Convergent evolution raises the issue of evolutionary inevitability: that functional facets of life are constrained to emerge similarly.
The earliest-evolved animals include sponges, placozoa, cnidarians, and ctenophores (comb jellies). Though they somehow relate to the original metazoan, their lineage remains uncertain. Though comb jellies are more complex than sponges, modern ones retain the most ancient DNA. These 4 may well all be basal: originators of their respective clades.
Sponges have unspecialized cells which can migrate and transform into the needed type. Sponges do not have circulatory, digestive, or nervous systems. They do possess an immune system.
Placozoa are a basal invertebrate: one of the simplest non-parasitic metazoa. These small flattened animals have specialized cells, including fibers to interconnect cells for communication and coordination. But they lack neurons.
Cnidarians include jellyfish and anthozoa (coral, sea anemone and sea pens). Cnidaria have various cell types, including nerve cells.
Comb jellies (ctenophores) have an elementary brain with nerve cells which are linked by complex synapses to muscles. These jellies have a nervous system like no other, with unique neurons and communication signaling molecules.
Cnidarians and ctenophores do not share a common ancestor. Nervous systems evolved at least twice.
Everyone thinks this kind of complexity cannot be done twice. But ctenophores suggest that it happens. There is more than one way to make a brain, a complex neural circuit and behaviors. ~ Russian biologist Leonid Moroz
The comb jelly genome is radically different from other animals. Besides nervous systems, comb jellies are distinct from other genomes in their immune system and developmental genes.
They are the aliens of the sea. ~ Leonid Moroz