For evolutionary biologists, what most distinguishes the marine creatures called cnidarians and ctenophores is not the peculiar spelling of their names, nor the fact that they tend to be beautiful, but that they are among the oldest known groups of complex animals. Cnidarians (the “c” is silent) include jellyfish, corals and sea anemones, and their colorful tendrils and fronds adorn oceans all over the globe. Ctenophores (another silent “c”), otherwise known as comb jellies, are mostly translucent and gelatinous, and they glide through the marine world like ectoplasm.
Thought to have arisen between about 740 million and 520 million years ago, both phyla of marine invertebrates were among the first multicellular animals to evolve several different tissue types. Their appearance ramped up the complexity of the animal world: They are more similar to us in terms of size and organization than they are to the single-celled organisms that came before them. The evolutionary debut of cnidarians and ctenophores thus represented one of the most significant steps in the journey from primordial slime to humans.
What made it possible?
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