Comb jellies are these beautiful, otherworldly creatures that sparkle gently in the sea. And now, if a study in the journal Science and another one in the journal Nature hold up, they may not be so gentle on evolution or the tree of life. These “aliens of the sea” are fundamentally changing how we think about both.
The standard line for evolution has been that all the complicated stuff evolved once. Way back when some common ancestor evolved a nervous system, muscles and so on and all of our systems are built on those first ones.
Seems reasonable given how hard it probably was to cobble together all the components to get these systems to work. And there was a lot of evidence to support this idea too. For example, it looked like a subset of parts of the nervous system were shared by all the animals that have a nervous system.
This no longer seems to be the case. Back in December, a group of researchers took a close look at the DNA of the sea walnut (Mnemiopsis leidyi) and found that it lacked the usual set of genes animals have to make a nervous system. They also found that this comb jelly lacked almost all of the genes needed to make muscles. This was even though this comb jelly has both muscles and a nervous system.
This result has now been confirmed in a study out on May 21 on a second comb jelly, the Pacific sea gooseberry (Pleurobrachia bachei). The researchers not only found that this comb jelly lacks the same set of genes, but they also showed that its nervous system works in a unique way too.
Most animals use a very similar set of chemicals to communicate from one nerve cell to another. The authors found that the Pacific sea gooseberry uses hardly any of these shared neurotransmitters. No dopamine, serotonin or any of the other common ones you may have heard of.
Instead, this comb jelly appears to have its own unique set of neurotransmitters. And because these signaling chemicals are captured by their own specific set of receptors, this means that the Pacific sea gooseberry has its own set of unique receptors too. The DNA confirms this result.
The easiest explanation for this is that the comb jelly nervous system evolved independently of every other animal’s nervous system. The other explanation that it had one like ours, lost it, and then invented a new one seems way less likely. Something similar probably happened with comb jelly muscles too.
All animals to date that have muscles use the same subset of genes to make them. As these studies show, comb jelly (or ctenophore) DNA has almost none of these genes. It looks like these beautiful sea creatures have reinvented the wheel on this one as well. They have their own set of genes that cause their muscles to develop.
So complicated systems can evolve more than once. This is mind blowing stuff that reshapes how we think about evolution.
Apparently evolving a nervous system isn’t so hard that there is only one way to do it. It also isn’t so hard that once something does it, that animal outcompetes everyone else before they can make their own nervous system. There is (or was) room in nature for many paths to complicated systems.
These findings have also caused scientists to remake the tree of life. Comb jellies now have their own branch, separate from all other animals. In other words, our common ancestor split into a group that led to comb jellies and another group that led to all other animals.
Looking at the DNA of lots of different beasts is causing us to rethink how evolution happens. Results like this make it imperative that we sequence as many living things as we can get our hands on especially since looking at DNA has become so cheap and easy. Of course this all depends on the government giving scientists the money they need to do these studies.