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. Seaweeds, 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.

Then, in the 20m-year blink of a geological eye, animals arrived in force. Most of the main groups of the animal kingdom—arthropods, brachiopods, coelenterates, echinoderms, molluscs and even chordates, the branch from which vertebrates 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.

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.

The animals’ carnival

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.

Portents of the modern world 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. 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. 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. 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.

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.

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 other Lagerstätten—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 various Lagerstätten relate to one another, to try to determine exactly when different groups of organisms emerged.

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.

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.