人類祖先並非古代非洲哺乳類滅絕的元兇

原文網址：https://unews.utah.edu/human-ancestors-not-to-blame-for-ancient-mammal-extinctions-in-africa/

人類祖先並非古代非洲哺乳類滅絕的元兇

新研究發現最近460萬年以來草原擴張造成大型哺乳類逐漸減少

有個歷時已久的說法認為最近數百萬年來非洲大型哺乳類逐漸滅絕，是由我們最早會使用工具的祖先造成。不過一項新的研究對此提出質疑；反之，他們主張長期的環境變遷，主要是草原的擴張才是哺乳類滅絕的元兇。而環境變遷可能是由大氣二氧化碳濃度下降所造成。

肯亞西部挖出的河馬(Hippopotamus amphibius，左)和白犀牛(Ceratotherium simum，右)牙齒化石，年代為更新世晚期。這兩種動物是現存為數不多的巨型草食動物(megaherbivore)。來源：J. Tyler Faith

領導這項研究的是猶他州自然歷史博物館的考古學研究員Tyler Faith，他也是猶他大學人類學系的助理教授。研究團隊還包括了麻省大學阿姆赫斯特校區的John Rowan、芝加哥大學的Andrew Du，以及加州大學聖克魯茲分校的Paul Koch。

這項研究今日發表於《科學》(Science)期刊。

「儘管數十年來皆有文獻指稱早期人族影響了古代非洲的動物相，卻少有研究試著實際驗證該理論，或是尋找不同的可能性。」Faith表示。「在瞭解人類對大型哺乳類群集的影響這方面，我認為我們的研究是十分重大的進展。我們也提出另一種可信的觀點來看待人類的早期祖先，跟長久以來所持有的角度恰好相反。」

為了考證古代人族造成的影響，研究人員彙整出長達700萬年東非草食動物滅絕事件的紀錄。他們特別著重於最為大型的物種，也就是所謂的「巨型草食動物」(megaherbivore，超過2000磅(將近1噸)的動物)。雖然非洲現在只有五種巨型草食動物，但過去卻種類繁多。舉例來說，三萬年前阿法南方古猿「露西」在伊索比亞哈達爾的林地生活時，與她毗鄰而居的就有三種長頸鹿、兩種犀牛、一種河馬以及四種類似大象的動物。

雖然這些動物的消失最常歸咎於人族演化出使用工具與吃肉的行為，但對於考古學家和古生物學家來說牠們消失的時間和原因仍是懸宕許久的謎題。

Faith表示：「我們的分析顯示大約460萬年前，巨型草食動物的多樣性開始長期而穩定的下降。相較於最早顯示人類祖先會製造器物或是支解動物屍體的時間，此滅絕過程的起始時間還早了一百萬年以上。而能夠實際獵殺這些動物的人族物種出現，比方說直立人，更是許久之後的事了。」

詳加檢視

Faith和他的團隊定量了東非巨型草食動物的長期變化，他們利用的資料庫包含了一百多個化石組合，時間涵蓋了最近七百萬年。此外，團隊也檢視了幾種氣候環境的趨勢與其影響的獨立紀錄，具體來說包括全球大氣的二氧化碳濃度、顯示植被結構的穩定碳同位素紀錄、東非草食動物牙齒化石的穩定碳同位素組成，以及其他項目。

他們的分析揭露出最近七百萬年來巨型草食動物經歷了慘重的滅絕過程：有28個譜系就此絕種，造成現在的動物群集缺乏大型動物。結果凸顯出整體來看，古代非洲大陸各地的巨型草食動物群集有非常高的多樣性，其中許多群集含有的種類比今日多出許多。

進一步的分析顯示巨型草食動物的減少大約起始於距今460萬年前，而且多樣性降低的速率並沒有隨著直立人的出現而跟著變化。人類的祖先之一直立人時常被指責為造成這些動物絕種的兇手，不過Faith的團隊主張氣候的嫌疑更大。

「巨型草食動物在上新世至更新世減少的關鍵因素似乎是草原擴張，可能跟最近五百萬年來全球大氣二氧化碳濃度下降有關。」麻省大學阿姆赫斯特校區的博士後科學家John Rowan表示。「二氧化碳濃度降低使熱帶的草原比起樹木更適合生長，結果造成莽原的樹木變少而越加開闊。我們知道滅絕的巨型草食動物中有許多仰賴木本植群為生，因此牠們可能是跟著牠們的食物來源一同消失了。」

其他本來歸因於古代人族的滅絕事件，或許也能用大型草食動物的衰亡來解釋。有些科學家提出越來越偏好肉食的人族物種和肉食動物互相競爭，造成最近數百萬年來許多肉食動物消失無蹤。但Faith和他的團隊提出了另外一種解釋。

「我們知道這段期間非洲也有許多肉食動物絕種。其中某些種類，像是劍齒虎，可能只會獵捕非常大型的動物，比方說未成年的大象。」Paul Koch表示。「某些肉食動物可能是跟著牠們的巨型草食獵物一起消失了。」

「綜觀所有可能造成巨型草食動物減少的因素，我們的分析指出氣候和環境變遷在非洲過往發生的滅絕事件中佔有關鍵地位。」Faith表示。「因此，在尋找古代人族過去對非洲生態系造成的影響時，我們應當聚焦在已知有能力辦到這件事的物種之一――也就是我們所屬的智人――在最近這30萬年來的所作所為。」

Human ancestors not to blame for ancient mammal extinctions in Africa

New research finds grassland expansion drove the decline of giant mammals over the last 4.6 million years

New research disputes a long-held view that our earliest tool-bearing ancestors contributed to the demise of large mammals in Africa over the last several million years. Instead, the researchers argue that long-term environmental change drove the extinctions, mainly in the form of grassland expansion likely caused by falling atmospheric carbon dioxide (CO₂) levels.

Tyler Faith, curator of archaeology at the Natural History Museum of Utah and assistant professor in the Department of Anthropology at the University of Utah, led the study. The research team also includes John Rowan from the University of Massachusetts Amherst, Andrew Du from the University of Chicago, and Paul Koch from the University of California, Santa Cruz.

The study is published today in the journal Science.

"Despite decades of literature asserting that early hominins impacted ancient African faunas, there have been few attempts to actually test this scenario or to explore alternatives," Faith says. "We think our study is a major step towards understanding the depth of anthropogenic impacts on large mammal communities, and provides a convincing counter-argument to these long-held views about our early ancestors."

To test for ancient hominin impacts, the researchers compiled a seven-million-year record of herbivore extinctions in eastern Africa, focusing on the very largest species, the so-called 'megaherbivores' (species over 2,000 lbs.) Though only five megaherbivores exist in Africa today, there was a much greater diversity in the past. For example, three-million-year-old 'Lucy' (Australopithecus afarensis) shared her woodland landscape with three giraffes, two rhinos, a hippo, and four elephant-like species at Hadar, Ethiopia.

When and why these species disappeared has long been a mystery for archaeologists and paleontologists, despite the evolution of tool-using and meat-eating hominins getting most of the blame.

"Our analyses show that there is a steady, long-term decline of megaherbivore diversity beginning around 4.6 million years ago. This extinction process kicks in over a million years before the very earliest evidence for human ancestors making tools or butchering animal carcasses and well before the appearance of any hominin species realistically capable of hunting them, like Homo erectus," says Faith.

Taking a Closer Look

Faith and his team quantified long-term changes in eastern African megaherbivores using a dataset of more than 100 fossil assemblages spanning the last seven million years. The team also examined independent records of climatic and environmental trends and their effects, specifically global atmospheric CO₂, stable carbon isotope records of vegetation structure, and stable carbon isotopes of eastern African fossil herbivore teeth, among others.

Their analysis reveals that over the last seven million years substantial megaherbivore extinctions occurred: 28 lineages became extinct, leading to the present-day communities lacking in large animals. These results highlight the great diversity of ancient megaherbivore communities, with many having far more megaherbivore species than exist today across Africa as a whole.

Further analysis showed that the onset of the megaherbivore decline began roughly 4.6 million years ago, and that the rate of diversity decline did not change following the appearance of Homo erectus, a human ancestor often blamed for the extinctions. Rather, Faith's team argues that climate is more likely culprit.

"The key factor in the Plio-Pleistocene megaherbivore decline seems to be the expansion of grasslands, which is likely related to a global drop in atmospheric CO₂ over the last five million years," says John Rowan, a postdoctoral scientist from University of Massachusetts Amherst. "Low CO₂ levels favor tropical grasses over trees, and as a consequence savannas became less woody and more open through time. We know that many of the extinct megaherbivores fed on woody vegetation, so they seem to disappear alongside their food source."

The loss of massive herbivores may also account for other extinctions that have also been attributed to ancient hominins. Some scientist suggest that competition with increasingly carnivorous species of Homo led to the demise of numerous carnivores over the last few million years. Faith and his team suggest an alternative.

"We know there are also major extinctions among African carnivores at this time and that some of them, like saber-tooth cats, may have specialized on very large prey, perhaps juvenile elephants" says Paul Koch. "It could be that some of these carnivores disappeared with their megaherbivore prey."

"Looking at all of the potential drivers of the megaherbivore decline, our analyses suggest that changing climate and environment played the key role in Africa's past extinctions," said Faith. "It follows that in the search for ancient hominin impacts on ancient African ecosystems, we must focus our attention on the one species known to be capable of causing them -- us, Homo sapiens, over the last 300,000 years."

原始論文：J. Tyler Faith, John Rowan, Andrew Du, Paul L. Koch. Plio-Pleistocene decline of African megaherbivores: No evidence for ancient hominin impacts. Science, 2018; 362 (6417): 938 DOI: 10.1126/science.aau2728

引用自：University of Utah. "Human ancestors not to blame for ancient mammal extinctions in Africa”