Complex Homology and the Evolution of Nervous Systems

Abstract

We examine the complex evolution of animal nervous systems and discuss the ramifications of this complexity for inferring the nature of early animals. Although reconstructing the origins of nervous systems remains a central challenge in biology, and the phenotypic complexity of early animals remains controversial, a compelling picture is emerging. We now know that the nervous system and other key animal innovations contain a large degree of homoplasy, at least on the molecular level. Conflicting hypotheses about early nervous system evolution are due primarily to differences in the interpretation of this homoplasy. We highlight the need for explicit discussion of assumptions and discuss the limitations of current approaches for inferring ancient phenotypic states.

Figure 1. Two Alternative Hypotheses of the Origins of Nervous Systems Arising from the Proposed Position of Ctenophores as the Earliest Diverging Animal Phylum

Either the common ancestor of all animals had a nervous system and neurons were lost twice in placozoa and sponges or the nervous system evolved twice, in ctenophores and in the common ancestor of cnidarians and bilaterians. Animal illustrations by Annika L. Smith.

Figure 2. Time Tree of Animal Diversification with Key Events in Nervous System Evolution Annotated on their Respective Branches

The precise timing of these events is mostly unknown but is bounded by the bifurcations, estimates for which are derived from [20,21,78]. The ages of important fossils and the timing of the major glaciation events are also given and are derived from [79,80]. No macroscopic animal fossils are available from the time period when the first animal lineages were diverging. When this is considered along with the length of the terminal branches, it seems more plausible that large-scale convergences of animal tissues might have occurred at the same time as the convergent molecular events. Animal illustrations by Annika L. Smith.