早期動物演化過程中的興衰更迭與氧氣大有關係
複雜動物迅速演化的關鍵時期大約始於5.4億年前。新的研究發現了和這段時期有關的重要線索。
研究探討了一段動物迅速演化的時期――「寒武紀大爆發」，結果指出大氣氧含量的劇烈變動，可以對應到演化導致動物多樣性的激增以及生物滅絕。
這段動物早期歷史中的關鍵階段起因為何？生物學家對此仍眾說紛紜。
由英國、中國、俄羅斯的科學家聯合發表在《自然―地球科學》(Nature Geoscience)的這項研究，強烈支持了下列理論：大氣氧含量是動物演化過程中相當重要的控制因素。
研究由英國利茲大學和倫敦大學學院主持，結果首度指出寒武紀大爆發期間，氧含量激增可以清楚對應至動物演化速度與多樣性遽增的時期；而滅絕事件發生時，氧氣含量也比較低。
研究主要作者是利茲大學的博士後研究員Tianchen
He博士，他在倫敦大學學院時開始進行這項研究。他說：「寒武紀大爆發時出現的複雜動物是現今許多動物的祖先。」
「但我們沒有那時大氣氧含量的直接紀錄，因此長久以來都很難確定這段演化上的關鍵時期，可能的起因是什麼。」
「然而，透過分析古代岩石中的碳硫同位素，我們可以找出寒武紀大爆發期間，地球大氣和淺海的氧氣含量如何變化。」
「比較同一時間的動物化石，我們可以清楚看見適應輻射的興衰更迭，跟氧濃度的高低漲落有一致關係。」
「這強烈顯示出早期動物的出現過程中，氧氣具有相當重要的地位。」
研究共同作者，倫敦大學學院地球科學系的教授Graham
Shields表示：「這是首度有研究可以清楚指出，我們最早的動物祖先曾經因為大氣氧濃度的極端變化，而經歷了一連串的適應輻射和演化瓶頸。」
「結果是在寒武紀的其中1300萬年，新的動物型態大量演化出來，成為名副其實的大爆發。」
「地球過往的居民為型態簡單、由單細胞組成且無法動彈的生物。寒武紀大爆發之後，地球開始擁有各式各樣複雜精巧且活力十足的生物，就像我們今日所見的一般。」
研究團隊從西伯利亞阿爾丹河和勒拿河沿岸的地層，採集了海洋碳酸鹽的岩石樣本，並分析其中的碳硫同位素。
寒武紀大爆發時此區的環境是一片淺海，當時地球上的多數生物都住在淺海。
西伯利亞的下部寒武系地層具有連續堆積的石灰岩層，其中的化石含量相當豐富並具備可信的定年結果，使這些岩石樣品成為良好的地球化學分析對象。
岩石中的同位素訊號反映了全球氧氣的產量，因此團隊可以藉此得出寒武紀的淺海與大氣含有多少氧氣。
共同作者，利茲大學地球與環境科學院的Benjamin
Mills博士說：「西伯利亞地台給了我們一個難得的機會觀察早期海洋的生態系。在寒武紀大爆發時變得多樣化的動物化石，已知的種類有半數以上都能在此發現。」
「結合同位素的測量結果跟數值模型，我們可以找出在這個重要的演化搖籃，哪些時期有大量的碳和硫進到沉積物裡面。
我們的模型利用這些資訊可以估計全球氧氣的收支情形，使我們能以全新的觀點看出氧氣如何塑造出現今活在地球上的生物。」
共同作者，中國科學院南京地質古生物研究所的朱茂炎表示：「想了解寒武紀大爆發的成因需要進行跨領域研究。我們花了很長一段時間才得到結果，早在2008年我們就已經從西伯利亞取得了樣品。」
「這些位在西伯利亞的岩層很難到達。我們花了許多時間規劃實地考察的行程並採集樣品。如果沒有俄羅斯研究人員的協助，這項計畫絕對不可能完成。」
Oxygen linked with the boom and bust
of early animal evolution
New research reveals clues to a crucial
period of rapid evolution in complex animals that began roughly 540 million
years ago.
The research examines the period of rapid evolution,
known as the Cambrian explosion, and indicates extreme fluctuations in
atmospheric oxygen levels corresponded with evolutionary surges and extinctions
in animal biodiversity.
The trigger for this fundamental phase in the early
history of animal life is a subject of ongoing biological debate.
The study, published today in Nature Geoscience by scientists from the UK, China and Russia,
gives strong support to the theory that oxygen content in the atmosphere was a
major controlling factor in animal evolution.
Led by the University of Leeds and UCL, the study is
the first to show that during the Cambrian explosion there was significant
correlation between surges in oxygen levels and bursts in animal evolution and
biodiversity, as well as extinction events during periods of low oxygen.
Dr Tianchen He, study lead author and postdoctoral
researcher at the University of Leeds, began this research while at UCL. He
said: “The complex creatures that came about during the Cambrian explosion were
the precursors to many of the modern animals we see today.
"But because there is no direct record of
atmospheric oxygen during this time period it has been difficult to determine
what factors might have kick started this crucial point in evolution.
“By analysing the carbon and sulphur isotopes found
in ancient rocks, we are able to trace oxygen variations in Earth’s atmosphere
and shallow oceans during the Cambrian Explosion.
"When compared to fossilised animals from the
same time we can clearly see that evolutionary radiations follow a pattern of
‘boom and bust’ in tandem with the oxygen levels.
“This strongly suggests oxygen played a vital role in
the emergence of early animal life.”
Study co-author Professor Graham Shields from UCL
Earth Sciences, said: “This is the first study to show clearly that our
earliest animal ancestors experienced a series of evolutionary radiations and
bottlenecks caused by extreme changes in atmospheric oxygen levels.
“The result was a veritable explosion of new animal
forms during more than 13 million years of the Cambrian Period.
"In that time, Earth went from being populated
by simple, single-celled and immobile organisms to hosting the wonderful
variety of intricate, energetic life forms we see today.”
The team analysed the carbon and sulphur isotopes
from marine carbonate samples collected from sections along the Aldan and Lena
rivers in Siberia.
During the time of the Cambrian explosion this area
would have been a shallow sea and the home for the majority of animal life on
Earth.
The lower Cambrian strata in Siberia are composed of
continuous limestone with rich fossil records and reliable age constraints,
providing suitable samples for the geochemical analyses.
The isotope signatures in the rocks relate to the
global production of oxygen, allowing the team to determine oxygen levels
present in the shallow ocean and atmosphere during the Cambrian Period.
Study co-author Dr Benjamin Mills, from the School of
Earth and Environment at Leeds, said: “The Siberian Platform gives us a unique
window into early marine ecosystems. This area contains over half of all
currently known fossilised diversity from the Cambrian explosion.
“Combining our isotope measurements with a mathematical
model lets us track the pulses of carbon and sulphur entering the sediments in
this critical evolutionary .
Our model uses this information to estimate the
global balance of oxygen production and destruction, giving us new insight into
how oxygen shaped the life we have on the planet today.”
Study co-author Maoyan Zhu from Nanjing Institute of
Geology and Palaeontology, Chinese Academy of Sciences, said: “Understanding
what triggered the Cambrian explosion requires multidisciplinary study. It took
a long time to get this result. We already got samples from Siberia in 2008.
"The sections in Siberia are difficult to
access. It took time for us to organize the expedition and collect the samples
there. Without support from Russian colleagues, we could not have done the
project.”
原始論文：Tianchen
He, Maoyan Zhu, Benjamin J. W. Mills, Peter M. Wynn, Andrey Yu. Zhuravlev,
Rosalie Tostevin, Philip A. E. Pogge von Strandmann, Aihua Yang, Simon W.
Poulton and Graham A. Shields. Possible
links between extreme oxygen perturbations and the Cambrian radiation of
animals. Nature Geoscience, 2019
DOI: 10.1038/s41561-019-0357-z
引用自：University of Leeds. "Oxygen linked with
the boom and bust of early animal evolution."
原文網址：http://www.leeds.ac.uk/news/article/4411/oxygen_linked_with_the_boom_and_bust_of_early_animal_evolution
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