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. And that would support a rewrite of the animal tree.

Comb jellies sit alongside two other major groups near the base of the tree, but their relative positions remain contentious. Normally, sponges are identified as the first to evolve, followed by the cnidaria — jellyfish, sea anemones and their kin — and then by the comb jellies.

" Eoandromeda puts a little piece of weight in favour of a more basal position for comb jellies," says Stefan Bengtson, a palaeontologist at the Swedish Museum of Natural History and a co-author on the paper.

Family squabbles
That evidence comes from the fossil's shape: it has octoradial symmetry, meaning its body can be sliced into eight identical pieces. This is in stark contrast to modern comb jellies, which, like humans, flies and sea anemones, have biradial or bilateral symmetry — their body plan can be sliced into only two identical pieces.

If Eoandromeda appeared after the cnidarians, the authors argue, bilateral symmetry would have to have evolved twice — once for the cnidarians and again for the bilateral organisms that came after Eoandromeda . Far simpler is the idea that Eoandromeda evolved first (see 'Simplest solution'). "This model of animal relationships calls for the least number of origins of bilateral symmetry," says Bengtson.

The proposal is in tune with DNA studies that place comb jellies closer to the root of the evolutionary tree. "It's great to have concordance between what these guys see in the fossil record and what's coming out of the genome," says Andy Baxevanis, a genomicist at the National Human Genome Research Institute in Bethesda, Maryland. His team recently sequenced the genome of the comb jelly Mnemiopsis leidyi , and is now comparing it to sequences from sponges, cnidarians, worms and other animals to sort out which lineages came first. So far, he says, the results suggest that sponges and comb jellies appeared before cnidarians.

The matter is far from settled, however. Some scientists even doubt that the fossil is in fact that of a comb jelly. The eight spiral arms are reminiscent of the eight iridescent rows, or combs, along the sides of modern comb jellies, but the fossil lacks some key characteristics of modern comb jellies, such as tentacles and a mouth. Differences between living animals and ancient fossils are expected, but the differences also allow for debate.

A galaxy of possibilities
" Eoandromeda fossils are excellent and very important, but the trouble is that the interpretation reflects the ideology of the person giving it," says Dolph Seilacher, a retired palaeontologist at the University of Tübingen in Germany who studies fossils from Eoandromeda 's time.

In the 1980s, Seilacher argued that many of the bizarre fossils from this period were abnormally large, single-celled, amoeba-like organisms in a kingdom he named Vendobionta. Until the vendobionts went extinct, he argues, multicellular animals "lived in the shadow of these unicellular giants". To Seilacher, the golf-ball-sized Eoandromeda looks like one of these giants.

Bengtson says he can't prove the fossil is a comb jelly, but its comb-like arms indicate that it is one. "The only reason to suggest they are vendobionts," he says, "is that they happen to be of that age."

Claus Nielsen, a retired evolutionary biologist at the Natural History Museum of Denmark in Copenhagen, doesn't think Eoandromeda represents comb jellies either. "I can't visualize it swimming at all," he says, noting the ancient fossil's spiraled arms. "I would say it's a beautiful and very exciting fossil, but what the heck?"

This article is reproduced with permission from the magazine Nature. The article was first published on September 7, 2011.