Fossil embryos deep in the fossil record

Embryos have been found in ‘old’ rocks on four continents, especially in Siberia, Nevada, Australia and China. They have been discovered in sedimentary rocks of even Late Neoproterozoic age, assuming the evolutionary geological column1 (figure 1). The Neoproterozoic is the period, so called, between 1 billion and 550 million years ago. This discovery was in the Doushantuo Formation, Guizhou Province, southwest China and shows Metazoan diversity 40 million years before the beginning of the Cambrian!2,3 ‘In addition, a large number of embryos display a variety of developmental patterns and different morphological types, implying that metazoans may have been diverse 40 million years before the Cambrian [emphasis mine].’4 Raff et al.5 analyzed 162 embryos in different bu t ea r ly s t ages o f embryonic development. These embryos were of large size compared to modern embryos, but size has been ruled out as causing their unusual preserva t ion . They were still encased in a fertilization envelope that likely preserved them during fossilization. Other fossils of sponges, cnidarians, acritarchs and possibly bilaterians have also been found in this formation.4,6 Some scientists have recently challenged the identification of the Neoproterozoic fossils, claiming instead that they are giant sulfur bacteria.7 The challenge is likely motivated by the problems caused by the existence of such animal embryos so early in the rock record.8 Although some observations support this new idea, other data are contrary to the bacteria hypothesis.9 For instance, the suggested bacteria have not been shown to undergo more than three rounds of cell division, while the Doushantuo Formation fossils have undergone at least 7 rounds of cell division. Furthermore, the fossils are found in an enveloping membrane, which is rare in this type of bacteria. The location of vacuoles in the cells are also anomalous, if the cells are from bacteria. Finally, the cells show preserved nuclei, unlike bacteria. Even more recently, Chinese scient is ts have vindicated the embryonic origin of these fossils, and shown that they are not giant bacteria.6 They have discovered embryos within eukaryotic architarchs, a metazoan animal. Other features of the fossils support the embryo interpretation. It is interesting that the architarchs are even found 30 m above what are believed to be glaciogenic rocks, from a time, supposedly, when the earth was totally covered in ice.