中國的古生物學家希望解釋出動物的起源

AMONG the mysteries of evolution, one of the most profound is what exactly happened at the beginning of the Cambrian period. Before that period, which started 541m years ago and ran on for 56m years, life was a modest thing. Bacteria had been around for about 3 billion years, but for most of this time they had had the Earth to themselves. Seaweed s, jellyfish -like creatures, sponges and the odd worm do start to put in an appearance a few million years before the Cambrian begins. But red in tooth and claw the Precambrian was not—for neither teeth nor claws existed.

進化懸案中，最具深遠意義的是寒武紀的起源問題。寒武紀之前（即5.41億年前，連續的5600萬年）物種較少。細菌已經出現了30億年，在這段時間裏地球只有它們相伴。在寒武紀到來前的幾百萬年前，海草、水母類生物、海綿和怪蚯蚓開始露面。但紅牙利爪生物在寒武紀前沒有出現過。

Then, in the 20m-year blink of a geological eye, animals arrived in force . Most of the main groups of the animal kingdom—arthropod s, brachiopod s, coelenterates , echinoderm s, molluscs and even chordate s, the branch from which vertebrate s went on to develop—are found in the fossil beds of the Cambrian. The sudden evolution of this megafauna is known as the Cambrian explosion. But two centuries after it was noticed, in the mountains of Wales after which the Cambrian period is named, nobody knows what detonated it.

接着，2000萬年的一眨眼時間裏，大規模動物降臨。人們在寒武紀化石牀裏發現了動物王國的主要動物集羣，包括腕足、腔腸、棘皮、軟體和脊索（脊椎動物進化的分支）。寒武爆發以巨型動物羣的急劇進化著稱。當其被發現的兩個世紀以後，在寒武紀命名的威爾士山，沒有人知曉爆發的原因。

A group of Chinese scientists, led by Zhu Maoyan of the Nanjing Institute of Geology and Palaeontology, plan to change that with a project called “From the Snowball Earth to the Cambrian explosion: the evolution of life and environment 600m years ago”. The “Snowball Earth” refers to a series of ice ages that happened between 725m and 541m years ago. These were, at their maxima, among the most extensive glaciations in the Earth’s history. They alternated, though, with periods that make the modern tropics seem chilly: the planet’s average temperature was sometimes as high as 50°C. Add the fact that a supercontinent (illustrated above, viewed from the Earth’s south pole) was breaking up at this time, and you have a picture of a world in chaos. Just the sort of thing that might drive evolution. Dr Zhu and his colleagues hope to find out exactly how these environmental changes correspond to changes in the fossil record.

一箇中國科學家團隊——以朱茂炎爲首的南京地質古生物所，計劃啓動名爲“從冰雪地球至寒武爆發：6億年前的生命進化與環境”，以改變現場。“冰雪地球”指發生在7.25億至5.41億年前的一系列冰河時代。這些時期的高峯段是地球歷史上冰川運動最廣泛的時候。儘管冰川活動在這些時期交替發生，使現代熱帶地區看起來很冷，但地球的平均溫度有時也會高達50攝氏度。加上超大陸（如上圖所示，位於南極附近）在這段時期分裂，便可想象得到此時的世界該是多麼混亂的一幅圖畫。只有一些物種會進行進化。朱博士和他的同僚們希望準確找出這些環境變化在化石記錄中所對應的變化。

The animals’ carnival 動物嘉年華

Fortunately, China’s fossil record for this period is rich. Until recently, the only known fossils of Precambrian animals were what is called the Ediacaran fauna—a handful of strange creatures found in Australia, Canada and the English Midlands that lived in the Ediacaran period, between 635m and 541m years ago, and which bear little resemblance to what came afterwards. In 1998, however, a team led by Chen Junyuan, also of the Nanjing Institute, and another led by Xiao Shuhai of the Virginia Polytechnic Institute, in America, discovered a 580m-year-old Lagerstätte—a place where fossils are particularly well preserved—in a geological formation called the Doushantuo, which spreads out across southern China.

幸運的是，中國這一地期的化石記錄倒豐富。直到最近，唯一被人們發現的前寒武紀的動物化石叫做埃迪卡拉動物羣——在澳大利亞、加拿大和英國內陸發現的生活在埃迪卡拉紀時期（6.35億至5.41億年間）的奇怪生物，它們被證實與後來的生物沒有一點相似之處。然而，1998年，南京地質古生物所的陳均遠隊和美國弗吉尼亞工學院帶肖書海隊，發現了另一個5.8億年前的寒武紀寶庫，該地化石保存良好，叫做陡山沱組地層，其分佈在中國南部地區。

Portents of the modern world 現代世界的先兆

This Lagerstätte has yielded many previously unknown species, including microscopic sponges, small tubular organisms of unknown nature, things that look like jellyfish but might not be and a range of what appear to be embryos that show bilateral symmetry (pictured right). What these embryos would have grown into is unclear. But some might be the ancestors of the Cambrian megafauna.

該化石寶庫發現了許多不爲人所知的物種，包括微型海綿動物，未知的自然小型管狀結構，像水母但又不是，顯示出兩個對稱的胚胎（右圖所示）。這些胚胎將演變成什麼目前尚未可知。但是有些可能是寒武紀巨型動物羣的先祖。

To try to link the evolution of these species with changes in the environment, Chu Xuelei of the Institute of Geology and Geophysics in Beijing and his colleagues have been looking at carbon isotopes in the Doushantuo rocks. They have found that the proportion of ¹²C—a light isotope of carbon that is more easily incorporated by living organisms into organic matter than its heavy cousin, ¹³C—increased on at least three occasions during the Ediacaran period. They suggest these increases mark moments when the amount of oxygen in seawater went up, because more oxygen would mean more oxidisation of buried organic matter. That would liberate its ¹²C, for incorporation into rocks.

爲了將這些物種的進化與環境的變化聯繫起來，北京地質和物理研究的所楚學雷和他的同僚們在陡山沱組岩石裏看到了碳同位素。他們已經發現，有機體中的碳12比其同胞碳13更易於與有機化合物結合，在埃迪卡拉紀可增加至少3倍。他們認爲，這些增長記錄了氧在海水中上升的數量，因爲更多的氧氣意味着更多的有機化合物氧化。那將釋放碳12，與岩石結合。

Each of Dr Chu’s oxidation events corresponds with an increase in the size, complexity and diversity of life, both plant and animal. What triggered what, however, is unclear. There may have been an increase in photosynthesis because there were more algae around. Or eroded material from newly formed mountains may have buried organic matter that would otherwise have reacted with oxygen, leading to a build-up of the gas.

楚博士的每一個氧化活動都伴隨着植物和動物的生命形狀、複雜性和多樣性的增加。然而，觸發的原因仍是個謎。由於周圍的水藻變多了，可能會增加光合作用。或者是來自新形成的山體中腐蝕的材料在消化有機化合物時，會呼出氧氣，形成氣體。

The last—and most dramatic—rise in oxygen took place towards the end of the Ediacaran. Follow-up work by Dr Zhu, in nine other sections of the Doushantuo formation, suggests this surge started just after the final Precambrian glacial period about 560m years ago, and went on for 9m years. These dates overlap with those of signs of oxidation found in rocks in other parts of the world, confirming that whatever was going on affected the entire planet. Dr Zhu suspects this global environmental shift propelled the evolution of complex animals.

埃迪卡拉紀末期出現了最後且是最劇烈的氧氣增加。根據朱博士對陡山沱組地質層其他九個部分的研究，他認爲氧氣的增加正好發生在5.6億年前的前寒武紀冰河時期，並持續了約900萬前。這些日期正好世界其他地區發現的岩石氧化跡象吻合，證實了所發生的一切影響了整個地球。朱博士猜想全球的環境變化促使了複雜動物的進化。

Dr Zhu also plans to push back before the Ediacaran period. Other researchers have found fossils of algae and wormlike creatures in rocks in northern China that pre-date the end of the Marinoan glaciation, 635m years ago, which marks the boundary between the Ediacaran and the Cryogenian period that precedes it. (The Cryogenian began 850m years ago.) Such fossils are hard to study, so Dr Zhu will use new imaging technologies that can look at them without having to clean away the surrounding rock, and are also able to detect traces of fossil organic matter invisible to the eye.

朱博士還計劃將研究倒推至埃迪卡拉紀以前。其他研究者已在中國北部地區的岩石裏發現水藻化石和蚯蚓類生物，是6.35億年前的冰河世紀末，標誌埃迪卡拉紀和成冰系時期的界限前移。（成冰系開始於8.5億年前）。這樣的化石很難研究，所以朱博士將運用新型影像工程學看到它們，無需清楚化石周圍的岩石，並能肉眼找到有機化合物化石的足跡。

Besides digging back before the Ediacaran, the new project’s researchers also intend to analyse the unfolding of the Cambrian explosion itself by taking advantage of otherLagerstätten—for China has several that date from the Cambrian. Dr Chen, indeed, first made his name in 1984, when he excavated one at Chengjiang in Yunnan province. It dates from 525m years ago, which make it 20m years older than the most famous Cambrian Lagerstätte in the West, the Burgess shale of British Columbia, in Canada. The project’s researchers plan to see how, evolutionarily speaking, the variousLagerstätten relate to one another, to try to determine exactly when different groups of organisms emerged.

除了挖掘出埃迪卡拉紀以前的項目，新的項目的研究者們還想要利用化其他化石寶庫（因爲中國存在着許多來自寒武紀的化石寶庫）分析寒武紀爆發的演變。陳博士最早因1984年考察到了雲南省澄江生物羣而出名。雲南澄江生物羣起源於5.25年前，比西方最著名的寒武紀化石寶庫“不列顛哥倫比亞伯吉斯生物羣”（加拿大）早了2000萬年。項目的研究者們想知道不同的化石寶庫如何與其他相關聯，並試圖準確確定不同有機生物羣體出現的時間。

They will also look at the chemistry of elements other than carbon and oxygen—particularly nitrogen and phosphorous, which are essential to life, and sulphur, which often indicates the absence of oxygen and is thus antithetical to much animal life. Dr Zhu hopes to map changes in the distribution of these chemicals across time and space. He will assess how these changes correlate, whether they are related to weathering, mountain building and the ebb and flow of glaciers, how they could have affected the evolution of life, and how plants and animals might themselves have altered the chemistry of air and sea.

他們也想研究除了碳和氧的其他化學元素——尤其氮和磷，哪一個對生命來說是必要的；且硫表明其缺少氧將對大部分動物有害。朱博士希望能繪製出這些化學元素在時間和空間中的分佈變化圖。他也將估定這些變化如何相互關聯，它們是否與風化、山體建造和冰川的潮汐有關，它們如何能影響到生命的進化，植物和動物如何改變空氣和大海的化學成分。

Most ambitiously, Dr Zhu, Dr Xiao and their colleagues hope to drill right through several fossiliferous sites in southern China where Ediacaran rocks turn seamlessly into Cambrian ones. Such places are valuable because in most parts of the world there is a gap, known as an unconformity, between the Ediacaran and the Cambrian. Unconformities are places where rocks have been eroded before new ones are deposited, and the widespread Ediacaran-Cambrian unconformity has been a big obstacle to understanding the Cambrian explosion. With luck, then, a mystery first noticed in the Welsh mountains in the early 19th century will be solved in the Chinese ones in the early 21st. If it is, the origin of the animal kingdom will have become clear, and an important gap in the history of humanity itself will have been filled.

朱博士、肖博士及他們的同僚們最有抱負的是，希望在中國南部地區，在埃迪古拉紀岩石與寒武紀岩石交替的化石遺址中進行鑽測。由於世界的大部分的此地區都有裂縫，即不整合面，因此埃迪古拉紀和寒武紀之間的這些地方是變化多樣的。不整合面的岩石早在新岩石產生前就已經腐蝕，埃迪古拉紀－寒武紀之間廣佈不整合面，它們是研究寒武爆發的巨大阻礙。幸運的是，19世紀早期首先在威爾士山脈被發現的一個祕密將解答21世紀中國人想要知道的問題。果真如此，動物王國的起源將逐漸清晰，人類歷史的一個重要鴻溝將被填平。