家埃米莉婭·休爾塔–桑切斯解釋說，該頜骨揭示出，曾被視為生活于低海拔的人群與其現代高海拔適應基因的未知影響之間存在聯系，但這一似有若無的聯系仍不清晰。她帶隊首次發現這種遺傳聯系，并于2014年將研究成果發表在《自然》雜志上。“我和其他作者都認為，很可能是這個原始人群體具有高海拔適應基因。”她說，“但我們不很確定。”

休爾塔-桑切斯解釋說，被認為有助于現代西藏人在低氧環境下繁衍生息的基因變化，不存在于編碼蛋白質的序列內，但會控制產生特定蛋白質的數量。雖然發現頜骨的地方氧氣稀薄，但沒有DNA，科學家無法確定，頜骨的主人是否具有在稀薄空氣里生存的適應基因。

雖然仍有很多未解之處，但該頜骨對認識亞洲人的進化還有什么啟示令科學家激動不已。例如，由于人類譜系不斷擴大，越來越多的亞洲原始人化石不能與譜系的已知分支完全對應，該化石便可用來鑒別其中的丹尼索瓦人，深入研究亞洲人化石記錄的瑪麗亞·馬丁農-托雷斯說道。舉例來說，該頜骨上的一顆三根臼齒與以前發現的叫做“澎湖1”的頜骨上的牙齒相似，說明“澎湖1”可能屬于丹尼索瓦人。