地球表面之下的騷動或許讓人覺得像是另一個世界發生的事情，但這些活動卻可以幫助陸塊形成，進而決定海洋環流、氣候模式，甚至是動物的行動與演化。事實上，科學家相信數百萬年前一道從地函往上湧的高溫岩石柱，可能在人類的演化史中具有重要的地位。
一群正在渡河的大象，攝於波札那的喬貝國家公園。圖片來源：Getty Images
在發表於《自然綜述—地球與環境》(Nature
Reviews Earth & Environment)的論文中，一組國際研究團隊探討了距今2000萬年前，將現今的阿拉伯半島與安納托利亞連結起來，讓亞洲與非洲得以相通的大型陸橋是如何形成的。
他們運用了德州大學奧斯汀分校地球科學院與GFZ德國地球科學研究中心開發的模型，結合前人發表的研究而得出這篇論文的結果。
這塊逐漸抬升的陸地使得許多動物的早期祖先可以往返亞洲及非洲，像是長頸鹿、大象、犀牛、獵豹甚至是人類。這片陸地的出現也終結了7500萬年來非洲被其他大陸孤立的狀態。
「這項研究關係到下列問題：『一般而言，我們的星球是如何發生變化的？生物與構造運動之間有什麼樣的關聯？』」研究共同作者Thorsten
Becker表示。他是德州大學奧斯汀分校地球科學院地球與行星科學系以及地球物理研究所的教授。
故事開始於距今5000萬年到6000萬年前，當時有段板塊滑入地函而產生了一條「輸送帶」，使得地底的高溫岩石翻騰起來，沿著一道柱狀結構往上升並在3000萬年之後到達地表。這股地函內部的對流活動加上地表的板塊碰撞運動，使得陸地往上抬升而把古代的特提斯洋封閉起來，將其分隔成現今的地中海與阿拉伯海；於此同時，亞洲與非洲之間也首度出現了將彼此連結起來的陸橋。
研究主要作者Eivind
Straume在德州大學奧斯汀分校地球科學院擔任博士後研究員的時候，分析了這次地質活動在許多方面造成的後果。他說動物的演化與陸橋的出現之間有著攜手並進的關係。
「如果沒有這些特定的作用，也就是地函對流以及隨之產生的的活躍地形變化，這條位在淺海的海道可能要再多花數百萬年才會閉合，」Straume表示。他現在是挪威研究中心與皮耶克尼斯氣候研究中心的博士後研究員。「你可以說如果沒有這道地函柱的話，這兩座大陸之間的碰撞過程會截然不同。」
在此故事當中，時間決定了一切。如果亞洲跟非洲還要再晚個數百萬年才連在一起，那麼透過這條路進出非洲的動物就會走上完全不同的演化途徑，這也包括了現代的人類祖先在內。
在陸橋完全閉合的數百萬年前，人類的靈長類祖先從亞洲進入非洲。雖然這些靈長類最後在亞洲走向了滅絕，牠們在非洲的後代子孫卻開枝散葉。當陸橋完全形成之後，這些靈長類又重新拓展回亞洲。
「此範例顯示了地函對流的長期演變過程如何跟生物的演化有所關連，」Straume表示。
阿拉伯半島抬升對於海洋環流和地球氣候來說也有很顯著的影響。它造成附近的海洋溫度升高，進而擴大了不同季節的溫差，並造成北非到中亞這片陸地變得更加乾燥。研究人員相信陸橋形成是造成撒哈拉地區變成沙漠的最後一根稻草。而地勢的變化也增強了亞洲季風的強度，使得東南亞變得更加濕潤。
此論文結合了許多議題現有的研究成果，包括板塊運動、地函對流、地形、古地理、演化人類學、哺乳類演化、氣候演變、海洋環流…….等等。這讓他們可以用一個完成且連貫的故事來訴說這次地函活動對許多方面的影響。
「這篇論文總結了近期的進展，雖然可能具有一些爭議，但至少對我們來說結果是很有說服力的，」Becker表示。
How activity in
Earth’s mantle led the ancient ancestors of elephants, giraffes, and humans
into Asia and Africa
What roils beneath the Earth’s surface
may feel a world away, but the activity can help forge land masses that dictate
ocean circulation, climate patterns, and even animal activity and evolution. In
fact, scientists believe that a plume of hot rocks that burst from the Earth’s
mantle millions of years ago could be an important part in the story of human
evolution.
In a paper published in Nature Reviews Earth & Environment, an international team of
researchers investigated the formation of a large land bridge that connected
Asia and Africa 20 million years ago, through what is now the Arabian Peninsula
and Anatolia.
The paper brings together previously published research
with new models created at The University of Texas at Austin Jackson School of
Geosciences and the GFZ Helmholtz Centre for Geosciences.
This gradual uplift of land enabled the early ancestors
of animals such as giraffes, elephants, rhinoceroses, cheetahs, and even
humans, to roam between Africa and Asia. The appearance of the land ended a
75-million-year-long isolation of Africa from other continents.
“This study has relevance to the question of ‘How did
our planet change, in general? What are the connections between life and
tectonics?’” said Thorsten Becker, a study co-author and professor at the
Jackson School’s Department of Earth and Planetary Sciences and Institute for
Geophysics.
The story begins 50-60 million years ago, when a slab
of rock sliding into the Earth’s mantle created a “conveyor belt” for hot rocks
to boil up in an underground plume that reached the surface some 30 million
years later. This convective activity in the mantle, coupled with the collision
of tectonic plates, created an uplift in land that contributed to closing the
ancient Tethys Sea, splitting it into what is now the Mediterranean and Arabian
Seas, and created a landmass that bridged Asia and Africa for the first time.
The study’s lead author Eivind Straume analyzed the
wide-ranging consequences of this geologic activity while he was a postdoctoral
fellow at the Jackson School. He said the appearance of the land bridge and
animal evolution go hand in hand.
“The shallow seaway closed several million years
before it otherwise likely would have due to these specific processes — mantle
convection and corresponding changes in dynamic topography,” said Straume, who
is now a postdoctoral fellow at NORCE Norwegian Research Centre and The
Bjerknes Centre for Climate Research. “Without the plume, you could argue that
the continental collision would have been different.”
In this case, timing is everything. If it had been an
additional million years before Africa and Asia were connected, the animals
that made their way into and out of Africa could have been on a different
evolutionary path. That includes the ancestors of today’s humans.
Several million years before the land bridge had
completely closed, the primate ancestors of humans came to Africa from Asia.
While those primates ended up going extinct in Asia, their lineages diversified
in Africa. Then when the land bridge fully emerged, these primates re-colonized
Asia.
“It’s an example of how the long-term convective
evolution of the planet talks to the evolution of life,” Straume said.
This uplift of the Arabian Peninsula also had
significant impacts on ocean circulation and the Earth’s climate. Nearby ocean
temperatures warmed, which in turn widened seasonal temperature ranges, and
made a swath of land from north Africa to central Asia more arid. Researchers
believe the formation of this land bridge was a final trigger in making the
Sahara a desert. And these topographical changes enhanced monsoon season in
Asia, making southeast Asia wetter.
This paper brings together existing research spanning
plate tectonics, mantle convection, topography and paleogeography, evolutionary
anthropology, mammal evolution, climate evolution, and ocean circulation, among
other topics, to tell a cohesive story of the wide-ranging effects of these
mantle dynamics.
“To us at least, this is a compelling — perhaps a
little bit provocative — summary of recent advancements,” Becker said.
原始論文：Eivind O.
Straume, Claudio Faccenna, Thorsten W. Becker, Bernhard Steinberger, Alexis
Licht, Andrea Sembroni, Zohar Gvirtzman, Paolo Ballato. Collision,
mantle convection and Tethyan closure in the Eastern Mediterranean. Nature
Reviews Earth & Environment, 2025; 6 (4): 299 DOI: 10.1038/s43017-025-00653-2
引用自：University of Texas at Austin. "How
activity in Earth's mantle led the ancient ancestors of elephants, giraffes,
and humans into Asia and Africa."
原文網址：https://www.jsg.utexas.edu/news/2025/04/how-activity-in-earths-mantle-led-the-ancient-ancestors-of-elephants-giraffes-and-humans-into-asia-and-africa/
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