Enough oxygen on Earth long
before animals rose
地球在動物出現許久之前就已經擁有相當充足的氧氣
Oxygen is crucial for the existence of
animals on Earth. But, an increase in oxygen did not apparently lead to the rise
of the first animals. New research shows that 1.4 billion years ago there was
enough oxygen for animals -- and yet over 800 million years went by before the
first animals appeared on Earth.
動物能生活在地球上氧氣扮演了至關重要的角色。然而，大氣氧含量的提升似乎並未促成第一批動物出現。新研究顯示雖然在14億年前大氣氧含量已經多到足以使動物生存，但還要再過8千萬年地球上首批動物才會誕生。
Animals evolved by about 600 million years ago, which was late in Earth's history. The late evolution of animals, and the fact that oxygen is central for animal respiration, has led to the widely promoted idea that animal evolution corresponded with a late a rise in atmospheric oxygen concentrations.
動物大約在6000萬年前演化出來，在地球史上算是相當近代的事件。動物於晚近才演化而成，加上氧氣是動物呼吸作用中的核心原料，使得一項廣為宣揚的想法認為動物的演化歷程可對應至大氣氧濃度於晚期才提升。
"But sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies," say postdoc Emma Hammarlund and Professor Don Canfield, Nordic Center for Earth Evolution, University of Southern Denmark.
「然而，我們的研究指出，僅靠充足的氧氣本身似乎並不足以讓動物出現。」南丹麥大學北歐地球演化中心的博士後研究員Emma Hammarlund和教授Don Canfield說。
Together with colleagues from the China National Petroleum Corporation and the University of Copenhagen, Hammarlund and Canfield have analyzed sediment samples from the Xiamaling Formation in China. Their analyses reveal that a deep ocean 1.4 billion years ago contained at least 4% of modern oxygen concentrations.
Hammarlund和Canfield跟中國石油天然氣集團以及哥本哈根大學合作，分析了來自中國下馬齡層的沉積物樣品。他們的分析結果顯示14億年前深海的氧濃度至少已達到現今氧濃度的4%。
The new study is published in the journal Proceedings of National Academy of Sciences.
此篇新研究刊登於《美國國家科學院院刊》(Proceedings of National Academy of Sciences)。
Usually it is very difficult to precisely determine past oxygen concentrations. The new study, however, combines several approaches to break new ground in understanding oxygen concentrations 1.4 billion years ago.
要精確分析過往的氧濃度通常是件十分困難的工作。然而，此篇研究結合了多種方法而開創出可以得知14億年前氧濃度的新方式。
The study uses trace metal distributions to show that the bottom waters where the Xiamaling Formation sediments deposited contain oxygen. The distribution of biomarkers, molecules derived from ancient organisms, demonstrate that waters of intermediate depth contain no oxygen. Therefore, the Xiamaling Formation deposited in an ancient oxygen-minimum zone, similar to (but also different) from those found off the present coasts of Chile and Peru.
此篇研究利用稀有金屬元素的分布模式來顯示下馬齡層沉積物沉積時的底層水含有氧氣。而生物標記(biomarker，古代有機體產生的分子)則顯現出中層水並未存有氧氣。因此下馬齡層的沉積環境為遠古時候的低氧帶(oxygen-minimum zone)，與現今在祕魯和智利外海發現的環境頗為相似(但仍有所差異)。
With this backdrop, the researchers used a simple ocean model to estimate the minimum concentrations to atmospheric oxygen required to reproduce the distribution of water-column oxygen in the Xiamaling Formation.
有了這項背景資料，研究人員便可以利用簡化的海洋模型來預估大氣中的氧濃度最低要有多少，才能重現下馬齡層沉積時水體中的氧含量分布情況。
"The water column had an oxygen concentration at least 4 % of present atmospheric levels (PAL). That should be sufficient for animals to exist and evolve," says Canfield.
「當時水體中的氧含量至少已經有現今量(present atmospheric levels，PAL)的4%，而這應該足以使動物誕生並開始演化。」Canfield說。
"Having determined the lowest concentration of oxygen in the air almost one and a half billion years ago is unique," says Hammarlund, adding:
「能夠確認將近15億年前大氣的最低氧含量是項十分獨特的成果。」Hammarlund說。他補充：
"Researchers know of simple animals, such as sponges and worms, that today are capable of managing with less than 4% PAL, even much less."
「研究人員知道現存的一些簡單生物，像是海棉與蠕蟲，可以生活在氧濃度小於PAL 4%，甚至是濃度低上許多的環境中。」
"Sponges probably resemble some of the first animals on Earth. If they manage with less than 4 % today's oxygen levels, it is likely that the first animals could do with these concentrations or less," says Canfield.
「海綿可能跟地球上最早出現的生物十分相似。如果牠們可以在氧含量小於4%現今氧濃度的環境中苟活，那麼首批生物也很可能可以在同樣或者更低的氧濃度下生存。」Canfield說。
The results differ from other studies and raise several questions, such as: Why then did animals rise so late in Earth's history?
這項結果跟其他的研究並不相同，且引起了某些疑問，比如說：那為什麼在地球史中動物這麼晚才出現？
"The sudden diversification of animals probably was a result of many factors. Maybe the oxygen rise had less to do with the animal revolution than we previously assumed," says Hammarlund.
「動物急遽多樣化可能是由許多因素造成。或許氧濃度的升高在動物演化中扮演的腳色比我們先前認為的還要渺小。」Hammarlund說。
引用自:University of Southern Denmark.
"Enough oxygen on Earth long before animals rose." ScienceDaily.
ScienceDaily, 4 January 2016.
沒有留言:
張貼留言