發表在期刊《科學》(Science)的新研究發現大約在112萬年前，北大西洋發生了一場嚴重的冷化事件，連帶產生的氣候、植被與食物來源變化中斷了早期人類移居歐洲的過程。
112萬7000年前北大西洋的冷化事件造成了歐洲南部的人口減少。(直立人頭骨的圖片由Axel Timmermann提供)。地圖上的粉紅色區域為早期人類棲地的適居程度，因為變得乾冷與食物來源減少而大幅下降的地方。
由英國、南韓和西班牙組成的國際科學團隊發表了這項研究。透過觀察與模擬得出的證據，他們證明了前所未有的氣候壓力改變了人類早期的歷史軌跡。
稱為直立人的古人類大約在180萬年前從非洲遷移至歐亞大陸中部。接著他們繼續往歐洲西部擴張，大約在150萬年前到達伊比利半島。從幾副化石的定年結果以及來自該時期的石器可以得證，起初在較為溫和的氣候條件之下，這些族群最後在歐洲南部建立了穩固的基礎。但是在距今120萬年前開始，歐洲的冰河循環逐漸加劇的情形之下，我們並不清楚早期人類在此區域生活了多久，以及移居歐洲的過程是否受到氣候條件惡化而中斷。
為了更加瞭解早期人種在歐洲經歷的環境條件，由花粉專家、海洋學家、氣候模擬專家、考古學家與人類學家組成的團隊，從副熱帶大西洋東部的深海沉積物岩芯取得了相關數據，並且在新型的超級電腦上，利用氣候模型與人類棲地模型來模擬這段人口減少事件的始末。
科學家從海洋沉積物岩芯篩出數以千計、十分細小的植物花粉，並且分析一百多萬年前的微小藻類殘留至今易受溫度影響的有機物。他們發現大約在112萬7000千年，北大西洋東部以及鄰近陸地的氣候突然下降了7℃。
「在古氣候紀錄中，這起劇烈的冷化事件代表了最初幾個晚期冰階(stadial)事件之一。冰階發生在一個冰期循環的最後階段，當冰層消融、釋出大量淡水到海洋，海洋環流會因此而改變，海冰也跟著往南擴張，」本研究的資深作者，倫敦大學學院的Chronis
Tzedakis表示。
從海洋沉積物岩芯萃取出來的花粉數據為當時的狀況增添了更多資訊。「微小的花粉被河流和海洋從附近的陸地帶到海洋，然後往下沉到深海並逐漸累積。根據我們分析深海沉積物岩芯裡花粉的結果，北大西洋的冷化事件讓歐洲的植被轉變成不適合居住的半乾旱環境，」研究主要作者，倫敦大學學院的Vasiliki
Margari博士表示。
為了量化早期人類遭遇這種前所未見的氣候異常時會有什麼樣的反應，南韓IBS氣候物理學中心(ICCP)的科學家運用新的電腦模型來模擬該時期。將北大西洋加入來自冰河的淡水之後，ICCP的Kyung-Sook
Yun 和Hyuna
Kim博士成功重現出晚期冰階事件的關鍵特徵，像是歐洲南部的氣候變成又冷又乾。「接著我們把全球氣候模型的模擬結果輸入到人類棲地模型當中，它可以判斷某個環境條件是否適合早期的直立人生存。我們發現對於直立人這類早期人種來說，當時歐洲南部許多區域都沒辦法居住，」研究共同通訊作者，釜山大學ICCP的主任Axel Timmermann如此敘述。
雖然冷化事件只持續了4000年左右，但是接下來的20萬年歐洲缺少石器與人類遺骸，加深了人類移居歐洲的過程出現了長期中斷的可能性。大概在90萬年前，一群通常被稱為前人(Homo
antecessor)的族群才再次移入歐洲。該族群和後代能夠適應歐洲越來越強烈的冰河環境，顯示他們的適應能力強韌了許多。
「我們對於過去氣候的研究佐證了在歐洲南部，植被與人類的食物來源容易受到北大西洋溫度變化的影響。這項結果連同越來越多的證據都指出，過往的氣候變遷塑造了我們人類的歷史，」Timmermann教授說。
How a massive North Atlantic
cooling event disrupted early human occupation in Europe
A new study published in the journal Science finds that around 1.12 million
years ago a massive cooling event in the North Atlantic and corresponding
shifts in climate, vegetation and food resources disrupted early human
occupation of Europe.
The study published by an international group of
scientists from the UK, South Korea and Spain presents observational and
modelling evidence documenting that unprecedented climate stress changed the
course of early human history.
Archaic humans, known as Homo erectus moved from
Africa into central Eurasia around 1.8 million years. From there on they spread
towards western Europe, reaching the Iberian peninsula around 1.5 million years
ago (Ma). Experiencing initially rather mild climatic conditions, these groups
eventually established a foothold in southern Europe, as documented by several
dated fossils and stone tools from this period. But given the increasing
intensity of glacial cycles in Europe from 1.2 Ma onwards, it remains unknown
for how long early humans lived in this area and whether the occupation was
interrupted by worsening climate conditions.
To better understand the environmental conditions,
which early human species in Europe experienced, the team of pollen experts,
oceanographers, climate modelers, archeologists, and anthropologists combined
data of a deep ocean sediment cores from the eastern subtropical Atlantic with
new supercomputer climate model and human habitat model simulations covering
the period of the depopulation event.
Sieving through thousands of small plant pollen
stored in the ocean sediment core and analyzing preserved temperature-sensitive
organic compounds left by tiny algae, which lived over a million years ago, the
scientists discovered that around 1.127 million years ago, the climate over the
eastern North Atlantic and the adjacent land suddenly cooled by 7oC.
“This massive cooling marks one of the first terminal
stadial events in the paleoclimatic record. It occurred during the last phase
of a glacial cycle, when ice-sheets disintegrated, releasing large amounts of
freshwater into the ocean, and causing ocean circulation changes and a
southward expansion of sea ice”, says Prof. Chronis Tzedakis from University
College London (UCL), senior author of the study.
The pollen data extracted from the ocean sediment
core further add to this scenario “Rivers and winds bring tiny pollen from the
adjacent land to the ocean, where they sink and get deposited in the deep
ocean. According to our ocean sediment core pollen analysis, the North Atlantic
cooling event switched western European vegetation to an inhospitable
semi-desert landscape.”, adds Dr. Vasiliki Margari from UCL, lead author of the
study.
To quantify how early humans may have reacted to such
an unprecedented climate anomaly, scientists from the IBS Center for Climate
Physics (ICCP) in South Korea, conducted new computer model simulations for
this period. By adding glacial freshwater to the North Atlantic, Dr. Kyung-Sook
Yun, and Ms. Hyuna Kim from the ICCP were able to reproduce key features of the
terminal stadial event, such as the cooling (Fig. 1) and drying over southern
Europe. “We then used this global climate model simulation as an input for a human
habitat model, which determines whether certain environmental conditions were
suitable for early Homo erectus or not. We found that over many areas of
southern Europe, early human species such as Homo erectus would have not been
able to survive” describes Prof. Axel Timmermann, Director of the ICCP at Pusan
National University and co-corresponding author of the study.
Even though the cooling event only lasted for about
4,000 years, a lack of stone tools and human remains over the next 200,000
years further raises the possibility of a long-lasting hiatus in European
occupation. Europe was again repopulated around 900 thousand years ago by a
group that is often referred to as Homo antecessor. This group and its
descendants were much more resilient, because they were able to adapt to the
increasing intensity of glacial conditions over Europe.
“Our study on past climates documents the sensitivity
of Southern European vegetation and human food resources to North Atlantic
temperature changes. This result adds to the mounting evidence that our human
history has been shaped by past climate changes.”, adds Prof. Timmermann.
原始論文：Vasiliki
Margari, David A. Hodell, Simon A. Parfitt, Nick M. Ashton, Joan O. Grimalt,
Hyuna Kim, Kyung-Sook Yun, Philip L. Gibbard, Chris B. Stringer, Axel
Timmermann, Polychronis C. Tzedakis. Extreme glacial cooling likely led
to hominin depopulation of Europe in the Early Pleistocene. Science,
2023; 381 (6658): 693 DOI: 10.1126/science.adf4445
引用自：Institute for Basic Science. "How a
massive North Atlantic cooling event disrupted early human occupation in
Europe."
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