火星並不像我們所見的如此乾涸

原文網址：http://www.ox.ac.uk/news/2017-12-20-mars-not-dry-it-seems

火星並不像我們所見的如此乾涸

兩篇英國牛津大學的新研究闡明了火星為何沒有生命

科學家搜索生命時的首要任務便是尋找維持生命的要素之一：淡水。

雖然現在火星的表面一片貧瘠、寒冷而不適合生命居住，但有一絲證據指出火星過去是一顆較為溫暖濕潤且表面有流水的星球。究竟發生了什麼事情讓火星上的水消失？這項謎題長久以來仍然懸而未解。發表在《自然》的最新研究提出這些水分現今是被封存在火星的岩石當中。

牛津大學地球科學系的科學家提出火星表面的岩石跟水反應後將水給吸收了。這個過程造成岩石的氧化程度增加，也讓火星變得不適合生命居住。

過往研究曾提出由於火星磁場的衰敗造成大部分的水分往外太空流失，當時水分被強烈的太陽風帶走，或者是以冰塊的形式封存於地底。然而，這些理論並無法解釋為何所有水分都消失了。

由牛津大學地球科學系的NERC(自然環境研究委員會)研究員Jon Wade博士領導的團隊確信這道難題的解答就藏在火星的礦物裡面。他們運用平常分析地球岩石組成的模擬方法，來計算火星表面的水分有多少可以透過岩石跟水之間的反應而流失。團隊評估了岩石溫度、地底壓力和火星整體組成對行星表面會有什麼樣的影響。

研究指出火星的玄武岩所能蘊含的水分比地球的玄武岩還多了將近25%，結果就是火星表面的水分有許多都被岩石吸收到火星內部。

Wade博士表示：「雖然科學家從很久以前就一直在思索這項問題，但卻從來沒有人去驗證水單純是因為跟岩石之間發生了反應而被吸收掉的理論。我們將現有的零星證據拼湊之後得到的結果，使我們開始相信要讓火星地函氧化必定需要一種(跟地球)不同的作用。舉例來說，火星隕石相較於其表面的岩石來說在化學上較為還原，而且它們的成分看起來也相差甚遠。這些現象的其中一個原因，以及火星為何會喪失所有水分的答案，可能都跟它的礦物學有關。」

「地球目前的板塊構造系統可以防止地表水的水位發生劇烈變化，這是因為含水的岩石在進到地球相對乾燥的地函以前會充分脫水的緣故。但早期地球和火星都沒有這種可以讓水循環的系統。在火星，形成玄武岩地殼的新鮮岩漿噴發出來時就會跟水反應，造成的效果就跟海綿一樣。火星的水跟岩石反應後形成了許多種類的含水礦物。這種水和岩石之間的反應不但改變了岩石本身的礦物組成，也讓火星的地表逐漸乾涸而變得不適合生物居住。」

那地球為什麼從未經歷此種變化？Wade博士表示：「火星的體積比地球小上許多，而且它的矽質地函的溫度曲線跟地球不同，含鐵量也比較高。雖然這些差異相當微小，但隨著時間經過卻會累積成為重大影響，使得火星表面更容易跟地表水發生反應並形成含水礦物。火星的地球化學因為這些因素自然會將水分帶往地函內部，然而早期地球的含水岩石卻傾向於在脫水之前持續浮在地函上方。」

Wade博士論文中的主旨――行星的組成基本上決定了其未來的適居性――呼應了另一篇同樣發表在《自然》探討地球鹽度的新研究。由牛津大學地球科學系的教授Chris Ballentine共同著作的研究中，顯示要讓生命形成並繼續生存下去，地球的鹵素(氯、溴、碘)濃度必須要恰到好處才行，太高或者太低都會讓地球變成不毛之地。過往研究對隕石鹵素濃度的估算得出相當高的數值。相較於組成地球的隕石樣品，它們的含鹽比例對地球來說實在是太高了。

有許多科學家提出理論來解釋此種差異是如何形成。這兩篇研究結合起來提升了證據的可信度，同時也支持進一步研究的必要。Wade博士表示：「太陽系內行星的成分大體來說相去不遠，但是差之毫釐卻會失之千里――岩石化學便是一個例子。火星跟地球岩石化學的最大差異是火星地函岩石的鐵多了一些，其來自於火星形成時的環境稍微偏向氧化條件。」

雖然我們知道火星一度擁有水且有機會讓生命繼續發展下去，但是我們對於其他行星的狀況實在所知不多，而研究團隊正在努力改變這種情況。

Wade博士表示：「要發展這項研究我們想先測試其他敏感因子對各個行星造成的影響――比方說我們所知甚少的金星。我們的問題像是：如果地球地函的鐵含量更多或者更少，會對環境造成什麼樣的變化？那如果地球的體積較大或者較小呢？這些答案有助於我們了解岩石化學對一顆行星的宿命會有多大程度的影響。」

「在其他星球上尋找生命時不能只看它的總體化學組成是否合宜，其他像是行星如何組合的細節也會深深影響到其表面是否擁有水分。但是對於其他星球來說，這些細節造成的影響及其後果卻還沒有被真正地好好探討過。」

Mars: Not as dry as it seems

Two new Oxford University papers have shed light on why there is no life on Mars.

When searching for life, scientists first look for an element key to sustaining it: fresh water.

Although today’s Martian surface is barren, frozen and inhabitable, a trail of evidence points to a once warmer, wetter planet, where water flowed freely. The conundrum of what happened to this water is long standing and unsolved. However, new research published in Nature suggests that this water is now locked in the Martian rocks.

Scientists at Oxford’s Department of Earth Sciences, propose that the Martian surface reacted with the water and then absorbed it, increasing the rocks oxidation in the process, making the planet uninhabitable.

Previous research has suggested that the majority of the water was lost to space as a result of the collapse of the planet’s magnetic field, when it was either swept away by high intensity solar winds or locked up as sub-surface ice. However, these theories do not explain where all of the water has gone.

Convinced that the planet’s minerology held the answer to this puzzling question, a team led by Dr Jon Wade, NERC Research Fellow in Oxford’s Department of Earth Sciences, applied modelling methods used to understand the composition of Earth rocks to calculate how much water could be removed from the Martian surface through reactions with rock. The team assessed the role that rock temperature, sub-surface pressure and general Martian make-up, have on the planetary surfaces.

The results revealed that the basalt rocks on Mars can hold approximately 25 per cent more water than those on Earth, and as a result drew the water from the Martian surface into its interior.

Dr Wade said: ‘People have thought about this question for a long time, but never tested the theory of the water being absorbed as a result of simple rock reactions. There are pockets of evidence that together, leads us to believe that a different reaction is needed to oxidise the Martian mantle. For instance, Martian meteorites are chemically reduced compared to the surface rocks, and compositionally look very different. One reason for this, and why Mars lost all of its water, could be in its minerology.

‘The Earth’s current system of plate tectonics prevents drastic changes in surface water levels, with wet rocks efficiently dehydrating before they enter the Earth’s relatively dry mantle. But neither early Earth nor Mars had this system of recycling water. On Mars, (water reacting with the freshly erupted lavas’ that form its basaltic crust, resulted in a sponge-like effect. The planet’s water then reacted with the rocks to form a variety of water bearing minerals. This water-rock reaction changed the rock mineralogy and caused the planetary surface to dry and become inhospitable to life.’

As to the question of why Earth has never experienced these changes, he said: ‘Mars is much smaller than Earth, with a different temperature profile and higher iron content of its silicate mantle. These are only subtle distinctions but they cause significant effects that, over time, add up. They made the surface of Mars more prone to reaction with surface water and able to form minerals that contain water. Because of these factors the planet’s geological chemistry naturally drags water down into the mantle, whereas on early Earth hydrated rocks tended to float until they dehydrate.’

The overarching message of Dr Wade’s paper, that planetary composition sets the tone for future habitability, is echoed in new research also published in Nature, examining the Earth’s salt levels. Co-written by Professor Chris Ballentine of Oxford’s Department of Earth Sciences, the research reveals that for life to form and be sustainable, the Earth’s halogen levels (Chlorine, Bromine and Iodine) have to be just right. Too much or too little could cause sterilisation. Previous studies have suggested that halogen level estimates in meteorites were too high. Compared to samples of the meteorites that formed the Earth, the ratio of salt to Earth is just too high.

Many theories have been put forward to explain the mystery of how this variation occurred, however, the two studies combined elevate the evidence and support a case for further investigation. Dr Wade said ‘Broadly speaking the inner planets in the solar system have similar composition, but subtle differences can cause dramatic differences – for example, rock chemistry. The biggest difference being, that Mars has more iron in its mantle rocks, as the planet formed under marginally more oxidising conditions.’

We know that Mars once had water, and the potential to sustain life, but by comparison little is known about the other planets, and the team are keen to change that.

Dr Wade said: ‘To build on this work we want to test the effects of other sensitivities across the planets – very little is known about Venus for example. Questions like; what if the Earth had more or less iron in the mantle, how would that change the environment? What if the Earth was bigger or smaller? These answers will help us to understand how much of a role rock chemistry determines a planet’s future fate.

When looking for life on other planets it is not just about having the right bulk chemistry, but also very subtle things like the way the planet is put together, which may have big effects on whether water stays on the surface. These effects and their implications for other planets have not really been explored.’

原始論文：(1) Jon Wade, Brendan Dyck, Richard M. Palin, James D. P. Moore, Andrew J. Smye. The divergent fates of primitive hydrospheric water on Earth and Mars. Nature, 2017; 552 (7685): 391 DOI: 10.1038/nature25031

(2) Patricia L. Clay, Ray Burgess, Henner Busemann, Lorraine Ruzié-Hamilton, Bastian Joachim, James M. D. Day, Christopher J. Ballentine. Halogens in chondritic meteorites and terrestrial accretion. Nature, 2017; 551 (7682): 614 DOI: 10.1038/nature24625

引用自：University of Oxford. "Mars: Not as dry as it seems: Water on Mars absorbed like a sponge, new research suggests."

研究爭議十足的化石指出生命可能在40億年前就已經出現在地球上

原文網址：http://www.sciencemag.org/news/2017/12/life-may-have-originated-earth-4-billion-years-ago-study-controversial-fossils-suggests

研究爭議十足的化石指出生命可能在40億年前就已經出現在地球上

By Elizabeth Pennisi

1992年，當時的研究人員發現了可能是由地球最原始生物留下的跡象：被包覆在澳洲岩石裡年代為35億年的細微曲線。然而，從那時起科學家就持續爭論著這些印痕是否真的代表了遠古微生物；就算是，它們的年代是否真的有這麼久遠。現今一項對這些微體化石的詳盡分析結果認為這些結構確實代表了一群遠古微生物，由於它們可能相當複雜，因此地球上的生命必定要比他們早5億年前左右就已經誕生了。

未參與這項新研究工作的澳洲伯斯科廷大學的地質生物學家Birger Rasmussen表示，研究成果暗示這群原始微生物擁有令人驚訝的複雜程度，它們能夠進行光合作用並利用其他化學作用來獲取能量。同樣沒有參與這項成果的美國堪薩斯大學勞倫斯市校區的地質生物學家Alison Olcott Marshall也表示：「這項研究或許會引發一股風潮，刺激其他研究人員對這些岩石進行更新的研究以找出數據來支持或者反對這項論點。」

美國加州大學洛杉磯分校的William Schopf是發現這些澳洲微體化石的古生物學家。在此新研究中，他和美國威斯康辛大學麥迪遜分校的地質學家John Valley組成了研究團隊。Valley的專長是運用二次離子質譜儀(SIMS)這種分析技術來測定樣品中不同形式的碳之間的比例，藉此他們可以判定樣品是否為有機物。

為了找到具有足夠化石樣品來讓SIMS進行分析的岩石薄片，Schopf在顯微鏡底下工作了4個月之久。最後他找到的樣品中具有11副微體化石，從它們的形狀和大小差異Schopf認為其中有5個不同的微生物種類。同時他也提供了不含這些可能為化石的岩石樣品作為比對。

新的證據支持這些曲線代表了原始生命。Credit: J. William Schopf, UCLA

Schopf、Valle和其他研究人員在今日發表於《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的文章中表示他們對這些材料的分析結果得到了幾種不同的碳同位素比例。其中兩種微體化石的碳同位素比例跟一種現存的細菌一樣，它可以利用陽光來合成進行活動所需的碳化合物――這種原始光合作用並不需要氧氣。另為兩種微體化石的碳同位素比例則跟一種古菌相同，其仰賴甲烷做為它的能量來源――並且在多細胞生物的形成過程中具有重要地位。最後一種微體化石的碳同位素類型則指出這種生物的的代謝過程中會產生甲烷。

Schopf表示有這麼多種不同類型的碳同位素強化了它們確實為化石的論點。如果這些曲線是由任何一種無機作用造成，則應該會留下一致的碳同位素訊號。而在地球歷史中的那個時間點微生物已經如此多樣，也代表地球生命的起源應該可以追溯至距今40億年以前。雖然已經有其他研究人員找到的生命訊號至少可以追溯到如此遙遠以前，但這些發現甚至比Schopf的還要更具爭議性。

Rasmussen同意「新成果更加鞏固了這些細微構造為生物來源的說法」，但他也擔心這些微體化石的保存情況可能相當差勁。Olcott Marshall完全不認為這些岩石中的痕跡是化石，而是一種地質作用的產物。她的評論更加尖銳：「這種分析技術造成的誤差相當大。」因此她認為這些數據並沒有辦法確切證明岩石中含有不同類型的微生物。

但是SIMS的專家則相當讚賞這項成果。「這是個十分縝密且設計良好的實驗。」未參與此研究的美國華盛頓大學西雅圖分校的化學家Lara Gamble表示，「他們相當努力確保所有的數據都有經過嚴格校正。」

Rasmussen希望後續工作會分析更多的微體化石。「由於我們正在關注的這些構造確實有可能是生命留下的最古老痕跡，所以值得我們進行研究來證明它們。」他說，「精進我們辨識地球上遠古生命訊號的技術相當重要，因為我們也正在放眼火星以及更遙遠的星球尋找生命。」

Life may have originated on Earth 4 billion years ago, study of controversial fossils suggests

By Elizabeth Pennisi

In 1992, researchers discovered evidence of what was then potentially the earliest life on Earth: 3.5-billion-year-old microscopic squiggles encased in Australian rocks. Since then, however, scientists have debated whether these imprints truly represent ancient microorganisms, and even if they do, whether they’re really that old. Now, a comprehensive analysis of these microfossils suggests that these formations do indeed represent ancient microbes, ones potentially so complex that life on our planet must have originated some 500 million years earlier.

The new work indicates these early microorganisms were surprisingly sophisticated, capable of photosynthesis and of using other chemical processes to get energy, says Birger Rasmussen, a geobiologist at Curtin University in Perth, Australia, who was not involved with the work. The study “will probably touch off a flurry of new research into these rocks as other researchers look for data that either support or disprove this new assertion,” adds Alison Olcott Marshall, a geobiologist at the University of Kansas in Lawrence who was not involved in the effort.

In the new study, William Schopf, a paleobiologist at the University of California, Los Angeles—and the discoverer of the Australian microfossils—teamed up with John Valley, a geoscientist at the University of Wisconsin in Madison. Valley is an expert in an analytical technique called secondary ion mass spectrometry (SIMS), which can determine the ratio of different forms of carbon in a sample—key to gauging whether it’s organic.

Schopf spent 4 months working with microscopes to find a thin slice of the rock that contains the fossils with specimens accessible enough to study with SIMS; that sample contained 11 microfossils whose diversity of shapes and sizes suggested they represented five species of microbes. He also provided samples of rock containing no putative fossils for comparison.

New evidence supports that these “squiggles” represent early life. Credit: J. William Schopf, UCLA

The analysis detected several distinct carbon ratios in the material, Schopf, Valley, and colleagues report today in the Proceedings of the National Academy of Sciences. Two types of microfossils had the same carbon ratio as modern bacteria that use light to make carbon compounds that fuel their activities—a primitive photosynthesis that did not involve oxygen. Two other types of microfossils had the same carbon ratios as microbes known as archaea that depend on methane as their energy source—and that played a pivotal role in the development of multicellular life. The ratio of a final type of microfossil indicated that this organism produced methane as part of its metabolism.

That there are so many different carbon ratios strengthens the case that these are real fossils, Schopf says. Any inorganic processes that could have created the squiggles would be expected to leave a uniform carbon ratio signature, he says. The fact that microbes were already so diverse at this point in Earth’s history also suggests that life on our planet may date back to 4 billion years ago, he says. Other researchers have found signs of life dating back at least that far, but those findings are even more controversial than Schopf’s.

“The new results add weight to the idea that the microstructures are biological,” Rasmussen agrees. But he is concerned that the microfossils may have been badly preserved. Olcott Marshall, who thinks the rock impressions are not fossils at all, but the product of geological processes, is even more critical: “The errors produced by this analytical technique are so large” that the data are not clear enough to say there are different types of microbes in rock, she says.

But SIMS experts praise the work. “It was a really careful, well thought out experiment,” says Lara Gamble, a chemist at the University of Washington in Seattle who was not involved in the study. “They put in a lot of effort to try to make sure everything was calibrated properly.”

Rasmussen hopes there will follow-up work that analyzes more microfossils. “It’s worth getting this right, given that we are looking at some of the oldest possible traces of life,” he says. “Honing our skills at recognizing ancient biosignatures on Earth is important as we cast our eyes to Mars and beyond.”

原始論文：J. William Schopf, Kouki Kitajima, Michael J. Spicuzza, Anatoliy B. Kudryavtsev, John W. Valley. SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions. Proceedings of the National Academy of Sciences, 2017; 201718063 DOI: 10.1073/pnas.1718063115

引用自：Elizabeth Pennisi. Life may have originated on Earth 4 billion years ago, study of controversial fossils suggests. Science, 2017; 358(6370). doi:10.1126/science.aar7944

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."