Ctenophores, also known as the "comb jellies", are an ancient phyla of animals. They have no HOX genes, at least some of which are present in every other animal except themselves and sponges. They lack many of the basic immune system adaptations common to all other animals, including sponges.
But comb jellies have nerves and muscles, which sponges do not have. It's natural to assume that something as complicated as nerves and muscles only evolved once. But a new paper in Nature took a closer look at the nerves of comb jellies, and found that they basically have nothing in common with the nerves of any other animal. Which strongly suggests that nerves and muscles evolved twice in animals.
Your nerve cells use a suite of chemical neurotransmitters to communicate with one another. Glutamate, serotonin, dopamine, adrenaline, (just to name a few) . . . and these neurotransmitters are present across all nerve-bearing animals, from insects to mammals down to starfish, jellyfish and coral. According to these researchers, comb jellies "do not use serotonin, acetylcholine, dopamine, noradrenaline, adrenaline, octopamine, histamine or glycine as intercellular messengers". Only one "normal" neurotransmitter, glutamate, was also found in comb jellies.
So what does this mean? The researchers present two possibilities:
- Nerves and muscles evolved twice in animals, once in the ctenophores, and once in the ancestor to the cnidarians (jellyfish, anemones, coral, etc.) after the sponges had branched off, or
- The common ancestor to all living animals had ctenophore-like nerves and muscles, but subsequently lost most/all of their function, and then later re-evolved new nerves.
As crazy-complex as nerves are, the first theory makes the most sense. And the researchers agree. Nerve cells (and muscles) evolved twice, and the comb jellies (now known as the oldest surviving branch of the animal family tree) have a neuromuscular system all their own.
Comb jelly image is in the public domain.