日趨乾燥且反覆無常的環境可能加速了人類演化

原文網址：https://www.sciencedaily.com/releases/2018/10/181008183342.htm

日趨乾燥且反覆無常的環境可能加速了人類演化

證據指出氣候變得反覆無常且越來越乾燥的同時，人類打造石器的技術有了重大改變，而現代智人也跟著出現。

根據10月8日發表在《美國國家科學院院刊》(Proceedings of the National Academy of Science)線上版的研究，在氣候變得越來越乾燥卻又偶爾多雨的情況下，促成了我們的人族祖先迅速演化成解剖上的現代人。

乾季馬加迪湖的湖水蒸發使湖床長出一大片天然鹼的(trona)晶體。背景裡本研究用的鑽井平台矗立在乾涸的湖床之上。圖片來源：Robin Renaut

科學家在經常稱為人類搖籃的東非裂谷發現種類繁多的人類化石、石器與其他考古證據之後，便一直試著拚湊出有關人類起源的複雜謎團，包括這段歷史的環境背景。

這項研究為「人族考古遺址與古湖泊鑽探計畫」(HSPDP)的一部份，主持人為美國亞歷桑納大學的Andrew Cohen教授。研究地點位在肯亞南部裂谷盆地的馬加迪湖(Lake Magadi)，他們根據從湖泊鑽出的沉積物岩芯得到第一個連續的環境資訊，可以指示附近挖出的諸多考古證據形成時的環境背景。

馬加迪湖的深度不深且會週期性乾涸，附近肯亞的歐羅結撒依立耶盆地(Olorgesailie basin)則是非洲找到最多人類演化考古證據的地點之一。作者提出劇烈的氣候變遷可能推動了人族演化，使現代智人誕生，揭開了中石器時代的序幕。

這篇論文題目為「東非過去50萬年來逐漸乾旱化，以及和人類演化的關聯」。作者於文中寫道雖然先前有假說認為人族演化和氣候變遷有關，多數前人研究卻缺乏區域證據可以把兩者連結起來。研究指出從距今575,000年前開始，此區域的乾燥化越趨劇烈，這是先前東非陸域連續沉積的岩芯都沒有紀錄到的。他們認為氣候變化可以對應到歐羅結撒依立耶地區紀錄到的動物滅絕事件以及石器製作技術的重大轉變。

「之前已經在此區域蒐集到許多人類演化的證據，但卻無法把這些紀錄和詳細的環境紀錄連結起來。」主要作者，香港浸會大學的的Richard Owen表示。「在50萬年前舊石器時代晚期的石器和32萬年前中石器時代序幕的石器之間，存在一大段空白的歷史紀錄。我們的研究結果運用連續環境紀錄把它給填補了起來。」

這段空白的中間某個時段發生了重大變化。考古學家挖出的證據顯示當時人類製作和運用石器的能力有大幅進展，石器的貿易也更加活絡。

馬家迪湖的岩芯首次呈現出氣候變遷和該區考古紀錄事件之間的詳細關聯。

「雖然不久之前我們已經瞭解到當時的氣候非常動盪不安，不過這份紀錄的關鍵在於它和考古證據出現轉變的時間相當接近。」亞歷桑納大學地質科學系的教授Cohen表示。「在歐羅結撒依立耶年代較早的石器於120萬年前至50萬年前並沒有太大變化。突然之間，在50萬年前至32萬年前考古證據有了重大轉變――器物變得更加精緻，運輸距離也更長。雖然我們不曉得確切時間，但可以確定是這在段時間內發生。」

Cohen說同一時間的湖泊岩芯指出氣候變得越來越乾且動盪不安，非洲其他地方也有現代智人出現的證據，這讓他們猜測兩者之間是否有深切的關聯。

「我們仍然無法完全確定演化讓腦容量增加和新的石器種類出現是否攜手併行。」他說。「不過從摩洛哥出土的最早現代智人化石可以追溯到325,000年前，於此同時我們也看到器物出現了變化。而兩者發生的時間點附近，我們的岩芯紀錄顯示出考古遺址的鄰近地區發生了嚴重乾旱。」

從馬加迪湖鑽出的岩芯最深可以到達200公尺，穿透了湖床堆積的沉積物直到火成岩岩床。每根岩心樣品的長度大約為10英尺(~3公尺)，直徑為2.5英吋(6.35公分)，研究人員把它們切成容易處理的5英尺(~1.5公尺)段落後，包裝起來空運到美國明尼蘇達大學的國家湖泊岩芯設施，以便管理、儲存並進行分析。

Owen說根據環境變異選擇理論(hypothesis of variability selection)，迅速變動的環境會製造選擇壓力迫使物種去適應。解剖學上的現代人在此情況下因為擁有較大的頭腦，使得他們能快速適應越來越難以預料的世界。

「現在我們有證據指出器物種類的變化、動物族群的改變以及氣候越趨乾燥都發生在同一時間。」Owen表示。「一連串的巧合會讓你想說：『它們之間可能真的有關。』當我們得到環境變動的證據並和此區的考古證據比對後，我們也有這種想法。」

當研究人員蒐集更多此區域的氣候資料時，HSPDP也在附近其他地點完成別的岩芯鑽探作業。這些資料讓他們可以繼續探討環境變動在人類演化歷程中的重要性有多高。

Dryer, less predictable environment may have spurred human evolution

Evidence of a variable but progressively drying climate coincides with a major shift in stone-tool-making abilities and the appearance of modern Homo sapiens

A progressively drying climate punctuated by variable wetter episodes may have precipitated the transition from our hominin ancestors to anatomically modern humans, according to research published on Oct. 8 in the online edition of the Proceedings of the National Academy of Science (PNAS).

Since the discovery of a rich assemblage of human fossils as well as stone tools and other archeological evidence in the rift valley of East Africa, a region often referred to as the cradle of humanity, scientists have attempted to piece together the complex puzzle that is the history of our human origins, including the environmental context of that history.

The study, based on lake sediment cores, is the first to provide a continuous environmental context for the diverse archeological evidence recovered from nearby localities in the rift valley basins of southern Kenya. The cores were sampled from Lake Magadi as part of the Hominin Sites and Paleolakes Drilling Project, or HSPDP, which is directed by University of Arizona professor Andrew Cohen.

Lake Magadi, a shallow, periodically dry lake, is close to the Olorgesailie basin in Kenya, one of the most productive sites for archaeological evidence of human evolution in Africa. The authors suggest that the profound climatic changes may have been driving forces behind hominin evolution, the origins of modern Homo sapiens and the onset of the Middle Stone Age.

While previous hypotheses have related hominin evolution to climate change, most prior studies lack regional-scale evidence for a link between environment and hominin evolution, the authors write in the paper, "Progressive aridification in East Africa over the last half million years and implications for human evolution." According to the study, a trend toward intense aridification in the area began 575,000 years ago. The change, not previously documented in continuous continental cores from East Africa, corresponds with faunal extinctions and a major transformation in stone tool technology documented in the Olorgesailie region.

"Much evidence for human evolution has been gathered from the area, but linking those records to detailed environmental records was missing until now," said the study's lead author, Richard Owen, of Hong Kong Baptist University. "There is a big gap in the records between the last Early Stone Age tools 500,000 years ago and the appearance of Middle Stone Age tools about 320,000 years ago. Our results plugged that gap with a continuous environmental record."

A critical transition occurred sometime during this gap, a period for which archeologists have unearthed evidence of a leap in early humans' abilities to make, use and trade stone tools.

The cores from Lake Magadi provide the first detailed link between climate change and events known from the region's archeological record.

"We have known for a while that the climate at the time was very varied, but the key here is that the records are in proximity to the archeological evidence for this transition," says Cohen, a professor in the University of Arizona's Department of Geosciences. "The older stone tools found at Olorgesailie did not change much between 1.2 million and a half-million years ago. And suddenly, after 500,000 and before 320,000 years ago - we don't know exactly when, but in that timespan - there was a critical transition in archeology when tools became more sophisticated and were transported over longer distances."

At the same time the lake core records point to the climate becoming drier and more variable, there is evidence elsewhere in Africa of the appearance of modern Homo sapiens, prompting much speculation whether the two are connected, Cohen said.

"Whether the evolution of bigger brains goes hand in hand with new toolkits is not entirely clear," he said. "But the earliest modern Homo sapiens fossils from Morocco date back 325,000 years, the same time we see this transition of tools. And both happened around the same time that our core record indicates severe drying very close to the archeological sites."

The deepest core drilled at Lake Magadi reached 200 meters (650 feet), penetrating all sedimentary layers down to the volcanic bedrock of the lake. The core samples, each about 10 feet long and 2 1/2 inches in diameter, are cut into manageable 5-foot segments, packaged and air-freighted to the National Lake Core Facility at the University of Minnesota for curation, analysis and storage.

According to the hypothesis of variability selection, a rapidly changing environment creates selective pressure that forces species to adapt to rapid change, Owen said. Under that scenario, the larger brains of anatomically modern humans would have allowed our ancestors to adapt quickly to an increasingly less predictable world.

"Now we have evidence that at the same time the toolkits were changing, the mammal fauna changed and the climate became more arid," Owen said. "So you have a series of coincidences that makes you think, 'This could be real.' Now we can say when the environment changed and then compare that to the archeological evidence of the region."

Drilling at other nearby sites by HSPDP has been completed as researchers gather more of the region's climate data to continue studying the importance of environmental variability in the course of human evolution.

原始論文：R. Bernhart Owen, Veronica M. Muiruri, Tim K. Lowenstein, Robin W. Renaut, Nathan Rabideaux, Shangde Luo, Alan L. Deino, Mark J. Sier, Guillaume Dupont-Nivet, Emma P. McNulty, Kennie Leet, Andrew Cohen, Christopher Campisano, Daniel Deocampo, Chuan-Chou Shen, Anne Billingsley, Anthony Mbuthia. Progressive aridification in East Africa over the last half million years and implications for human evolution. Proceedings of the National Academy of Sciences, 2018; 201801357 DOI: 10.1073/pnas.1801357115

引用自：University of Arizona. "Drier, less predictable environment may have spurred human evolution."