A morphological phylogenetic approach to the evolution of symmetry in animals

Animal body plans exhibit different forms of symmetry, which plays a major role in their physiology and interactions with the environment. However, there is considerable uncertainty regarding the evolutionary history of symmetry and how phylogeny and ecology have shaped its expression through time. To address this, we assembled a comprehensive morphological phylogenetic dataset for extant and extinct animals and analysed this using Bayesian inference to estimate the ancestral symmetry state for key nodes under competing topologies. The results suggest that the ancestral animal body plan was asymmetrical, in contrast with previous interpretations based on parsimony optimization of symmetry states. Additionally, we built an ecological dataset for phyla with variable symmetry states, which was analysed using phylogenetic comparative methods. Cnidarians show a correlation between symmetry and feeding strategy, whereas in echinoderms and sponges symmetry is best explained by phylogeny alone. Together, our results indicate that bilateral symmetry evolved prior to the widespread appearance of directed locomotion and may instead have been an adaptation to aid internal transport. We find there is no single answer as to why different types of symmetry evolved, and this was more likely driven by a complex interplay between environmental, ecological and phylogenetic constraints.