Comb jellies like this sea walnut are rocking the world of evolution.  (Wikimedia Commons/Bruno C. Vellutini)
Comb jellies like this sea walnut are rocking the world of evolution. (Wikimedia Commons/Bruno C. Vellutini)

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.

New work puts comb jellies closer to the base of the tree. (Wikimedia Commons)
New work puts comb jellies closer to the base of the tree. (Wikimedia Commons)

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.

Comb Jelly DNA Studies Are Changing How Scientists Think Animals Evolved 2 June,2014Dr. Barry Starr

  • Benrubi

    It actually makes sense. If there was only one way to make a nervous system, what would the odds be that our ancestors would take that specific path that made it possible? The fact that there are more than one way to make a nervous system makes you wonder what may exist on other worlds, maybe even in our own solar system if there is life on Europa.

    We don’t know what the earliest common ancestor of animals looked like, but we can be pretty sure that they were multicellular, did not have neurons or muscles, was free living at least at one stage in their life cycle, had primitive sense organs and was swimming around in the ocean with cilia. That describes sponge larvae pretty good (or Trichoplax, even if it behave more like an amebae).

    Evolution only works with the material that is already available, so the natural selection regarding the first animals would probably be to polish what was already there. A ciliated creature which was moving around in the ocean eating what it could find in the environment was already dealing with the processing of information. Every mutation that made the processing more efficient, faster or required less energy in total, would spread through natural selection if given the opportunity. In the end, we would have specialized cells dedicated to a single purpose alone. Neurons may be expensive and energy consuming, but you need far fewer of them to do the same job required if you don’t use them.

    For all we know, there was already an ecosystem back then with these ciliated creatures where some would hunt others for food in a world without any nervous system, triggering a traditional arms race. When someone came up with something new that gave them the upper hand (like the first cephalopods who could attack their prey from above thanks to their gas filled shells), there was no going back. And there is no reason to assume it could only happen once or in a single way.

    Next in line would be the origin of individual cells that could contract and relax depending on what signals they received from the neurons, instead of the “all or nothing” functions still seen in many animals today in addition to muscles. At first the muscles probably served the digestion system, but eventually they would also become the most important method for locomotion in most (but not all) animals. And faster movements and a specialized nervous system would in turn allow the evolution of more efficient sense organs, metabolism, inner organs and tissues, as seen in so many animals today.

    In addition to the unique way of making neurons and muscles (a nervous system without serotonin and dopamine and such sounds a little depressing), comb jellies also have the most reduced mitochondria of all animals studied so far, no microRNA and no HOX-genes. As already mentioned in the article, it sounds very unlikely that they should have evolved from animals who still used the same genes to make a nervous system and muscles, still had HOX-genes and micro-RNA and everything else that is absent or different in comb jellies, only to lose it all and/or evolve something new and different to replace it. It is also difficult to imagine the exact reason why natural selection would perform such rearrangement of their genome. Which is why seeing them as an example of convergent evolution makes a lot more sense.

  • Jason Miller

    Wait. Why is this revolutionary? I learned about convergent evolution in biology class 15 years ago.

    • Tim Hansen

      If you really think this is just about convergent evolution as a phenomena, you have misunderstood. It’s the extraordinary degree of convergent evolution we are talking about here, that nerves and muscles have evolved independently in two different lines. Sorry if you fail to see how special that is. It’s also interesting because it has long been assumed that comb jellies were closer related to Cnidarians, and that sponges were the first animals. This research suggest that Ctenophora are the living descendants of a line that branched off from the rest of the animal family tree before the sponges, before nerves and muscles and other traits had evolved. That’s what makes them so special and unique, and also a very isolated group in the animal kingdom.

  • VladimirJosephStephanOrlovsky

    #1 *simple: we have no clue, how Mother Nature works, Not yet — VJO
    #2 *We need to Learn More, and Definitely, we need Less `tinkering` with DNA. We acting Up like 3 years-old boy; we sticking finger in power outlet .. well, do Not cry! — VJO
    // VladimirJosephStephanOrlovsky //

  • Tim Hansen

    For most ctenophores, the cilia is their main method of locomotion. With the exception of some colonial animals, these are the largest pelagic organisms to do so, and certainly the fastest. In some species these cilia are 200 times longer than in other animals. Had they been of the same length that is normal in the animal kingdom, and not fused together into combs made up of hundreds of cilia, they could not have grown to such sizes and still be active predators as they would probably have been too slow, restricted to using their “fish nets” as some of them do. In the oceans of a young earth, where still no animals had evolved hard parts or efficient muscles, these animals must have been close to or even on the top of the food chain, bigger or faster, probably both, than most of the other prey and predators that lived in the seas back then. It was the era of the gelatinous creatures.


Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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