原文網址：https://www.cnrs.fr/en/when-variations-earths-orbit-drive-biological-evolution
鈣板藻(Coccolithophor)是一種微小的單細胞藻類，它們會在細胞外圍形成迷你的石灰岩板，稱為鈣板(coccolith)，其形狀以及大小依據種類不同而有差異。鈣板藻在死後沉到海底，身上的鈣板接著堆積在沉積物裡面，使得它們在地質時間中的詳細演化過程可以被沉積物忠實地記錄下來。
鈣板藻是浮游生物中重要的一份子。它們的演化過程依循著地球公轉軌道偏心率的變化節奏。圖片來源：Luc BEAUFORT / CNRS / CEREGE
在2021年12月1日發表於《自然》(Nature)的文章中，法國國家科學研究院主持的科學團隊指出，地球公轉軌道的某些變化影響了鈣板藻的演化方向。為了進行這項研究，他們在熱帶海域的數個地點採集了超過900萬個鈣板藻，年代涵蓋了280萬年。接著再利用自動化顯微鏡技術和人工智慧來測量並分類它們。
研究人員觀察到鈣板藻大小與形狀的多樣性高低會以40萬年和10萬年的規律來循環。他們也提出了原因：以同樣規律變化的，地球繞日軌道接近圓形的程度。因此，地球軌道跟目前一樣較接近圓形(稱作偏心率低)的時候，赤道地區沒有什麼季節變化，特化程度較低的種類便會佔據海洋各處。反之，偏心率增加使得赤道附近的季節變化較為顯著的時候，鈣板藻便會特化成許多不同的種類，但它們加總起來產生的石灰岩卻比較少。
這些生物至關重要的地方在於它們分布在全球各地且數量龐大，海洋中的石灰岩(也就是碳酸鈣，部分組成為碳)一半是由它們產生，這對於碳循環以及決定海洋化學的成分來說都具有相當大的影響。因為如此，這些石灰岩生產者數量來回變動的模式可能在過往的氣候變遷當中具有關鍵地位，或許還能解釋發生在過去的暖期，迄今尚未解開的某些謎題。換句話說，在沒有冰的時期，這些微體藻類的演化或許決定了氣候變化的節奏。
這道假說仍然有待確認。
When variations in Earth's orbit drive
biological evolution
Coccolithophores are microscopic algae
that form tiny limestone plates, called coccoliths, around their single cells.
The shape and size of coccoliths varies according to the species. After their
death, coccolithophores sink to the bottom of the ocean and their coccoliths
accumulate in sediments, which faithfully record the detailed evolution of
these organisms over geological time.
A team of scientists led by CNRS researchers show, in
an article published in Nature on the
1st December 2021, that certain variations in Earth’s orbit have influenced the
evolution of coccolithophores. To achieve this, no less that 9 million
coccoliths, spanning an interval of 2.8 million years and several locations in
the tropical ocean, were measured and classified using automated microscope
techniques and artificial intelligence.
The researchers observed that coccoliths underwent
cycles of higher and lower diversity in size and shape, with rhythms of 100 and
400 thousand years. They also propose a cause: the more or less circular shape
of Earth’s orbit around the Sun, which varies at the same rhythms. Thus, when
Earth’s orbit is more circular, as is the case today (this is known as low
eccentricity), the equatorial regions show little seasonal variation and
species that are not very specialised dominate all the oceans. Conversely, as
eccentricity increases and more pronounced seasons appear near the equator,
coccolithophores diversify into many specialised species, but collectively produce
less limestone.
Crucially, due to their abundance and global
distribution, these organisms are responsible for half of the limestone
(calcium carbonate, partly composed of carbon) produced in the oceans and
therefore play a major role in the carbon cycle and in determining ocean
chemistry. It is therefore likely that the cyclic abundance patterns of these
limestone producers played a key role in ancient climates, and may explain
hitherto mysterious climate variations in past warm periods. In other words, in
the absence of ice, the biological evolution of micro-algae could have set the
tempo of climates.
This hypothesis remains to be confirmed.
原始論文：Luc
Beaufort, Clara T. Bolton, Anta-Clarisse Sarr, Baptiste Suchéras-Marx, Yair
Rosenthal, Yannick Donnadieu, Nicolas Barbarin, Samantha Bova, Pauline
Cornuault, Yves Gally, Emmeline Gray, Jean-Charles Mazur, Martin Tetard. Cyclic
evolution of phytoplankton forced by changes in tropical seasonality. Nature,
2021; DOI: 10.1038/s41586-021-04195-7
引用自：CNRS.
"When variations in Earth's orbit drive biological evolution."
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