The Evolution of Centralised Nervous Systems in Metazoa: Molecular, Palaeontological and Phylogenetic Perspectives

The evolution of nervous systems represents one of the most significant innovations in metazoan history, fundamentally transforming animal complexity and behavioural repertoires (Anderson & Bucher, 2015; Northcutt, 2012). This comprehensive review examines the phylogenetic relationships amongst major metazoan groups and the evolutionary origins of nervous systems, with particular emphasis on the contentious debate surrounding the monophyletic versus polyphyletic origins of centralised nervous systems (Holland et al., 2013; Moroz, 2015). Through analysis of fossil evidence from the Ediacaran period (635-543 million years ago), comparative neuroanatomy across metazoan phyla, and molecular developmental data, we present a synthesis of current understanding regarding nervous system evolution (Glaessner, 1984; Chen et al., 2004a). The evidence strongly supports a single evolutionary origin of centralised nervous systems in the last common bilaterian ancestor, with subsequent elaboration and modification across protostome and deuterostome lineages (Holland et al., 2013). Key molecular signatures, including conserved expression patterns of BMP/chordin signalling pathways and neural patterning genes such as Pax6, Otx, and Hox genes, provide compelling evidence for homologous neural structures across bilaterians (Montgomery, 2025). The controversial phylogenetic position of Ctenophora, recently proposed as sister to all other metazoans, necessitates reconsideration of traditional models of nervous system evolution and raises important questions about the potential for independent neural evolution or secondary loss in certain lineages (Ryan et al., 2013; Moroz et al., 2014). This review integrates palaeontological, comparative anatomical, and molecular evidence to provide a comprehensive framework for understanding nervous system evolution in Metazoa, whilst identifying key areas requiring further investigation.