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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.
进化悬案中，最具深远意义的是寒武纪的起源问题。寒武纪之前（即5.41亿年前，连续的5600万年）物种较少。细菌已经出现了30亿年，在这段时间里地球只有它们相伴。在寒武纪到来前的几百万年前，海草、水母类生物、海绵和怪蚯蚓开始露面。但红牙利爪生物在寒武纪前没有出现过。
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，2000万年的一眨眼时间里，大规模动物降临。人们在寒武纪化石床里发现了动物王国的主要动物集群，包括腕足、腔肠、棘皮、软体和脊索（脊椎动物进化的分支）。寒武爆发以巨型动物群的急剧进化著称。当其被发现的两个世纪以后，在寒武纪命名的威尔士山，没有人知晓爆发的原因。
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.
一个中国科学家团队——以朱茂炎为首的南京地质古生物所，计划启动名为“从冰雪地球至寒武爆发：6亿年前的生命进化与环境”，以改变现场。“冰雪地球”指发生在7.25亿至5.41亿年前的一系列冰河时代。这些时期的高峰段是地球历史上冰川运动最广泛的时候。尽管冰川活动在这些时期交替发生，使现代热带地区看起来很冷，但地球的平均温度有时也会高达50摄氏度。加上超大陆（如上图所示，位于南极附近）在这段时期分裂，便可想象得到此时的世界该是多么混乱的一幅图画。只有一些物种会进行进化。朱博士和他的同僚们希望准确找出这些环境变化在化石记录中所对应的变化。
The animals’ carnival 动物嘉年华
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.
幸运的是，中国这一地期的化石记录倒丰富。直到最近，唯一被人们发现的前寒武纪的动物化石叫做埃迪卡拉动物群——在澳大利亚、加拿大和英国内陆发现的生活在埃迪卡拉纪时期（6.35亿至5.41亿年间）的奇怪生物，它们被证实与后来的生物没有一点相似之处。然而，1998年，南京地质古生物所的陈均远队和美国弗吉尼亚工学院带肖书海队，发现了另一个5.8亿年前的寒武纪宝库，该地化石保存良好，叫做陡山沱组地层，其分布在中国南部地区。
Portents of the modern world 现代世界的先兆
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.
该化石宝库发现了许多不为人所知的物种，包括微型海绵动物，未知的自然小型管状结构，像水母但又不是，显示出两个对称的胚胎（右图所示）。这些胚胎将演变成什么目前尚未可知。但是有些可能是寒武纪巨型动物群的先祖。
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.
为了将这些物种的进化与环境的变化联系起来，北京地质和物理研究的所楚学雷和他的同僚们在陡山沱组岩石里看到了碳同位素。他们已经发现，有机体中的碳12比其同胞碳13更易于与有机化合物结合，在埃迪卡拉纪可增加至少3倍。他们认为，这些增长记录了氧在海水中上升的数量，因为更多的氧气意味着更多的有机化合物氧化。那将释放碳12，与岩石结合。
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.
楚博士的每一个氧化活动都伴随着植物和动物的生命形状、复杂性和多样性的增加。然而，触发的原因仍是个谜。由于周围的水藻变多了，可能会增加光合作用。或者是来自新形成的山体中腐蚀的材料在消化有机化合物时，会呼出氧气，形成气体。
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.
埃迪卡拉纪末期出现了最后且是最剧烈的氧气增加。根据朱博士对陡山沱组地质层其他九个部分的研究，他认为氧气的增加正好发生在5.6亿年前的前寒武纪冰河时期，并持续了约900万前。这些日期正好世界其他地区发现的岩石氧化迹象吻合，证实了所发生的一切影响了整个地球。朱博士猜想全球的环境变化促使了复杂动物的进化。
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.
朱博士还计划将研究倒推至埃迪卡拉纪以前。其他研究者已在中国北部地区的岩石里发现水藻化石和蚯蚓类生物，是6.35亿年前的冰河世纪末，标志埃迪卡拉纪和成冰系时期的界限前移。（成冰系开始于8.5亿年前）。这样的化石很难研究，所以朱博士将运用新型影像工程学看到它们，无需清楚化石周围的岩石，并能肉眼找到有机化合物化石的足迹。
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纪以前的项目，新的项目的研究者们还想要利用化其他化石宝库（因为中国存在着许多来自寒武纪的化石宝库）分析寒武纪爆发的演变。陈博士最早因1984年考察到了云南省澄江生物群而出名。云南澄江生物群起源于5.25年前，比西方最著名的寒武纪化石宝库“不列颠哥伦比亚伯吉斯生物群”（加拿大）早了2000万年。项目的研究者们想知道不同的化石宝库如何与其他相关联，并试图准确确定不同有机生物群体出现的时间。
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.
他们也想研究除了碳和氧的其他化学元素——尤其氮和磷，哪一个对生命来说是必要的；且硫表明其缺少氧将对大部分动物有害。朱博士希望能绘制出这些化学元素在时间和空间中的分布变化图。他也将估定这些变化如何相互关联，它们是否与风化、山体建造和冰川的潮汐有关，它们如何能影响到生命的进化，植物和动物如何改变空气和大海的化学成分。
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、肖博士及他们的同僚们最有抱负的是，希望在中国南部地区，在埃迪古拉纪岩石与寒武纪岩石交替的化石遗址中进行钻测。由于世界的大部分的此地区都有裂缝，即不整合面，因此埃迪古拉纪和寒武纪之间的这些地方是变化多样的。不整合面的岩石早在新岩石产生前就已经腐蚀，埃迪古拉纪－寒武纪之间广布不整合面，它们是研究寒武爆发的巨大阻碍。幸运的是，19世纪早期首先在威尔士山脉被发现的一个秘密将解答21世纪中国人想要知道的问题。果真如此，动物王国的起源将逐渐清晰，人类历史的一个重要鸿沟将被填平。
