原始網址：www.sciencedaily.com/releases/2016/09/160923100751.htm
強烈證據顯示氧氣多寡是動物早期演化的關鍵
這項新研究首度將氧含量高低不同的水體區分探討。結果顯示寒武紀不久之前演化而成的複雜生物無法在氧含量低的水體中生存，代表氧氣的存在與否對這些動物的出現來說是十分重要的因素。
刊登於期刊《自然通訊》(Nature Communications)的這篇研究主要依據於奈米比亞Nama群岩層進行的野外調查結果。
現在任職於牛津大學地球科學系的第一作者Rosalie Tostevin博士以這項研究的分析結果作為她倫敦大學學院地球科學博士論文的一部分。她表示：「複雜的動物為何需要這麼久的時間才出現在地球上讓科學家困惑許久。一項主張認為這不過是因為演化並非一蹴可幾；但另一項較受偏好的假說則認為海洋氧濃度升高給予簡單的生命形式演化出骨骼、機動性以及其他現代生物具備的典型特徵所需的動力。」
「雖然有地球化學證據顯示海洋氧濃度升高跟較複雜的動物出現時間大約一致，但要證實兩者之間有因果關係真得相當困難。我們分別研究氧含量高低不同的水體，發現具有骨骼的原始動物只限於富含氧氣的水體中生存，此強力證據顯示這些動物的演化過程中氧氣是關鍵因素。但是富含氧氣的環境當時可能相當稀少，使得最早期的動物在海洋中的棲地十分有限。」
此團隊還包括來自倫敦大學學院、里茲大學、劍橋大學以及奈米比亞地質調查所的地球化學家、古生物學家和地質學家。他們分析來自Nama群代表古代海床的岩石樣品中的化學元素組成。Nama群岩石是奈米比亞一處保存狀況相當良好的岩石，富含早期Cloudina、Namacalathus及Namapoikia動物的化石。
研究人員從岩石中鐵和鈰這類的元素濃度發現，在富含氧氣的表層海水和完全缺氧的深層水之間，還有低氧濃度的環境。具有骨骼的早期動物雖然在氧氣充足的環境中數量眾多，卻未佔據氧氣不足的大陸棚地區，顯示要發展出以早期動物為主的生態系，擁有足夠的氧氣(>10微莫耳濃度)是關鍵所在。
Tostevin博士論文的指導教授，同時也是共同作者之一的倫敦大學學院地球科學系Graham Shields-Zhou教授說：「我們專注研究元古宙的最後1000萬年是因為現今存活的動物群就是在地球歷史上這段期間首度披上甲殼並開始攪動沉積物，而我們發現早期動物的發展跟環境條件的關係中，氧氣含量扮演了重要腳色。」
Oxygen levels were key to early animal evolution, strongest evidence now shows
It has long puzzled
scientists why, after 3 billion years of nothing more complex than algae,
complex animals suddenly started to appear on Earth. Now, a team of researchers
has put forward some of the strongest evidence yet to support the hypothesis
that high levels of oxygen in the oceans were crucial for the emergence of
skeletal animals 550 million years ago.
The new
study is the first to distinguish between bodies of water with low and high
levels of oxygen. It shows that poorly oxygenated waters did not support the
complex life that evolved immediately prior to the Cambrian period, suggesting
the presence of oxygen was a key factor in the appearance of these animals.
The
research, based on fieldwork carried out in the Nama Group in Namibia, is
published in the journal Nature Communications.
Lead
author Dr Rosalie Tostevin completed the study analyses as part of her PhD with
UCL Earth Sciences, and is now in the Department of Earth Sciences at Oxford
University. She said: 'The question of why it took so long for complex animal
life to appear on Earth has puzzled scientists for a long time. One argument
has been that evolution simply doesn't happen very quickly, but another popular
hypothesis suggests that a rise in the level of oxygen in the oceans gave
simple life-forms the fuel they needed to evolve skeletons, mobility and other
typical features of modern animals.
'Although
there is geochemical evidence for a rise in oxygen in the oceans around the
time of the appearance of more complex animals, it has been really difficult to
prove a causal link. By teasing apart waters with high and low levels of
oxygen, and demonstrating that early skeletal animals were restricted to
well-oxygenated waters, we have provided strong evidence that the availability
of oxygen was a key requirement for the development of these animals. However,
these well-oxygenated environments may have been in short supply, limiting
habitat space in the ocean for the earliest animals.'
The team,
which included other geochemists, palaeoecologists and geologists from UCL and
the universities of Edinburgh, Leeds and Cambridge, as well as the Geological
Survey of Namibia, analysed the chemical elemental composition of rock samples
from the ancient seafloor in the Nama Group - a group of extremely
well-preserved rocks in Namibia that are abundant with fossils of early
Cloudina, Namacalathus and Namapoikia animals.
The
researchers found that levels of elements such as cerium and iron detected in
the rocks showed that low-oxygen conditions occurred between well-oxygenated
surface waters and fully 'anoxic' deep waters. Although abundant in
well-oxygenated environments, early skeletal animals did not occupy
oxygen-impoverished regions of the shelf, demonstrating that oxygen
availability (probably >10 micromolar) was a key requirement for the
development of early animal-based ecosystems.
Professor
Graham Shields-Zhou (UCL Earth Sciences), one of the co-authors and Dr
Tostevin's PhD supervisor, said: 'We honed in on the last 10 million years of
the Proterozoic Eon as the interval of Earth's history when today's major
animal groups first grew shells and churned up the sediment, and found that
oxygen levels were important to the relationship between environmental
conditions and the early development of animals.'
原始論文：R. Tostevin, R. A. Wood, G. A. Shields, S. W. Poulton, R.
Guilbaud, F. Bowyer, A. M. Penny, T. He, A. Curtis, K. H. Hoffmann, M. O.
Clarkson. Low-oxygen waters
limited habitable space for early animals. Nature Communications,
2016; 7: 12818 DOI:10.1038/ncomms12818
引用自：University College London. "Oxygen levels were key to early animal
evolution, strongest evidence now shows." ScienceDaily. ScienceDaily, 23
September 2016.
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