Where Did Sea Sponges' Nervous Systems Go?

Researchers just sequenced the genome of a type of comb jelly called a sea walnut, and what they found could rewrite the earliest chapter of animal evolution.

Comb jellies are gelatinous ocean dwellers that belong to the phylum Ctenophora, and have eight iridescent paddles along their sides. In the last few years, new (very old) fossils of comb jellies have pushed the comb jelly’s starting point to earlier than jellyfish and sea anemones.

But this latest phylogenomic analysis of the sea walnut’s DNA suggests that comb jellies might be the earliest group to break off from the animal tree of life, perhaps even before sponges, which were thought to be the earliest branch.

This paper, published in Friday’s Science, is the second big study this year that looked through comb jelly genes and concluded they’re likely older than sponges. In January, a team from University of Washington in Seattle looked at the species Pleurobrachia bachei, also known as the sea gooseberry, and said in Nature that picturing comb jellies diverging before sponges was “wild to imagine." Nevertheless, that's what they presented at January’s Society for Integrative and Comparative Biology’s annual meeting.

Part of what makes this surprising is the comparative complexity of comb jellies. They’ve got muscle cells, a simple nervous system called a nerve net, tissue layers and light sensors, while sponges are “simple animals” that lack complex cell types.

However, no matter which diverged first, all these divergence obviously happened millions and millions of years ago, so whatever living examples we find today may be evolved since then. In spite of their muscle cells, comb jellies lack the genes that specify their muscle types, which researchers believe indicates that the muscle cells evolved independently, after comb jellies diverged from the rest of animals.

Sponges also share some of the same genes with comb jellies that are involved in the nervous system, which sponges don’t have. Perhaps they had a nervous system once, and evolved it away.

“These results present a newly supported view of early animal evolution that accounts for major losses and/or gains of sophisticated cell types, including nerve and muscle cells,” the researchers wrote of their sea walnut findings.

While we often imagine evolution as always a moving from simplicity to more complexity, but ironically it’s our understanding of evolution that is becoming more complex—with the realization that this isn’t always the case.