Molecular clocks and the early evolution of metazoan nervous systems

Abstract

One contribution of 16 to a discussion meeting issue 'Origin and evolution of the nervous system'. The timing of early animal evolution remains poorly resolved, yet remains critical for understanding nervous system evolution. Methods for estimating divergence times from sequence data have improved considerably, providing a more refined understanding of key divergences. The best molecular estimates point to the origin of metazoans and bilaterians tens to hundreds of millions of years earlier than their first appearances in the fossil record. Both the molecular and fossil records are compatible, however, with the possibility of tiny, unskeletonized, low energy budget animals during the Proterozoic that had planktonic, benthic, or meiofaunal lifestyles. Such animals would likely have had relatively simple nervous systems equipped primarily to detect food, avoid inhospitable environments and locate mates. The appearance of the first macropredators during the Cambrian would have changed the selective landscape dramatically, likely driving the evolution of complex sense organs, sophisticated sensory processing systems, and diverse effector systems involved in capturing prey and avoiding predation.

The genome of the contractile demospongeTethya wilhelmaand the evolution of metazoan neural signalling pathways

“…Properties of the earliest metazoans are unknown, including life cycle or number of cell types, but it is most parsimonious that the first obligate multicellular animals did not have anything resembling a modern bilaterian nervous system [Wray et al, 2015]. Yet, the genomic evidence shows that these animals have cellular capacity to respond to environmental or paracrine signals, regulate the cell internal ion concentrations and respond to changes in their concentrations, and secrete small molecules that could serve as effectors in unconnected (but proximal) cells.…”

“…Neural evolution has been discussed previously in the context of paleontology (reviewed in [Wray et al, 2015]) and metazoan phylogeny (reviewed in [Jékely et al, 2015]). Indeed, it has been suggested that many features of bilaterian neurons and nervous systems represent separate, parallel evolutionary events from a “simple” nervous system.…”

Physiology and Evolution of Voltage-Gated Calcium Channels in Early Diverging Animal Phyla: Cnidaria, Placozoa, Porifera and Ctenophora

“…Metazoans are divided into four major groups based on body symmetry (Figure 1): (1) Animals with bilateral body symmetry (bilaterians), proposed to make up over 99% of all animal species (Finnerty et al, 2004; Ryan and Chiodin, 2015), further subdivided into Deuterostomia (e.g., phyla Chordata, Hemichordata, and Echinodermata) and Protostomia (e.g., Arthropoda, Nematoda, Mollusca, and Annelida) (Wray, 2015); (2) Animals with radial body symmetry, from the phylum Cnidaria (e.g., jellyfish, corals, sea anemones, and hydra); (3) Animals that lack body symmetry, from the phyla Porifera (sponges) and Placozoa ( Trichoplax sp. ); and (4) Animals with bi-radial body symmetry, from the phylum Ctenophora (i.e., comb-jellies), which have a combination of bilateral and radial symmetry (Tamm, 2014a).…”

Introduction to ‘Origin and evolution of the nervous system’

“…Wray's paper [29] adds to the controversy about the timing of metazoan evolution and divergence, explaining that sequence data suggest metazoan origins far earlier than rstb.royalsocietypublishing.org Phil. Trans.…”