海洋生物的演化歷程在1.7億年前就此改變
Alan
Williams
新研究表示大約在1.7億年前，全球海洋生態系的演化規則發生轉變，因而塑造出我們今日所見的海洋。
鈣板藻是一種主要的鈣質浮游生物，上圖為它們製造的殼體。圖片來源：Tim
Bralower
在這起事件發生之前，海洋生物能否取得成功的關鍵取決於環境中的非生物因子，像是海洋化學和氣候。
然而，大約在1.7億年前的侏羅紀中期之後，生物有關的因子則越發重要，像是獵物和掠食者之間的關係。
科學家最近發表在《自然―地球科學》(Nature
Geoscience)的文章中，表示這項改變發生時，分泌碳酸鈣的浮游生物也變得相當繁榮，隨後大量堆積在海床上。
他們認為這類浮游生物的崛起穩定了海洋的化學成分，使得環境條件有利於發生地球歷史上最亮眼的海洋生物多樣化事件之一。
英國普利茅斯大學地理、地球與環境科學院，以及電腦科學與數學科學院的學者領導了這項研究。與其合作的科學家來自挪威的卑爾根大學以及德國的埃爾朗根―紐倫堡大學。
普利茅斯大學的博士候選人Kilian
Eichenseer是研究主要作者，他解釋了鈣質浮游生物帶來的影響：「現代許多海床表面覆蓋了成分跟粉筆一樣的物質，它們的組成是在侏羅紀中期崛起並統治海洋的微小生物。這種類似於粉筆的物質有助於平衡海洋的酸度，造成生物不用跟過往一樣，受到海洋化學的短期波動左右。在海洋化學穩定的情況下，不論生物殼體的礦物組成是什麼都可以很容易地分泌出來。」
研究目標是要測試一項理論：在地質歷史上隨著時間經過，跟生物無關的環境因素對演化的重要性也逐漸降低。
多細胞生物自5.4億年前出現之後，演化的過程就一直同時受到環境中的生物和非生物因素影響，但是這兩種因子的重要程度如何消長，大部分仍不為所知。
鈣質海洋生物的殼體是解決這項問題的理想材料，因為組成殼體的礦物――霰石和方解石――也能經由海中的非生物作用形成。
在此研究中，作者運用了全球各地的大量化石紀錄來探討分泌碳酸鈣的海洋生物，年代從西元前一萬年至距今五億年前，總計超過了40萬件樣品。
作者利用了古代海洋溫度與海水成分的重建結果，估計五億年來85個地質年代階中，有多少比例的霰石與方解石是透過無機作用形成。
他們透過一連串特別為此開發的統計方法，估計出海裡無機作用形成的霰石和方解石的變化模式，再和同一期間海洋生物殼體的礦物組成比較。
結果顯示早於約1.7億年前的侏儸紀中期，分泌殼體的海洋生物能在生態系中取得成功，和他們的殼體成分有密切關係。也就是說，分泌出來的殼體成分是環境偏好的生物，就具有演化優勢。
然而，鈣質浮游生物的崛起使得碳酸鈣的生成地點從大陸棚擴散至開放海洋。這項變革從此改變了地球的生物系統。
這確保了之後的劇烈氣候變遷以及連帶造成的海洋酸化，相較於地球歷史更早之前發生的類似事件，對演化的衝擊較為緩和。
普利茅斯大學的古生物學講師Uwe
Balthasar博士在2015年發表的一項研究中，首度探討了海洋環境中的霰石與方解石何者佔有優勢的問題。他說：「地球歷史上曾發生數次重大的事件，決定了地球生物的演化途徑，比方說五次大滅絕事件，或是『寒武紀大爆發』時複雜的生物大量出現。我們的研究證實了另一起之前受到忽略，卻有同等影響力的事件：過往的海洋生物在演化時受制於我們之前不知道的因素，但是分泌碳酸鈣的浮游生物大約在1.7億年前崛起而打破了這道限制，使得海洋生物的多樣性攀上了前所未見的高峰。」
Evolution of life in the ocean changed 170 million years ago
The ocean as we understand it today was
shaped by a global evolutionary regime shift around 170 million years ago,
according to new research.
Until that point, the success of organisms living
within the marine environment had been strongly controlled by non-biological
factors, including ocean chemistry and climate.
However, from the middle of the Jurassic period
onwards (some 170 million years ago), biological factors such as predator-prey
relationships became increasingly important.
Writing in Nature
Geoscience, scientists say this change coincided with the proliferation of
calcium carbonate-secreting plankton and their subsequent deposition on the
ocean floor.
They believe the rise of this plankton stabilised the
chemical composition of the ocean and provided the conditions for one of the
most prominent diversifications of marine life in Earth's history.
The research was led by academics from the University
of Plymouth's School of Geography, Earth and Environmental Sciences and School
of Computing, Electronics and Mathematics, in cooperation with scientists from
the University of Bergen in Norway, and the University of Erlangen-Nuremberg in
Germany.
PhD candidate Kilian Eichenseer, the study's lead
author, explained the impact of calcifying plankton: "Today, huge areas of
the ocean floor are covered with the equivalent of chalk, made up of
microscopic organisms that rose to dominance in the middle of the Jurassic
period. The chalky mass helps to balance out the acidity of the ocean and, with
that balance in place, organisms are less at the mercy of short-term perturbations
of ocean chemistry than they might have been previously. It is easier to
secrete a shell, regardless of its mineralogy, if the ocean chemistry is
stable."
The aim of the research was to test the hypothesis
that the evolutionary importance of the non-biological environment had declined
through geological time.
Since its emergence more than 540 million years ago,
multicellular life evolved under the influence of both the non-biological and
the biological environment, but how the balance between these factors changed
remained largely unknown.
Calcified seashells provide an ideal test to answer
this question, as aragonite and calcite -- the minerals making up seashells --
also form non-biologically in the ocean.
In their study, the authors used the vast global
fossil record of marine organisms that secreted calcium carbonate, which
encompasses more than 400,000 samples dating from 10,000 years BC up to around
500 million years ago.
Using reconstructions of the temperature and the
ocean water composition of the past, the authors estimated the proportion of
aragonite and calcite that formed inorganically in the ocean in 85 geological
stages across 500 million years.
Through a series of specially developed statistical
analyses, this inorganic pattern of aragonite-calcite seas was then compared
with seashell mineral composition over the same time.
The results show that up until the middle of the
Jurassic period, around 170 million years ago, the ecological success of
shell-secreting marine organisms was tightly coupled to their shell
composition: organisms that secreted the mineral that was environmentally
favoured had an evolutionary advantage.
However, the Earth-Life system was revolutionised
forever by the rise of calcifying plankton, which expanded the production of
calcium carbonate from continental shelves to the open ocean.
This ensured that the evolutionary impact of episodes
of severe climate changes, and resulting ocean acidification, was less severe
than comparable events earlier in Earth history.
Dr Uwe Balthasar, Lecturer in Palaeontology, first
published research exploring the dominance of aragonite and calcite in the
marine environment in 2015. He said: "During the Earth's history there
have been several major events that shaped the evolution of life on our planet,
such as the five big mass extinctions or the radiation of complex animals
during the 'Cambrian Explosion'. Our research identifies a previously
overlooked event of this magnitude around 170 million years ago when the
emergence of calcium carbonate-secreting plankton lifted constraints on the
evolution of other marine organisms that we did not know existed. As a result,
life in the ocean has diversified to levels far beyond what existed
before."
原始文章：Kilian
Eichenseer, Uwe Balthasar, Christopher W. Smart, Julian Stander, Kristian A.
Haaga, Wolfgang Kiessling. Jurassic shift from abiotic to biotic
control on marine ecological success. Nature Geoscience, 2019;
DOI: 10.1038/s41561-019-0392-9
引用自：University of Plymouth. "Evolution of life
in the ocean changed 170 million years ago."
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