2015/11/09 by Douglas H. Erwin · 155 citations
Agricultural and Biological Sciences · Earth and Planetary Sciences · #Cephalopods and Marine Biology #Marine Invertebrate Physiology and Ecology #Marine Biology and Ecology Research #Biology #Molecular clock #Ecology #Paleontology #Evolutionary biology #Fossil Record #Period (music) #Ecosystem #Clade #Phylogenetics #Gene
paper · pdf · doi:10.1098/rstb.2015.0036
published in Philosophical Transactions of the Royal Society B Biological Sciences 370(1684), 20150036 (Royal Society)
openalex publication_date 2015/11/09 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/22
Recent molecular clock studies date the origin of Metazoa to 750-800 million years ago (Ma), roughly coinciding with evidence from geochemical proxies that oxygen levels rose from less than 0.1% present atmospheric level (PAL) to perhaps 1-3% PAL O2. A younger origin of Metazoa would require greatly increased substitution rates across many clades and many genes; while not impossible, this is less parsimonious. Yet the first fossil evidence for metazoans (the Doushantuo embryos) about 600 Ma is followed by the Ediacaran fossils after 580 Ma, the earliest undisputed bilaterians at 555 Ma, and an increase in the size and morphologic complexity of bilaterians around 542 Ma. This temporal framework suggests a missing 150-200 Myr of early metazoan history that encompasses many apparent novelties in the early evolution of the nervous system. This span includes two major glaciations, and complex marine geochemical changes including major changes in redox and other environmental changes. One possible resolution is that animals of these still unknown Cryogenian and early Ediacaran ecosystems were relatively simple, with highly conserved developmental genes involved in cell-type specification and simple patterning. In this model, complex nervous systems are a convergent phenomenon in bilaterian clades which occurred close to the time that larger metazoans appeared in the fossil record.