If you’re reading these words right now, chances are you’re a human (if you’re not, shoot me an email–I have a LOT of questions for you). As humans, our tendency is to organize–to tease meaning out of data. Humans are wired to look for patterns; this is one of the traits that’s propelled our species from being a bit player in the global scheme of things to placing us at center stage for the next mass extinction (see: Anthropocene). We’ve used those patten-finding skills to classify the crap out of all living things.
You’re probably aware of the classification system we use–first off, all (currently known) life can be binned into 3 groups–either bacteria, archea, or eukaryota. Bacteria and archea are single-celled and tiny, while Eurkaryota encompasses everything from amoebas to Homo sapiens. Within these three groups you can subdivide and subdivide, it seems, ad infinitum.
Animals specifically can be classified as anything within the kingdom Animalia, and include everything from seafloor sponges to siberian tigers. All animals possess a common ancestor (way, way, way long ago), and as time progressed and this ancestor multiplied, evolved, and certain traits were selected for by the environment, a variety of different kinds of creatures emerged. Traits, such as having sausage-shaped fingers or hairy nostrils, can arise independently in different groups of organisms, but what we often see when we look at a “Tree of Life” is that traits are grouped among organisms that all had a common ancestor with that trait. This is especially true for traits that act are foundational for life–for example, in animals, the development of a nervous system. Which is what makes the recent discovery that ctenophores (aka comb jellies) may have developed a nervous system completely independently of ours, and of those of the rest of Animalia, a complete surprise. This discovery, by Dr. Leonid Moroz and other collaborators, upends our traditional ideas about what group of animals first evolved–the ctenophores, rather than the more visually simplistic sponges, could have been the first animal group to split off from our common ancestor.
Dr. Moroz’s lab looked specifically at the comb jelly Pleurobrachia bachei as well as 10 other ctenophore species in order to discover this. Ctenophores are also known as comb jellies because they propel themselves through the water with comblike cilia, which can glow with rainbow-hued biofluorescence as they move back and forth. Like the better-known jellyfish, ctenophores are mostly gelatinous, but unlike them all ctenophores are active predators. They can be pretty common in coastal areas, depending on the time of year. I actually used to catch the odd comb jelly in my fishing net during a summer spent doing research in Louisiana marshes. It was sad–the net itself would often tear their fragile jelly-bodies to pieces, but when I caught a whole one I would slip it back into the water and watch it sink, cilia glowing in the murky water.
Scientists a hundred years ago wanted to group ctenophores with jellyfish, since appearance-wise they look very similar. However, advances in genetics have allowed us to examine the genetic code of these organisms and, by comparing how similar sections of this code are, determine that ctenophores are actually animals! Using this same principle and comparing the genes present in ctenophores versus other animals, Dr. Moroz determined that many of the features we thought of as essential for animals are absent in ctenophores.
Because ctenophores lack so many of the components we use to create functioning nervous systems, it would stand to reason that these creatures would have abbreviated or nonfunctional nervous systems. However, researchers know that comb jellies exhibit complex behavior, such as predation and diurnal migrations, which would not be possible without the guidance of a nervous system. This, Dr. Moroz and his team concluded, means that ctenophores may have developed their own separate version of a nervous system which accomplishes the same thing as ours but in a very different way.
These findings are fantastic not just because they show us how boundless and innovative life can be, but also because they demonstrate the rambling, haphazard way evolution changes organisms over time. As humans, we like to think of ourselves as the end of the line–the tip of the uppermost tree branch on our Tree of Life. It’s nice to think that evolution is working towards a better future (and that we are that future) but in reality, evolution is more of an “meh, good enough” sort of system. Even if you aren’t as evolutionarily fit as you could be, as long as you’re good enough to survive and reproduce, you’ll stay viable and keep evolving. This process is continuing to happen to all organisms, all the time–in us, in katydids, and in comb jellies. Who knows what evolution has in store for us! I’m doubtful that Tyrannosaurus rex would have looked at the tiny, ratlike creatures scrabbling around in the underbrush beneath his feet and guessed that their descendants would one day be the dominant life forms on the planet. I for one am not excited about the prospect of cockroach overlords, but I guess I’ll just roll with whatever life has planned.