重新修改現代人散佈至歐亞大陸各處的故事

原文網址：http://www.shh.mpg.de/742617/human-dispersals-africa

重新修改現代人散佈至歐亞大陸各處的故事

日新月異的科技和跨領域研究團隊正在重新塑造我們認知中人類離開非洲大陸的方式和時間――以及他們沿途遇到了什麼樣的人們。

大多數人現在都對傳統的「出非洲說」(“Out of Africa” model)耳熟能詳：在非洲演化出的現代人大約在6萬年前發動了一波離開非洲的移民潮，之後往亞洲各地散佈並抵達澳洲。然而，DNA分析和其他化石辨認技術的進步，加上備受重視的跨領域研究成果，使得研究人員開始重新修訂這則故事。最近的研究指出早於6萬年前人類就已經數度離開過非洲，而且還和歐亞大陸許多處的其他人族雜交繁殖。

德國馬克斯普朗克人類歷史科學研究所和美國夏威夷馬諾大學的研究人員，在回顧關於早期現代人從非洲散佈至亞洲的近期研究之後，認定解剖學上的現代人從非洲散佈出去的事件僅在6萬年前左右發生過一次的傳統觀點，已經無法再當作故事的全貌。此篇發表於期刊《科學》( Science)的分析結果中，回顧了過去十年來在亞洲地區因為技術進步和跨領域合作而得以進行的大量新研究，顯示智人到達亞洲大陸偏遠地區以及近大洋洲的時間比過去認為的早上許多。此外，現代人曾與其他早就生存在亞洲的人族，像是尼安德塔人和丹尼索瓦人雜交的證據，也讓我們這個物種的演化史變得更加複雜。

新模型：現代人曾多次從非洲散佈出去，開始時間可以追溯至12萬年前

作者集結了多篇近期研究做出的發現，以更加精確地描繪人類從非洲散佈至亞洲的整體圖像。雖然科學家一度認為人類首度也是唯一一次離開非洲的移民潮發生在6萬年前左右，近期研究卻在亞洲的遠方疆域找到了年代可能古老許多的現代人化石。舉例來說，在中國南方和中部定年結果為7萬至12萬年前的多處遺址中有發現到智人的遺骸。其他發現則指出現代人在早於6萬年前就已經到達東南亞和澳洲。

然而，其他近期研究確實也證明今日所有非洲人以外的族群，都是從一個大約在6萬年前離開非洲的祖先族群分支出來。這可能意謂著早在12萬年前就開始有好幾波規模較小的移民從非洲散佈出去，之後在6萬年前發生了大型散佈事件。雖然近期散佈出去的族群提供了今日非洲人以外族群的整體基因組成，較早移民的痕跡仍然清晰可見。

馬克斯普朗克人類歷史科學研究所的Michael Petraglia解釋：「早於6萬年前最初幾波離開非洲的遷徙事件可能是由採集者小團體組成，而這些早期散佈事件中至少有些在今日人類族群的基因中留下了少許痕跡。接下來才發生的大型『出非洲』事件，時間最有可能是在6萬年前左右或者更晚。」

多次雜交事件

近期的基因研究已經解決了現代人是否有跟古代其他人族雜交的問題，答案是肯定的。現代人不只有跟尼安德塔人雜交過，還有最近發現的智人近親―—丹尼索瓦人，以及另外一種目前尚未辨明的前現代人族。一項研究估計現今所有非洲人以外的人類皆有1-4%的尼安德塔人血統；另一個團隊則估計現在的美拉尼西亞人平均有5%的尼安德塔人血統。總之，目前已經可以確定現代人、尼安德塔人、丹尼索瓦人，或許還有其他人族群落，在亞洲的生存年代與活動範圍可能有所重疊，而他們之間的互動頻率想必非常頻繁。

人族之間彼此交流的證據越來越多也代表物質文化的傳播過程其實比過往認為得還要更加複雜。夏威夷馬諾大學的Christopher Bae表示：「我們從行為方面的紀錄確實可以看出所謂的『現代人行為』並非是單純地隨著時間進展從西往東傳。反之，還需要同時考量到各地生態系的差異，以及晚更新世分布在亞洲各處不同人族群落之間的行為差異。」

從這些最新發現我們已經開始瞭解到人類在整個舊世界移動的足跡比我們之前認為的還要複雜許多，而且還有許多問題尚待解決。作者主張要發展出更為複雜的模型來解釋人類的擴張過程，並且要在亞洲多處迄今仍未進行研究的地方展開新的研究工作。此外，重新審視在現代分析方法發展出來以前採集到的樣品也很重要，或許我們能藉此溫故知新。同為馬克斯普朗克人類歷史科學研究所的Katerina Douka表示：「幸運的是過去數十年來亞洲發起了越來越多跨領域研究計畫。未來這些研究發表的資訊將有助於填補演化紀錄中的空缺。」

「能在這個時刻參與亞洲各地的跨領域研究計畫是再令人興奮不過了。」Bae補充。

Revising the story of the dispersal of modern humans across Eurasia

Technological advances and multidisciplinary research teams are reshaping our understanding of when and how humans left Africa -- and who they met along the way.

Most people are now familiar with the traditional “Out of Africa” model: modern humans evolved in Africa and then dispersed across Asia and reached Australia in a single wave about 60,000 years ago. However, technological advances in DNA analysis and other fossil identification techniques, as well as an emphasis on multidisciplinary research, are revising this story. Recent discoveries show that humans left Africa multiple times prior to 60,000 years ago, and that they interbred with other hominins in many locations across Eurasia.

A review of recent research on dispersals by early modern humans from Africa to Asia by researchers from the Max Planck Institute for the Science of Human History and the University of Hawai’i at Manoa confirms that the traditional view of a single dispersal of anatomically modern humans out of Africa around 60,000 years ago can no longer be seen as the full story. The analysis, published in the journal Science, reviews the plethora of new discoveries being reported from Asia over the past decade, which were made possible by technological advances and interdisciplinary collaborations, and shows that Homo sapiens reached distant parts of the Asian continent, as well as Near Oceania, much earlier than previously thought. Additionally, evidence that modern humans interbred with other hominins already present in Asia, such as Neanderthals and Denisovans, complicates the evolutionary history of our species.

New model: Multiple dispersals of modern humans out of Africa, beginning as early as 120,000 years ago

The authors brought together findings from multiple recent studies to refine the picture of human dispersals out of Africa and into Asia. While scientists once thought that humans first left Africa in a single wave of migration about 60,000 years ago, recent studies have identified modern human fossils in far reaches of Asia that are potentially much older. For example, H. sapiens remains have been found at multiple sites in southern and central China that have been dated to between 70,000 and 120,000 years ago. Additional finds indicate that modern humans reached Southeast Asia and Australia prior to 60,000 years ago.

However, other recent studies do confirm that all present-day non-African populations branched off from a single ancestral population in Africa approximately 60,000 years ago. This could indicate that there were multiple, smaller dispersals of humans out of Africa beginning as early as 120,000 years ago, followed by a major dispersal 60,000 years ago. While the recent dispersal contributed the bulk of the genetic make-up of present-day non-Africans, the earlier dispersals are still evident.

“The initial dispersals out of Africa prior to 60,000 years ago were likely by small groups of foragers, and at least some of these early dispersals left low-level genetic traces in modern human populations. A later, major ‘Out of Africa’ event most likely occurred around 60,000 years ago or thereafter,” explains Michael Petraglia of the Max Planck Institute for the Science of Human History.

Multiple interbreeding events

Recent genetic research has resolved the question of whether or not modern humans interbred with other ancient hominins – they definitely did. Modern humans interbred not only with Neanderthals, but also with our recently-discovered relatives the Denisovans, as well as a currently unidentified population of pre-modern hominins. One estimate is that all present-day non-Africans have 1-4% Neanderthal heritage, while another group has estimated that modern Melanesians have an average of 5% Denisovan heritage. In all, it is now clear that modern humans, Neanderthals, Denisovans and perhaps other hominin groups likely overlapped in time and space in Asia, and they certainly had many instances of interaction.

The increasing evidence of interactions suggests that the spread of material culture is also more complicated than previously thought. “Indeed, what we are seeing in the behavioral record is that the spread of so-called modern human behaviors did not occur in a simple time-transgressive process from west to east. Rather, ecological variation needs to be considered in concert with behavioral variation between the different hominin populations present in Asia during the Late Pleistocene,” explains Christopher Bae of the University of Hawai’i at Manoa.

In light of these new discoveries, our understanding of human movements across the Old World has become much more complex, and there are still many questions left open. The authors argue for the development of more complicated models of human dispersals and for conducting new research in the many areas of Asia where none has been done to date. Additionally, it will be important to review materials collected prior to the development of modern analytic methods, to see what more can now be learned from them. “Fortunately,” states Katerina Douka, also of the Max Planck Institute for the Science of Human History, “there have been an increasing number of multidisciplinary research programs launched in Asia over the past few decades. The information that is being reported is helping to fill in the gaps in the evolutionary records.”

“It is an exciting time to be involved with interdisciplinary research projects across Asia,” adds Bae.

原始論文：Christopher J. Bae, Katerina Douka, Michael D. Petraglia. On the origin of modern humans: Asian perspectives. Science, 2017; 358 (6368): eaai9067 DOI: 10.1126/science.aai9067

引用自：Max Planck Institute for the Science of Human History. "Revising the story of the dispersal of modern humans across Eurasia: Technological advances and multidisciplinary research teams are reshaping our understanding of when and how humans left Africa -- and who they met along the way."