Ocean Chemistry and Early Animals

During the Neoproterozoic Era (1000 to 542 million years ago), the prokaryote microbial communities that had previously dominated Earth were supplemented by the diverse communities of animals and other eukaryotes that have characterized Earth since then. Chemical changes in the world's oceans were central to this transition. After more than a billion years of euxinic (sulfidic anoxic) oceans, deep-water conditions turned ferruginous (iron-rich anoxic) in the late Neoproterozoic. This ferruginous ocean may have also been a substantial reservoir for dissolved organic carbon. On page 80 of this issue, Li et al. elucidate redox conditions in the late Neoproterozoic Doushantuo Formation of southern China. These strata contain superbly preserved eggs, embryos, and probable resting cysts that are generally regarded as the oldest-known microscopic animals.