Early metazoan life: divergence, environment and ecology

Abstract

One contribution of 16 to a discussion meeting issue 'Origin and evolution of the nervous system'. 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 O 2 . 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.

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

“…Molecular clock estimates indicate an origin for the metazoan crown group at 750 to 800 Ma (Erwin et al ., ; Erwin, ) which, if correct, would indicate that these early animals survived panglaciations, and that the divergence and proliferation of life began well before the Cambrian explosion of diverse skeletonized organisms. However, nearly all geological evidence for these early animals, prior to the Gaskiers glaciation (late Ediacaran) (Figs and ), is disputed.…”

Ediacaran pre-placozoan diploblasts in the Avalonian biota: the role of chemosynthesis in the evolution of early animal life

“…Molecular clock data suggest that the origin of metazoans predates the late Ediacaran period, with many major clades radiating during the Cryogenian and early Ediacaran (dos Reis et al 2015;Erwin 2015). There is no convincing direct evidence for triploblastic features and complex internal organs among the Ediacaran macrofossils and a simple diploblastic bauplan consisting of multiple modules along a polarized body axis is typically invoked for these organisms Budd & Jensen 2015), with the possible exception of the latest Ediacaran mollusc-like Kimberella (e.g.…”

The “Dirty Ice” of the McMurdo Ice Shelf: Analogues for biological oases during the Cryogenian

“…Sedimentary biomarker hydrocarbons derived from algae and demosponges provide further evidence for the presence of a productive biosphere during the Cryogenian Epoch (Grosjean, Love, Stalvies, Fike, & Summons, ; Love et al., ). Molecular clock data suggest that animal phyla, and specifically sponges, appeared in the time window just prior to the Snowball Earth Epoch (Dohrmann & Wörheide, ; Erwin, ) and well before the Doushantuo embryos which comprise the earliest body fossil evidence for metazoans, dated to about 600 Ma, and which precede the earliest documented Ediacaran biota of ~571 Ma (Erwin, ; Pu et al., ). If so, there must have been refugia capable of supporting complex ecosystems during Cryogenian times.…”

“…If sufficient in areal extent, cryoconite pans could then have played a significant role in the Cryogenian carbon cycle (Hoffman, ; Hoffman et al., ). Cryoconite pans, ice‐covered lakes, and other meltwater environments may have been habitats where early metazoa thrived (Christner, Kvitko, & Reeve, ) and evolved, potentially accounting for the absence of any apparent eukaryotic bottleneck during the Cryogenian Period (Erwin, ; Erwin et al., ). For a contrary view, see Cohen and Macdonald ().…”