When did the first animals appear, and what (if any) descendants did they leave? The best evidence suggests that “when” was roughly 600 million years ago, during the Ediacaran period. “What” is a harder question. There’s a rivalry going on among the earliest ancestors of sponges, cnidaria (jellyfish and sea anemones), and comb jellies (like jellyfish, only different).
A 580-million-year-old fossil is casting doubt on the established tree of animal life. The invertebrate, named Eoandromeda octobrachiata because its body plan resembles the spiral galaxy Andromeda, suggests that the earliest branches in the tree need to be reordered, say the authors of study in Evolution and Development.
The researchers, led by paleontologist Feng Tang of the Chinese Academy of Geological Sciences in Beijing, believe that Eoandromeda is the ancient ancestor of modern ocean dwellers known as comb jellies — gelatinous creatures similar to jellyfish, but rounder and with eight rows of iridescent paddles along their sides. If they are right, it would be the oldest known fossil of a comb jelly.