證據顯示地球上首次生物大滅絕的元凶並非重大災害，而是生物本身

原文網址：www.sciencedaily.com/releases/2015/09/150902123456.htm

Evidence that Earth's first mass extinction was caused by critters not catastrophe

證據顯示地球上首次生物大滅絕的元凶並非重大災害，而是生物本身

A powerful analogy for what is happening today

對當今情況的強烈類比

In the popular mind, mass extinctions are associated with catastrophic events, like giant meteorite impacts and volcanic super-eruptions.

在大眾認知中，大滅絕事件通常會跟重大天災扯上關聯，像是巨型隕石撞擊，以及超級火山噴發。

But the world's first known mass extinction, which took place about 540 million years ago, now appears to have had a more subtle cause: evolution itself.

然而現在卻發現在約5億4000萬年前發生，世上已知的首次大滅絕事件成因相當微妙：即演化本身。

"People have been slow to recognize that biological organisms can also drive mass extinction," said Simon Darroch, assistant professor of earth and environmental sciences at Vanderbilt University. "But our comparative study of several communities of Ediacarans, the world's first multicellular organisms, strongly supports the hypothesis that it was the appearance of complex animals capable of altering their environments, which we define as 'ecosystem engineers,' that resulted in the Ediacaran's disappearance."

「人們過了相當久才認知到生物本身也能驅使生物大滅絕發生。」范德堡大學地球與環境科學系的助理教授，Simon Darroch說。「地球上第一種多細胞生物群稱作埃迪卡拉動物(Ediacaran)。我們研究並比較不同的埃迪卡拉動物群落，結果強烈支持牠們之所以消失，是因為複雜動物出現而導致的假說。由於這些複雜動物能夠改變牠們生存的環境，因此我們也稱呼牠們為「生態工程師」(ecosystem engineer)。」

The study is described in the paper "Biotic replacement and mass extinction of the Ediacara biota" published Sept. 2 in the journal Proceedings of the Royal Society B.

他們的研究記述在以《埃迪卡拉動物群的生物群演替與大滅絕》(Biotic replacement and mass extinction of the Ediacara biota)為題的論文中，刊登於9月2日的《皇家學會報告B系列》期刊。

"There is a powerful analogy between the Earth's first mass extinction and what is happening today," Darroch observed. "The end-Ediacaran extinction shows that the evolution of new behaviors can fundamentally change the entire planet, and we are the most powerful 'ecosystem engineers' ever known."

「眼下發生之事與地球首度發生的大滅絕事件彼此之間有許多相似之處。」Darroch觀察到。「消滅埃迪卡拉動物的大滅絕事件展現了生物演化出的新行為模式，能從基本層面改變整個星球，而我們是已知的生態工程師中最強而有力者。」

The earliest life on Earth consisted of microbes -- various types of single-celled microorganisms. They ruled the Earth for more than 3 billion years. Then some of these microorganisms discovered how to capture the energy in sunlight. The photosynthetic process that they developed had a toxic byproduct: oxygen. Oxygen was poisonous to most microbes that had evolved in an oxygen-free environment, making it the world's first pollutant.

地球上最早的生命由微生物組成，包括了各種單細胞微生物。在超過30億年的時光當中，地球由它們統治著。接著某些微生物發明出新方法來汲取陽光中蘊含的能量。它們發展出光合作用的同時卻產生了一種有毒副產物：氧氣。對在無氧環境下演化而成的大多數微生物來說，氧氣具有致命的毒性，因此氧氣導致了世上第一起汙染事件。

But for the microorganisms that developed methods for protecting themselves, oxygen served as a powerful new energy source. Among a number of other things, it gave them the added energy they needed to adopt multicellular forms. Thus, the Ediacarans arose about 600 million years ago during a warm period following a long interval of extensive glaciation.

然而，對那些研發出防禦機制保護自己的微生物來說，氧氣反倒成為了一種強力新型能源。除了一些其他的革新，氧氣能提供它們要實行多細胞生活所需的額外能量。因此，經過了歷時漫長而範圍廣大的冰河期後，埃迪卡拉動物於6億年前在接續而至的溫暖時期崛起。

"We don't know very much about the Ediacarans because they did not produce shells or skeletons. As a result, almost all we know about them comes from imprints of their shapes preserved in sand or ash," said Darroch.

「因為埃迪卡拉動物並不會形成甲殼或者骨骸，使得我們對牠們所知甚少。我們對牠們的相關知識，幾乎來自於牠們的身形拓印在砂粒或者火山灰之上的痕跡。」Darroch說。

What scientists do know is that, in their heyday, Ediacarans spread throughout the planet. They were a largely immobile form of marine life shaped like discs and tubes, fronds and quilted mattresses. The majority were extremely passive, remaining attached in one spot for their entire lives. Many fed by absorbing chemicals from the water through their outer membranes, rather than actively gathering nutrients.

科學家所知的是，在那段太平盛世，埃迪卡拉動物幾乎遍布整個地球。牠們大多是無法移動的海洋生物，外型狀似餐盤、水管、植物或者床墊。多數種類終其一生固著在某個地點之上，過著相當被動的生活。許多以吸收海水流過牠們的外膜時順道帶來的化學物質過活，而非主動去攝取營養。

Paleontologists have coined the term "Garden of Ediacara" to convey the peace and tranquility that must have prevailed during this period. But there was a lot of churning going on beneath that apparently serene surface.

古生物學家創造出「埃迪卡拉花園」這個詞彙，來傳達出當時瀰漫的和平寧靜氛圍。然而在這平靜的表象之下，實則暗潮洶湧。

After 60 million years, evolution gave birth to another major innovation: animals. All animals share the characteristics that they can move spontaneously and independently, at least during some point in their lives, and sustain themselves by eating other organisms or what they produce. Animals burst onto the scene in a frenzy of diversification that paleontologists have labeled the Cambrian explosion, a 25-million-year period when most of the modern animal families -- vertebrates, molluscs, arthropods, annelids, sponges and jellyfish -- came into being.

6000萬年之後，演化又產生了另一項重大革新：動物。所有動物都擁有相同特點：至少在生命史上的某個階段，牠們能夠自發且主動移動；並以攝食其他生物本體或製造物為生。在短短2500萬年的時光中，大多數現存的動物門，像是脊索動物、軟體動物、節肢動物、環節動物、海綿、水母先後出現在地球上。這起動物以各式各樣的姿態赫然出現於生命舞台上的事件，古生物學家稱為「寒武紀大爆發」。

"These new species were 'ecological engineers' who changed the environment in ways that made it more and more difficult for the Ediacarans to survive," said Darroch.

「這些新物種為「生態工程師」，牠們改變了環境而使得埃迪卡拉動物越來越難以生存。」Darroch說。

He and his colleagues performed an extensive paleoecological and geochemical analysis of the youngest known Ediacaran community exposed in hillside strata in southern Namibia. The site, called Farm Swartpunt, is dated at 545 million years ago, in the waning one to two million years of the Ediacaran reign.

他和他的同僚詳盡分析了已知最年輕的埃迪卡拉生物群所處地層的古生物及地球化學特徵。此地層出露位置為奈米比亞南部，稱作Swartpunt農場的山丘；年代約為5億4500萬年前，約是埃迪卡拉時代的最後一至兩百萬年。

"We found that the diversity of species at this site was much lower, and there was evidence of greater ecological stress, than at comparable sites that are 10 million to 15 million years older," Darroch reported. Rocks of this age also preserve an increasing diversity of burrows and tracks made by the earliest complex animals, presenting a plausible link between their evolution and extinction of the Ediacarans.

「相較於其他早了1000萬至1500年的場址，我們發現這個場址的埃迪卡拉物種多樣性低了許多，且證據顯示出牠們受到了十分強大的生態壓力。」Darroch發表道。保存在此年代岩石中，由最古老的複雜動物挖掘而成的軌跡和洞穴，種類也變得越來越豐富，顯現牠們的演化與埃迪卡拉動物的消失間有一定關聯。

The older sites were Mistaken Point in Newfoundland, dating from 579 to 565 million years ago; Nilpena in South Australia, dating from 555 to 550 million years ago; and the White Sea in Russia, dating also from 555 to 550 million years ago million years ago.

這些年代較老的場址包括了5億7900萬至5億6500萬年前，紐芬蘭的Mistaken Point；5億5500萬至5億5000萬年前，南澳的Nilpena，以及相同時代，位於俄羅斯的White Sea。

Darroch and his colleagues made extensive efforts to ensure that the differences they recorded were not due to some external factor.

Darroch和他的同僚盡力確保這些場址的化石差異並非來自於其他外在因素。

For example, they ruled out the possibility that the Swartpunt site might have been lacking in some vital nutrients by closely comparing the geochemistry of the sites.

舉例來說，他們仔細比較了各場址的地球化學性質，而排除了Swartpunt的物種多樣性低落，是因為缺少某些重要營養素的可能性。

It is a basic maxim in paleontology that the more effort that is made in investigating a given site, the greater the diversity of fossils that will be found there. So the researchers used statistical methods to compensate for the variation in the differences in the amount of effort that had been spent studying the different sites.

有一條古生物學名言是：在一個場址調查越久，就會發現越多種化石。因此研究人員利用統計方法，來彌補花費在不同場址的心力不同，而導致化石發現數量的差異。

Having ruled out any extraneous factors, Darroch and his collaborators concluded that "this study provides the first quantitative palaeoecological evidence to suggest that evolutionary innovation, ecosystem engineering and biological interactions may have ultimately caused the first mass extinction of complex life."

屏除了所有外在因素後，Darroch和他的同僚總結說：「本研究首度以定量化的古生態學方法，證實了演化革新、生態工程和生物之間的互動，可能最終導致了發生在複雜生命上的首度大滅絕。」

引用自：Vanderbilt University. "Evidence that Earth's first mass extinction was caused by critters not catastrophe: A powerful analogy for what is happening today." ScienceDaily. ScienceDaily, 2 September 2015.