Complex Homology and the Evolution of Nervous Systems
Benjamin J. Liebeskind correspondence email, David M. Hillis, Harold H. Zakon, Hans A. Hofmann
Article Info Publication History Published Online: December 30, 2015
Summary
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.
Trends
New phylogenetic evidence suggests that nervous systems are not monophyletic. There is a debate about whether this indicates a single origin of nervous systems with several losses or multiple independent origins of neurons.
Comparative genomics studies have found that many of the gene families associated with extant nervous system function were present before the origin of animals. However, changes at the biophysical level and gene family expansions occurred independently in several animal lineages, suggesting widespread homoplasy in nervous systems.
Fossils of the first animals are microscopic, about the same size as the larvae of various extant marine species, which display complex behavior although only some have nervous systems. Together with colonial choanoflagellates, these larvae may provide the best model systems for understanding behavior and the origin of nervous systems in early animals.
