Doushantuo embryo fossils

In the Doushantuo Formation near Weng'an in Guizhou Province, China, phosphate-rich rocks about 609 million years old preserve millimetre-sized spheres whose insides are packed with cells. Some look like an egg splitting into 2, 4, 8 and 16 cells, the way an animal embryo does. Xiao Shuhai, Andrew Knoll and colleagues described them in 1998, and the fossils (Megasphaera, Parapandorina and others) became famous because they seemed to show animals, or their close relatives, long before the Cambrian.¹

The rocks formed in the warm shallow sea that followed the great glaciations of Snowball Earth, when oxygen was rising and multicellular life was already well established. Phosphate mineralised the tiny fossils so completely that cell walls survive in three dimensions. In 2011 Therese Huldtgren and colleagues scanned them with synchrotron X-rays and found structures that look like cell nuclei, and concluded that the stages are better explained as encysting single-celled protists than as animal embryos. Other researchers argue the balls are giant bacteria or algal cysts. The fossils remain the best window on how animal-like development might have begun, and a test for molecular clocks that put the origin of animals before 700 million years ago.²⁴