原始網址：www.sciencedaily.com/releases/2017/01/170102155018.htm

孵化一顆恐龍蛋要花多久時間？3-6個月

人類嬰兒通常要經過九個月才出生，而鴕鳥雛鳥則花上42天就能破卵而出。但是孵化一隻恐龍寶寶得用上多久的時間？

由佛羅里達州立大學(FSU)的教授進行的研究，確認了恐龍依據種類的不同，孵化所需的時間從三至六個月不等。

在此篇發表於《美國國家科學院院刊》的文章中，FSU的生物學教授Gregory Erickson和研究團隊破解了這些史前生物的複雜生理性質，並解釋如何利用胚胎的牙齒生長紀錄，來解開孵化恐龍蛋需要多久的謎團。

「關於恐龍的最大謎題有些在於牠們的胚胎學性質――事實上我們對此幾乎一無所知。」Erickson說。「牠們的蛋是慢慢孵化，與牠們的爬蟲類表親――鱷魚和蜥蜴類似？或者較為迅速，跟現存的恐龍，也就是鳥類相仿？」

科學家許久以來假設恐龍孵化所需時間跟鳥類較為相似。鳥類的蛋孵化所需時間為11至85天，

而類似大小的爬蟲類的蛋，一般來說則要花上兩倍以上的時間，從數週到數月不等。

因為恐龍蛋的尺寸相當大，某些種類甚至有4公斤重，大小跟排球差不多。所以，科學家認為它們的孵化歷程必定相當迅速，而鳥類則從牠們的恐龍祖先身上繼承了這項特色。

Erickson、FSU的研究生David Kay和來自卡爾加里大學與美國自然史博物館的其他研究人員，決定要實際證明這些理論是否正確。

為了達成此目的，他們取得了一些相當稀有的化石――恐龍的胚胎化石。

「對動物的發育來說在蛋中的時光是相當重要的時期，但由於恐龍的胚胎相當稀少，因此我們對恐龍最初的成長階段也所知不多。」共同作者，卡爾加里大學的地質科學助理教授Darla Zelenitsky表示。「胚胎或許可以告訴我們恐龍在生命史最早期的成長發育過程，以及在這方面牠們是近似於鳥類還是爬蟲類。」

研究人員探討的兩種恐龍胚胎分別來自於原角龍(Protoceratops)和亞冠龍(Hypacrosaurus)。前者是一種發現於蒙古戈壁沙漠，綿羊大小的恐龍，其產下的蛋相當小巧(194公克)；後者則是發現於加拿大亞伯達省的巨型鴨嘴龍類，蛋可重達4公斤以上。

Erickson和他的團隊將胚胎的下顎通過電腦斷層掃描儀以得到牙齒生長的影像。接著他們取出數顆牙齒，在高精密度的顯微鏡之下做進一步的觀察。

研究人員於顯微鏡切片當中發現了他們正在尋找的證據。牙齒上的生長線可以準確告訴研究人員這隻恐龍在蛋裡面已經發育了多久。

「這些線條是在動物牙齒生長時形成的，」Erickson表示。「這有點像是樹木的年輪，但它們每天就會形成一條。因此我們可以逐條計算它們的數目以得到每一隻恐龍已經發育了多久。」

他們的成果顯示嬌小的原角龍胚胎為3個月左右，而碩大的亞冠龍胚胎則是6個月左右。

「恐龍胚胎是世上最珍貴的幾種化石之一。」這篇研究的共同作者，美國自然史博物館的馬考利圖書館的館長Mark Norell說。「在此，我們利用了美國博物館於戈壁進行考察時採集的珍貴化石樣品，結合新技術與概念，而發現到一些關於恐龍的真正最新見解。」

恐龍蛋需要長時間來孵育的概念也帶來了眾多啟發。

除了發現恐龍的孵化過程跟原始爬蟲類比較相似之外，研究人員也可以從此研究當中推論出恐龍生物學當中的許多面向。

較長的孵化期會讓恐龍蛋和他們的雙親暴露在掠食者、飢餓和其他環境危險因子的威脅當中。另外，從孵蛋以及遷移所需的時間長短來看，認為某些恐龍會在較溫和的加拿大低緯地區築巢，之後夏季時遷移到極區的理論，現在似乎變得不太可行。

然而，從此研究中衍生出來的最重要概念則跟恐龍的滅絕有關。若這些溫血生物需要大量資源才能長到成年體型、一歲以上才能生育且孵化過程緩慢，跟其他度過大滅絕事件的動物相比，牠們可以說是處在明顯劣勢的位置。

「為何恐龍會在白堊紀結束時滅絕，然而兩棲類、鳥類、哺乳類以及其他爬蟲類卻能度過這場浩劫並在之後繁榮生長？我們認為我們的發現有助於釐清這個難題。」Erickson表示。

本研究由美國國家科學基金會資助。

 

How long did it take to hatch a dinosaur egg? 3-6 months

A human typically gives birth after nine months. An ostrich hatchling emerges from its egg after 42 days. But how long did it take for a baby dinosaur to incubate?

Groundbreaking research led by a Florida State University professor establishes a timeline of anywhere from three to six months depending on the dinosaur.

In an article in the Proceedings of the National Academy of Sciences, FSU Professor of Biological Science Gregory Erickson and a team of researchers break down the complicated biology of these prehistoric creatures and explain how embryonic dental records solved the mystery of how long dinosaurs incubated their eggs.

"Some of the greatest riddles about dinosaurs pertain to their embryology -- virtually nothing is known," Erickson said. "Did their eggs incubate slowly like their reptilian cousins -- crocodilians and lizards? Or rapidly like living dinosaurs -- the birds?"

Scientists had long theorized that dinosaur incubation duration was similar to birds, whose eggs hatch in periods ranging from 11-85 days. Comparable-sized reptilian eggs typically take twice as long -- weeks to many months.

Because the eggs of dinosaurs were so large -- some were about 4 kilograms or the size of a volleyball -- scientists believed they must have experienced rapid incubation with birds inheriting that characteristic from their dinosaur ancestors.

Erickson, FSU graduate student David Kay and colleagues from University of Calgary and the American Museum of Natural History decided to put these theories to the test.

To do that, they accessed some rare fossils -- those of dinosaur embryos.

"Time within the egg is a crucial part of development, but this earliest growth stage is poorly known because dinosaur embryos are rare," said co-author Darla Zelenitsky, assistant professor of geoscience at University of Calgary. "Embryos can potentially tell us how dinosaurs developed and grew very early on in life and if they are more similar to birds or reptiles in these respects."

The two types of dinosaur embryos researchers examined were those from Protoceratops -- a sheep-sized dinosaur found in the Mongolian Gobi Desert whose eggs were quite small (194 grams) -- and Hypacrosaurus, an enormous duck-billed dinosaur found in Alberta, Canada with eggs weighing more than 4 kilograms.

Erickson and his team ran the embryonic jaws through a CT scanner to visualize the forming dentition. Then, they extracted several of the teeth to further examine them under sophisticated microscopes.

Researchers found what they were looking for on those microscope slides. Growth lines on the teeth showed researchers precisely how long the dinosaurs had been growing in the eggs.

"These are the lines that are laid down when any animal's teeth develops," Erickson said. "They're kind of like tree rings, but they're put down daily. We could literally count them to see how long each dinosaur had been developing."

Their results showed nearly three months for the tiny Protoceratops embryos and six months for those from the giant Hypacrosaurus.

"Dinosaur embryos are some of the best fossils in the world," said Mark Norell, Macaulay Curator for the American Museum of Natural History and a co-author on the study. "Here, we used spectacular fossils specimens collected by American Museum expeditions to the Gobi Desert, coupled them with new technology and new ideas, leading us to discover something truly novel about dinosaurs."

The implications of long dinosaur incubation are considerable.

In addition to finding that dinosaur incubation was similar to primitive reptiles, the researchers could infer many aspects of dinosaurian biology from the results.

Prolonged incubation put eggs and their parents at risk from predators, starvation and other environmental risk factors. And theories that some dinosaurs nested in the more temperate lower latitude of Canada and then traveled to the Arctic during the summer now seem unlikely given the time frame for hatching and migration.

The biggest ramification from the study, however, relates to the extinction of dinosaurs. Given that these warm-blooded creatures required considerable resources to reach adult size, took more than a year to mature and had slow incubation times, they would have been at a distinct disadvantage compared to other animals that survived the extinction event.

"We suspect our findings have implications for understanding why dinosaurs went extinct at the end of the Cretaceous period, whereas amphibians, birds, mammals and other reptiles made it through and prospered," Erickson said.

This research was supported by the National Science Foundation.

原始論文：Gregory M. Erickson, Darla K. Zelenitsky, David Ian Kay, and Mark A. Norell. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. PNAS, 2017 DOI:10.1073/pnas.1613716114

引用自：Florida State University. "How long did it take to hatch a dinosaur egg? 3-6 months." ScienceDaily. ScienceDaily, 2 January 2017. 

化石燃料的形成是大氣擁有氧氣的關鍵？

原文網址：www.sciencedaily.com/releases/2016/12/161230185406.htm

化石燃料的形成是大氣擁有氧氣的關鍵？

在動物演化的過程中，除了DNA之外，恐怕沒有比大氣中的氧氣還重要的事物了。

氧氣讓動物能進行化學反應，獲取儲存在碳水化合物，也就是食物中的能量。因此5億年前左右，動物在「寒武紀大爆發」(Cambrian explosion)事件中大量出現並演化，同時大氣氧含量出現高峰，也許並不是單純的巧合。

現在能看到的動物型態，多半是於寒武紀大爆發期間開始出現。

在綠色植物體內，光合作用會將二氧化碳分解成氧氣(排放到大氣中)以及碳(儲存在碳水化合物內)。

但當時光合作用早就已經存在了至少25億年之久。所以是什麼原因造成了氧氣含量於寒武紀突然出現高峰？

刊登於《地球和行星科學通訊》(Earth and Planetary Science Letters )二月號線上版的研究，將氧氣含量的提高歸因於沉積物埋藏量的迅速增加，這些沉積物具有大量富含碳的有機物質。共同作者，威斯康辛大學麥迪遜分校的地質科學教授Shanan Peters表示，關鍵在於瞭解沉積物可以將碳封存起來防止其被氧化。

如果沒有埋藏作用，地球表面死亡的植物殘骸會因為氧化作用而燃燒。它們體內原本來自大氣中的碳，就會跟氧氣結合而形成二氧化碳。因此，若要讓氧氣在大氣中持續累積，就必須要防止植物的有機物質受到氧化。

而這就是有機物質—也就是木炭、石油和天然氣的原始成分—透過地質作用被埋藏時所發生的。

為了證明這項理論，Peters和的博士後研究員Jon Husson從Macrostrat中發掘出一組全新的資料。Peters用了10年策劃並建立了Macrostrat，此資料庫彙整了北美各處的地質資訊。

他們根據沉積岩層提供的資訊，建立了同時顯示大氣氧含量和沉積物埋藏量變化的圖表，從中可以看出氧氣和沉積物之間有關聯性存在。它們都在23億年前有個較小的高峰，另一個較大的高峰則出現在5億年前。

「雖然這僅顯示出兩者之間有相關性，但我們可以宣稱地質作用和大氣氧含量的變化之間確實有因果關係存在。」Husson表示。「光合作用會把二氧化碳轉換成生質(biomass)並釋放氧氣到大氣當中。當你把沉積物儲存起來，也會連帶隔絕由光合作用產生的有機質。埋藏作用可以帶走地表的碳，防止它們從大氣帶走氧分子並結合在一起。」

Husson和Peters辨識出來沉積物大量埋藏的時間點，有些和今日仍在開採的大型化石燃料礦場的形成時間一致。這些礦場包括了德州形成於二疊紀石油蘊藏豐富的盆地，以及阿帕拉契地區晚石炭紀時形成的煤田。

「埋藏那些會成為化石燃料的沉積物，是高等動物生命出現在地球的關鍵。」Peters表示，並強調多細胞生物大多是誕生於寒武紀。

今日，燃燒化石燃料中數十億噸的碳正持續將大量氧氣從大氣中移除，這跟使大氣氧含量提升的方式恰好相反。因此，隨著大氣二氧化碳的濃度增加，氧氣含量便會下降。

Macrostrat中跟北美有關的資料代表了超過一世紀以來數千名地質學家的研究成果。目前的研究僅考慮到北美地區，這是因為涵蓋地球陸地表面其餘80%的綜合資料庫還尚未建立起來。

導致這兩次氧氣增加的沉積物加速埋藏事件，根本的地質成因仍然不清楚。「有許多概念可以用來解釋不同階段的氧氣濃度變化。」Husson承認。「我們猜測在板愧運動、地函熱傳導或對流中發生的基本變化可能具有一定作用，但現階段我們還未能給出解釋。」

Peters拿著一塊大約於4.5億年前形成，嵌有數隻三葉蟲的奧陶紀頁岩說：「為什麼大氣中會有氧氣？高中教科書给的答案是『光合作用』。但從威斯康辛州的地質學家Thomas Chrowder Chamberlin(1843-1928，曾擔任威斯康辛大學的校長)那時起，我們長久以來已經了解到氧氣的增加需要黑色頁岩這類的岩層形成，它們含有大量本該燃燒殆盡的碳。這些頁岩當中的有機碳是經由光合作用而固定住，將它們埋藏並保存在岩石當中才能真正將氧分子釋放出來。」

Husson表示此研究的創新之處，在於從涵蓋地球陸地20%的豐富資料庫中，可以找到這種關聯性存在的證據。

必須要持續地將碳埋藏起來才能使大氣氧含量不斷提高。Husson強調地表發生的許多作用，像是鐵氧化而產生的鐵鏽會消耗自由氧。「大氣有氧氣的秘訣在於將當時存在的一小部分生質移除並封存在沉積物當中。這就是化石燃料沉積時可以辦到的事情。」

Fossil fuel formation: Key to atmosphere’s oxygen?

For the development of animals, nothing -- with the exception of DNA -- may be more important than oxygen in the atmosphere.

Oxygen enables the chemical reactions that animals use to get energy from stored carbohydrates -- from food. So it may be no coincidence that animals appeared and evolved during the "Cambrian explosion," which coincided with a spike in atmospheric oxygen roughly 500 million years ago.

It was during the Cambrian explosion that most of the current animal designs appeared.

In green plants, photosynthesis separates carbon dioxide into molecular oxygen (which is released to the atmosphere), and carbon (which is stored in carbohydrates).

But photosynthesis had already been around for at least 2.5 billion years. So what accounted for the sudden spike in oxygen during the Cambrian?

A study now online in the February issue of Earth and Planetary Science Letters links the rise in oxygen to a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter. The key, says study co-author Shanan Peters, a professor of geoscience at the University of Wisconsin-Madison, is to recognize that sediment storage blocks the oxidation of carbon.

Without burial, this oxidation reaction causes dead plant material on Earth's surface to burn. That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. And for oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation.

And that's exactly what happens when organic matter -- the raw material of coal, oil and natural gas -- is buried through geologic processes.

To make this case, Peters and his postdoctoral fellow Jon Husson mined a unique data set called Macrostrat, an accumulation of geologic information on North America whose construction Peters has masterminded for 10 years.

The parallel graphs of oxygen in the atmosphere and sediment burial, based on the formation of sedimentary rock, indicate a relationship between oxygen and sediment. Both graphs show a smaller peak at 2.3 billion years ago and a larger one about 500 million years ago.

"It's a correlation, but our argument is that there are mechanistic connections between geology and the history of atmospheric oxygen," Husson says. "When you store sediment, it contains organic matter that was formed by photosynthesis, which converted carbon dioxide into biomass and released oxygen into the atmosphere. Burial removes the carbon from Earth's surface, preventing it from bonding molecular oxygen pulled from the atmosphere."

Some of the surges in sediment burial that Husson and Peters identified coincided with the formation of vast fields of fossil fuel that are still mined today, including the oil-rich Permian Basin in Texas and the Pennsylvania coal fields of Appalachia.

"Burying the sediments that became fossil fuels was the key to advanced animal life on Earth," Peters says, noting that multicellular life is largely a creation of the Cambrian.

Today, burning billions of tons of stored carbon in fossil fuels is removing large amounts of oxygen from the atmosphere, reversing the pattern that drove the rise in oxygen. And so the oxygen level in the atmosphere falls as the concentration of carbon dioxide rises.

The data about North America in Macrostrat reflects the work of thousands of geoscientists over more than a century. The current study only concerns North America, since comprehensive databases concerning the other 80 percent of Earth's continental surface do not yet exist.

The ultimate geological cause for the accelerated sediment storage that promoted the two surges in oxygen remains murky. "There are many ideas to explain the different phases of oxygen concentration," Husson concedes. "We suspect that deep-rooted changes in the movement of tectonic plates or conduction of heat or circulation in the mantle may be in play, but we don't have an explanation at this point."

Holding a chunk of trilobite-studded Ordovician shale that formed approximately 450 million years ago, Peters asks, "Why is there oxygen in the atmosphere? The high school explanation is 'photosynthesis.' But we've known for a long time, going all the way back to Wisconsin geologist (and University of Wisconsin president) Thomas Chrowder Chamberlin, that building up oxygen requires the formation of rocks like this black shale, which can be rich enough in carbon to actually burn. The organic carbon in this shale was fixed from the atmosphere by photosynthesis, and its burial and preservation in this rock liberated molecular oxygen."

What's new in the current study, Husson says, is the ability to document this relationship in a broad database that covers 20 percent of Earth's land surface.

Continual burial of carbon is needed to keep the atmosphere pumped up with oxygen. Many pathways on Earth's surface, Husson notes, like oxidation of iron -- rust -- consume free oxygen. "The secret to having oxygen in the atmosphere is to remove a tiny portion of the present biomass and sequester it in sedimentary deposits. That's what happened when fossil fuels were deposited."

原始論文：Jon M. Husson, Shanan E. Peters. Atmospheric oxygenation driven by unsteady growth of the continental sedimentary reservoir. Earth and Planetary Science Letters, 2017; 460: 68 DOI:10.1016/j.epsl.2016.12.012

引用自：University of Wisconsin-Madison. "Fossil fuel formation: Key to atmosphere’s oxygen?." ScienceDaily. ScienceDaily, 30 December 2016. 

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.