A 580-million-year-old fossil suggests that evolution of bilateral symmetry may be simpler than previously thought, Nature reports. The fossil, an eight-armed swirl named Eoandromeda after the galaxy Andromeda, is believed by researchers to be the ancestor of modern comb jellies, which populate oceans worldwide and swim using rows of cilia, or combs. On the established tree of animal evolution, comb jellies branch off after the cnidarians, other jelly-filled creatures like sea anemones and jellyfish.
Like vertebrates, cnidarians exhibit bilateral or biradial symmetry—meaning they can be bisected into two mirror images. The ancestors of comb jellies, however, were not. The current version of the animal tree, which has the comb jelly lineage arising after bilateral cnidarians but before bilateral flatworms suggests that bilateral symmetry must have evolved twice—once in the cnidarian lineage, and again in animals that evolved after the comb jellies branched away. The new fossil, however, may shake up that view. According to the paper published recently in Evolution and Development, the new fossils of Eoandromeda would place comb jellies lower down on the tree than cnidarians, meaning bilateral symmetry would only have evolved once in the animal lineage—after comb jellies evolved.
"Eoandromeda puts a little piece of weight in favour of a more basal position for comb jellies," co-author Stefan Bengtson, a palaeontologist at the Swedish Museum of Natural History, told Nature. The new theory is also supported by emerging DNA evidence that suggests comb jellies emerged earlier than previously thought.