科學家在非洲最新發現的「蹺蹺板型氣候」驅動了人類演化

 原文網址：https://www.shh.mpg.de/2005624/kaboth-bahr-scerri-african-climate-seesaw?c=1935799

科學家在非洲最新發現的「蹺蹺板型氣候」驅動了人類演化

德國波茨坦大學的Stefanie Kaboth-Bahr博士領導的科學團隊，發現過去有種類似聖嬰現象的氣候模式，主導了撒哈拉以南的非洲過去62萬年的環境變化，這段期間也是人類演化史上的重要時期。之前科學家較常認為人類演化與冰期―間冰期循環有一定的關聯，但團隊發現這個氣候模式對古代撒哈拉以南的非洲造成的影響要來得更大。

研究人員主張低緯度的氣候作用掌管了非洲西部和東部動植物的演化與擴散過程，方法是透過增加資源豐富的環境與穩定的生態過度帶，一般認為這些環境條件對於早期人類來說也很重要。圖片來源：Hans Sell

雖然科學家已經廣為接受人類在非洲的演化過程是由氣候變遷驅動，但是當時氣候變遷的特性以及造成的影響都還沒有完全了解。在過去100萬年這段人類演化的關鍵時期之中，冰期―間冰期循環對全世界許多地方的氣候變遷模式都有深刻影響，因此科學家之前推測非洲的環境變遷也是受此調控。一般認為冰河期循環促成的生態系變化，刺激了早期人類的演化速度並讓他們往外擴散。

本周發表在《美國國家科學院院刊》( Proceedings of the National Academy of Sciences)的一篇文章質疑了這道觀點。Kaboth-Bahr博士參與的跨領域國際團隊發現，過去一種類似聖嬰現象的天氣模式主導了非洲的氣候變遷，促使他們重新審視人類演化過程中對於氣候背景的現有理解。

逐水而居

Kaboth-Bahr博士和她的同僚整理了過去62萬年涵蓋全非洲大陸的11種氣候紀錄，藉此得出清晰的圖像 顯示非洲大陸各地在什麼時候普遍處於乾燥或者濕潤的狀態。「我們驚訝地發現東部和西部的氣候呈現出一種明顯的『蹺蹺板』模式，這和目前對於非洲降雨分佈有深遠影響的聖嬰現象非常類似，」主持這項研究的Kaboth-Bahr博士解釋。

作者推測造成這種蹺蹺板型氣候的主因是熱帶太平洋對「沃克環流」造成的影響，這道位在赤道的對流胞會影響熱帶地區的旱澇。數據清楚顯示乾燥和濕潤的區域以大約10萬年的時間尺度在非洲大陸的東部和西部來回移動，每次氣候轉換的時候就會大幅改變植物和哺乳類的組成。

「乾溼時期的交替變換似乎掌管了非洲東部和西部的植物與哺乳類如何演化並擴散，」Kaboth-Bahr.博士解釋。「這造成環境呈現塊狀的分布，對於人類演化與早期的人口組成來說可能是很重要的影響因素 。」

這些科學家強調雖然氣候變遷肯定不是驅動早期人類演化的唯一因素，不過這篇研究呈現了一種全新的觀點，顯示我們老祖宗的起源與環境波動之間有緊密的關聯。

「我們看到非洲許多哺乳類的分布情形都和我們辨識出來的模式相符，牠們的演化史看起來就像是受到非洲東西部之間的乾濕交替所牽連。」共同作者之一 Eleanor Scerri表示。她是德國馬克斯·普朗克人類歷史科學研究所的演化考古學家。「這些動物保存了人類是在什麼樣的環境中演化的訊息。我們的祖先可能像牠們一樣，受到同樣的環境壓力而被分隔在非洲大陸各處。」

生態過渡帶：不同生態區之間的過渡地帶

這群科學家的成果也提出像是蹺蹺板一般輪替的非洲東西部降雨模式可能會形成極為重要的「生態過渡帶」，也就是不同生態區之間的緩衝地帶，例如草原和森林的交界。

「生態過渡帶提供了多樣化、資源豐富與穩定的環境，一般認為對於早期人類來說這些條件是很重要的，」Kaboth-Bahr博士加以解釋。「就其他動物群集來說必定也是如此。」

科學家認為這代表非洲的內陸區域在支持人口可以長期延續的方面具有很重要的功能。「我們在非洲各地都可以看到人族的早期成員留下來的考古痕跡，」Scerri博士表示。「但我們也看見創新的事物出現之後接著消失，然後又經常被重新發明出來，意味著在人類許久以前的歷史當中，地方族群興衰交替的鋸齒狀模式是種常態。生態過渡帶或許能提供讓族群長期延續的地方，確保就算地方族群經常邁入滅絕，但總體來說人類族群仍然可以繼續繁衍。」

「以新的氣候模式為背景來重新審視這些靜滯、變化與滅絕的模式，可以讓我們對人類許久以前的歷史有新的洞見，」Kaboth Bahr博士表示。「這代表人類在面對氣候變遷的時候並非全然無助，不過棲地的適居程度有所改變勢必會對人口組成造成衝擊，最後也會影響到人類演化的基礎――也就是不同族群之間的基因交流。」

Newly discovered African ‘Climate Seesaw’ drove human evolution

A scientific consortium led by Dr Stefanie Kaboth-Bahr of the University of Potsdam has found that ancient El Niño-like weather patterns were the primary drivers of environmental change in sub-Saharan Africa over the last 620 thousand years – the critical timeframe for the evolution of our species. The group found that these ancient weather patterns had more profound impacts in sub-Saharan Africa than glacial-interglacial cycles more commonly linked to human evolution.

While it is widely accepted that climate change drove the evolution of our species in Africa, the exact character of that climate change and its impacts are not well understood. Glacial-interglacial cycles strongly impact patterns of climate change in many parts of the world, and were also assumed to regulate environmental changes in Africa during the critical period of human evolution over the last ~1 million years. The ecosystem changes driven by these glacial cycles are thought to have stimulated the evolution and dispersal of early humans.

A paper published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) this week challenges this view. Dr. Kaboth-Bahr and an international group of multidisciplinary collaborators identified ancient El Niño-like weather patterns as the drivers of major climate changes in Africa. This allowed the group to re-evaluate the existing climatic framework of human evolution.

Walking with the rain

Dr. Kaboth-Bahr and her colleagues integrated 11 climate archives from all across Africa covering the past 620 thousand years to generate a comprehensive spatial picture of when and where wet or dry conditions prevailed over the continent. “We were surprised to find a distinct climatic east-west ‘seesaw’ very akin to the pattern produced by the weather phenomena of El Niño, that today profoundly influences precipitation distribution in Africa,” explains Dr. Kaboth-Bahr, who led the study.

The authors infer that the effects of the tropical Pacific Ocean on the so-called “Walker Circulation” – a belt of convection cells along the equator that impact the rainfall and aridity of the tropics - were the prime driver of this climate seesaw. The data clearly shows that the wet and dry regions shifted between the east and west of the African continent on timescales of approximately 100,000 years, with each of the climatic shifts being accompanied by major turnovers in flora and mammal fauna.

“This alternation between dry and wet periods appeared to have governed the dispersion and evolution of vegetation as well as mammals in eastern and western Africa,” explains Dr. Kaboth-Bahr. “The resultant environmental patchwork was likely to have been a critical component of human evolution and early demography as well.”

The scientists are keen to point that although climate change was certainly not the sole factor driving early human evolution, the new study nevertheless provides a novel perspective on the tight link between environmental fluctuations and the origin of our early ancestors.

“We see many species of pan-African mammals whose distributions match the patterns we identify, and whose evolutionary history seems to articulate with the wet-dry oscillations between eastern and western Africa,” adds Dr. Eleanor Scerri, one of the co-authors and an evolutionary archaeologist at the Max Planck Institute for the Science of Human History in Germany. “These animals preserve the signals of the environments that humans evolved in, and it seems likely that our human ancestors may have been similarly subdivided across Africa as they were subject to the same environmental pressures.”

Ecotones: the transitional regions between different ecological zones

The scientists’ work suggests that a seesaw-like pattern of rainfall alternating between eastern and western Africa probably had the effect of creating critically important ecotonal regions – the buffer zones between different ecological zones, such grassland and forest.

“Ecotones provided diverse, resource-rich and stable environmental settings thought to have been important to early modern humans,” adds Dr. Kaboth-Bahr. “They certainly seem to have been important to other faunal communities.”

To the scientists, this suggests that Africa’s interior regions may have been critically important for fostering long-term population continuity. “We see the archaeological signatures of early members of our species all across Africa,” says Dr. Scerri, “but innovations come and go and are often re-invented, suggesting that our deep population history saw a constant saw-tooth like pattern of local population growth and collapse. Ecotonal regions may have provided areas for longer term population continuity, ensuring that the larger human population kept going, even if local populations often went extinct.”

“Re-evaluating these patterns of stasis, change and extinction through a new climatic framework will yield new insights into the deep human past,” says Dr. Kaboth Bahr. “This does not mean that people were helpless in the face of climatic changes, but shifting habitat availability would certainly have impacted patterns of demography, and ultimately the genetic exchanges that underpin human evolution.”

原始論文：Stefanie Kaboth-Bahr, William D. Gosling, Ralf Vogelsang, André Bahr, Eleanor M. L. Scerri, Asfawossen Asrat, Andrew S. Cohen, Walter Düsing, Verena E. Foerster, Henry F. Lamb, Mark A. Maslin, Helen M. Roberts, Frank Schäbitz, Martin H. Trauth. Paleo-ENSO influence on African environments and early modern humans. PNAS, 2021 DOI: 10.1073/pnas.2018277118

引用自：Max Planck Institute for the Science of Human History. "Newly discovered African 'climate seesaw' drove human evolution."