缺乏肥料使得動物演化受阻了數十億年之久?

原文網址：www.sciencedaily.com/releases/2016/12/161221132815.htm

缺乏肥料使得動物演化受阻了數十億年之久?

一則對地球歷史其中35億年的研究指出，淺海缺乏磷的現象結束，跟演化長足進展的時刻一致

第一隻動物已經準備好要演化出來，就只差最後一步，卻等了三十億年甚至更久。因為可供呼吸的氧氣還未出場，或許還要歸咎於缺乏某些簡單的營養成分。

接著，整個星球激烈地轉變成另外一種樣貌。根據喬治亞理工學院和耶魯大學的地球化學家進行的新研究，在大約8億年前的元古宙(Proterozoic Eon)晚期，所有生物皆需要的化學元素―磷，開始廣泛地在全球海岸線旁的淺海地帶累積，而形成了動物和其他複雜生物誕生的溫床。

隨著磷的累積，全球發生了一連串包含其他養分在內的化學連鎖反應，而助長生物將氧氣挹注至大氣與海洋當中。一則可信度高的假説認為，在這項轉變不久之後，發生了兩次極端氣候席捲全球，造成整個地球被冰封數億年的事件。可以取得更多養分，加上氧濃度提升，可能也有助於引發演化史上最大的躍進。

經過數十億年生命幾乎僅由單細胞生物構成的狀態之後，動物終於演化出來。起初牠們相當原始，跟今日的海綿或水母相去不遠；但代表地球已經從數十億年以來，幾乎不適合複雜生命生長的環境，開始邁入成為充滿各式各樣複雜生物的星球。

地球的真實創生過程

在接下來數億年，生物多樣性開枝散葉，形成了茂密的叢林與草原，迴盪著各種動物的鳴叫聲；水裡則有外型千變萬化，顏色五花八門的生物悠游其中。在這演變過程中的大多階段都在化石記錄中留下了痕跡。

雖然研究人員很小心地不傳遞出磷是整個連鎖反應必要因素的暗示，但是從海岸地區採集到的沉積岩中，磷確實標出了生命大躍進和氣候變化發生的時間點。喬治亞理工學院地球和大氣科學院的助理教授Chris Reinhard說：「這個時間點確實十分引人注目。」

Reinhard和耶魯大學的地球化學家Noah Planavsky共同主持了這項研究。他們層層往下挖掘形成於遠古海岸地區，直至年代為35億年前的沉積岩。利用這些岩石記錄，他們計算磷這種重要肥料的化學循環演變過程，而它又是如何在地球真正的創生過程中占據重要章節。

當他們隨著頁岩岩層往上進入到動物出現的時刻，也就是元古宙晚期，他們注意到某種相當明顯的一致性變化。

「最根本的改變是在海洋表層水體當中，磷的供應從本來相當有限的狀況變得提高許多。」Reinhard表示。「而轉變發生的時機，似乎剛好在海洋-大氣氧含量大幅變化，以及動物正要出現之前的時間點附近。」

海邊的磷

Reinhard、Planavsky以及他們領導的研究團隊，提出的理論認為在厭氧(anoxic，幾乎沒有氧分子存在)條件的世界中，缺乏養分或是其他形式的毒害抑制了光合作用生物生長，使得氧氣無法累積至少20億年之久。接著這個系統的平衡狀態被推翻，讓海洋中的磷得以到達淺海水體。

科學家將他們的發現刊登於期刊《自然》(Nature)之中。此研究的經費來自美國國家科學基金會、NASA天體生物學研究所、史隆基金會和日本學術振興會。

這項成果對於是什麼因素讓生命得以重塑地球大氣提供了嶄新觀點，也有助於科學家建立理論基礎，來預測生命改造地外行星的大氣層時，需要什麼樣的條件。或許還能啟發科學家更深入探討地球的海洋-大氣化學變化如何造成氣候動盪，以及對漫長歷史中生命的興衰有何影響。

藍綠菌，氧氣之母

包含動物在內的複雜生命形式，通常都具有龐大的代謝系統。這需要大量氧氣來驅動，因此難以想像動物能在沒有氧氣的情況下演化出來。

缺乏養分為何會妨礙可供呼吸的氧氣產生？要了解這番脈絡得先從一種非常特別的細菌說起。這種稱作藍綠菌的細菌為地球氧氣之母。

「我們之所以能住在一顆充滿氧氣的星球，唯一原因便是地球具備產氧光合作用(oxygenic photosynthesis)生物。」Planavsky說。「像藍綠菌這類可行使光合作用的細胞在結合二氧化碳和水以產生糖分時，排出的廢物即為氧氣。」

而光合作用為演化唯一性(evolutionary singularity)，意味者在地球歷史中其只演化出一次—就在藍綠菌體內。

在地球歷史不同年代，其他一些生命現象曾在數十起或數百起的獨立事件中重複演化出來，比方單細胞生物演化成具有雛型的多細胞生物。但科學家相當有信心，產氧光合作用在地球歷史上就僅在藍綠菌體內演化出一次，而地球上的植物和行光合作用的生物，都是從這次演化衍生出來的。

被鐵錨所固定

藍綠菌的功勞便是使地球大氣充滿氧氣，而它們已存在了將近25億年甚至更久。

這帶出了一項問題：為什麼要經歷這麼久的時間?科學家提出的假說為藍綠菌無法輕易取得它們所需的基本養分。Planavsky和Reinhard特別鎖定的磷雖然也已經在海洋中存在了數十億年，但它們卻被封存在錯誤的地方。

在那數十億年之中，曾經充滿整個海洋的礦物質―鐵，可能會跟磷結合而一同沉往黑暗的海洋深處。這使得磷被帶離稱作大陸邊緣的淺海地帶，此為藍綠菌生存並產生氧氣所需的環境。即使到了今日，人們在處理被肥料污染的水體時，也是利用鐵會跟磷形成沉澱的原理來去除磷。

研究人員同時也運用地球化學模型來證實在低氧濃度的世界中，當淺海環境處於高鐵濃度和可供利用的磷與氮相當稀少的狀態，地球系統可以在這種情況下長期維持自身恆定。

「這個行星系統看起來相當穩定，」Reinhard說。「但很明顯我們現在居住的星球並非如此，因此問題便是：我們要如何把低氧狀態轉變成我們現今所處的狀態？」

導致此變化的根本因素有待未來研究來解答。

由磷開響第一槍

但在8億年前確實發了某些變化，此時大陸邊緣生態系中的藍綠菌和其他微小生物獲得了更多的磷。磷是構成DNA和RNA的骨架，也是細胞代謝作用中的要角之一。這些細菌變得更加活躍且繁殖更加快速，吞食大量的磷並產生更多氧氣。

「磷不僅只是生命的必需成份。」Planavsky說。「所有證據皆清楚表明：它可以控制地球生命的多寡。」

當大量新生細菌死亡之後會沉到淺海海床。它們在此層層堆積並逐漸分解，使得泥巴磷含量大大增加。最終，這些淺海海床的泥巴會被壓縮成岩石。

「隨著生物體內的磷含量增加，就會有更多磷進入到沉積岩層當中。」Reinhard表示。「對於科學家來說，頁岩就是紀錄海床歷史文獻的頁面。」

科學家數十年來不停翻閱這些頁岩以蒐集資訊。Planavsky和Reinhard為了進行這項研究而分析了將近15,000個岩石紀錄。

「起初我們能蒐集到的僅有600份樣品。」Planavsky說。Reinhard接著表示：「但之後就像你看到的。當時磷的供應拮据可說顯而易見。隨著資料庫持續擴充，也越來越確定這個現象曾經發生。」

代表地球淺海有磷出現的首筆訊號出現於頁岩紀錄之時，也鳴槍宣佈了豐富地球生命的競賽正式開始。

A fertilizer dearth foiled animal evolution for eons?

End of phosphorus dearth in ocean shallows coincides with evolutionary surge in study of 3.5 billion years of Earth's history

For three billion years or more, the evolution of the first animal life on Earth was ready to happen, practically waiting in the wings. But the breathable oxygen it required wasn't there, and a lack of simple nutrients may have been to blame.

Then came a fierce planetary metamorphosis. Roughly 800 million years ago, in the late Proterozoic Eon, phosphorus, a chemical element essential to all life, began to accumulate in shallow ocean zones near coastlines widely considered to be the birthplace of animals and other complex organisms, according to a new study by geoscientists from the Georgia Institute of Technology and Yale University.

Along with phosphorus accumulation came a global chemical chain reaction, which included other nutrients, that powered organisms to pump oxygen into the atmosphere and oceans. Shortly after that transition, waves of climate extremes swept the globe, freezing it over twice for tens of millions of years each time, a highly regarded theory holds. The elevated availability of nutrients and bolstered oxygen also likely fueled evolution's greatest lunge forward.

After billions of years, during which life consisted almost entirely of single-celled organisms, animals evolved. At first, they were extremely simple, resembling today's sponges or jellyfish, but Earth was on its way from being, for eons, a planet less than hospitable to complex life to becoming one bursting with it.

Earth's true genesis

In the last few hundred million years, biodiversity has blossomed, leading to dense jungles and grasslands echoing with animal calls, and waters writhing with every shape of fin and color of scale. And most every stage of development has left its mark on the fossil record.

The researchers are careful not to imply that phosphorus necessarily caused the chain reaction, but in sedimentary rock taken from coastal areas, the nutrient has marked the spot where that burst of life and climate change took off. "The timing is definitely conspicuous," said Chris Reinhard, an assistant professor in Georgia Tech's School of Earth and Atmospheric Sciences.

Reinhard and Noah Planavsky, a geochemist from Yale University, who headed up the research together, have mined records of sedimentary rock that formed in ancient coastal zones, going down layer by layer to 3.5 billion years ago, to compute how the cycle of the essential fertilizer phosphorus evolved and how it appeared to play a big part in a veritable genesis.

They noticed a remarkable congruency as they moved upward through the layers of shale into the time period where animal life began, in the late Proterozoic Eon.

"The most basic change was from very limited phosphorus availability to much higher phosphorus availability in surface waters of the ocean," Reinhard said. "And the transition seemed to occur right around the time that there were very large changes in ocean-atmosphere oxygen levels and just before the emergence of animals."

Phosphorus at the beach

Reinhard and Planavsky, together with an international team, have proposed that a scavenging of nutrients in an anoxic (nearly O2-free) world stunted photosynthetic organisms that otherwise had been poised for at least two billion years to make stockpiles of oxygen. Then that balanced system was upset and oceanic phosphorus made its way to coastal waters.

The scientists published their findings in the journal Nature. Their research was funded by the National Science Foundation, the NASA Astrobiology Institute, the Sloan Foundation and the Japan Society for the Promotion of Science.

The work provides a new view into what factors allowed life to reshape Earth's atmosphere. It helps lay a foundation that scientists can apply to make predictions about what would allow life to alter exoplanets' atmospheres, and may inspire deeper studies, here on Earth, of how oceanic-atmospheric chemistry drives climate instability and influences the rise and fall of life through the ages.

Cyanobacteria, the mother of O2

Complex living things, including animals, usually have an immense metabolism and require ample O2 to drive it. The evolution of animals is unthinkable without it.

The path to understanding how a nutrient dearth would starve out breathable oxygen production leads back to a very special kind of bacteria called cyanobacteria, the mother of oxygen on Earth.

"The only reason we have a well-oxygenated planet we can live on is because of oxygenic photosynthesis," Planavsky said. "O2 is the waste product of photosynthesizing cells, like cyanobacteria, combining CO2 and water to build sugars."

And photosynthesis is an evolutionary singularity, meaning it only evolved once in Earth's history -- in cyanobacteria.

Some other biological phenomena evolved repeatedly in dozens or hundreds of unrelated incidents across the ages, such as the transition from single-celled organisms to rudimentary multicellular organisms. But scientists are confident that oxygenic photosynthesis evolved only this one time in Earth's history, only in cyanobacteria, and all plants and other beings on Earth that photosynthesize coopted the development.

The iron anchor

Cyanobacteria are credited with filling Earth's atmosphere with O2, and they've been around for 2.5 billion years or more.

That begs the question: What took so long? Basic nutrients that fed the bacteria weren't readily available, the scientist hypothesize. The phosphorus, which Planavsky and Reinhard specifically tracked, was in the ocean for billions of years, too, but it was tied up in the wrong places.

For eons, the mineral iron, which once saturated oceans, likely bonded with phosphorus, and sank it down to dark ocean depths, far away from those shallows -- also called continental margins -- where cyanobacteria would have needed it to thrive and make oxygen. Even today, iron is used to treat waters polluted with fertilizer to remove phosphorus by sinking it as deep sediment.

The researchers also used a geochemical model to show how a global system with high iron concentration and low phosphorus availability combined with low nitrogen availability in ocean shallows could perpetuate itself in a low-oxygen world.

"It looks to have been such a stable planetary system," Reinhard said. "But it's obviously not the planet we live on now, so the question is, how did we transition from this low-oxygen state to where we are now?"

What ultimately caused that change is a question for future research.

Phosphorus starting pistol

But something did change about 800 million years ago, and cyanobacteria and other minute organisms in continental margin ecosystems got more phosphorus, the backbone of DNA and RNA, and a main actor in cell metabolism. The bacteria became more active, reproduced more quickly, ate lots more phosphorus and made loads more O2.

"Phosphorus is not only essential for life," Planavsky said. "What's implicit in all this is: It can control the amount of life on our planet."

When the newly multiplied bacteria died, they fell to the floor of those ocean shallows, stacking up layer by layer to decay and enrich the mud with phosphorus. The mud eventually compressed to stone.

"As the biomass increased in phosphorus content, the more of it landed in layers of sedimentary rock," Reinhard said. "To scientists, that shale is the pages of the sea floor's history book."

Scientists have thumbed through them for decades, compiling data. Planavsky and Reinhard analyzed some 15,000 rock records for their study.

"The first compilation we had of this was only 600 samples," Planavsky said. Reinhard added, "But you could already see it then. The phosphorus jolt was as clear as day. And as the database grew in size, the phenomenon became more entrenched."

That first signal of phosphorus in Earth's coast shallows pops up in the shale record like a shot from a starting pistol in the race for abundant life.

原始論文：Christopher T. Reinhard, Noah J. Planavsky, Benjamin C. Gill, Kazumi Ozaki, Leslie J. Robbins, Timothy W. Lyons, Woodward W. Fischer, Chunjiang Wang, Devon B. Cole, Kurt O. Konhauser. Evolution of the global phosphorus cycle. Nature, 2016; DOI: 10.1038/nature20772

引用自：Georgia Institute of Technology. "A fertilizer dearth foiled animal evolution for eons? End of phosphorus dearth in ocean shallows coincides with evolutionary surge in study of 3.5 billion years of Earth's history." ScienceDaily. ScienceDaily, 21 December 2016.