As a new article in Nature (title below) notes, there are five major groups of animals that arose early in animal evolution and persist today: ctenophores (comb jellies), sponges (Porifera), placozoans (small, simple multicellular organisms), cnidarians (jellyfish, corals, sea anemones), and bilaterians (all other animals ranging rom molluscs to vertebrates). We have a pretty good notion of when their ancestors branched off from each other in evolution (this is in effect their relatedness, expressed in their phylogeny, or family tree), except for one question: which group’s ancestors branched off first? That group would be called the “sister group” of all living animals. (It could also be called “the outgroup among all groups of animals”.)
DNA sequencing has shown that it’s either the ctenophores or the sponges (the most common candidate), but it’s been very difficult to decide between the two because there’s been so much time since the ancestors of modern sponges and ctenophores branched off from the other groups—700-800 million years—that too many DNA changes have accumulated to allow a firm DNA-based resolution. (DNA is now the best way to go to resolve these trees.) Every few years then, someone attempts another DNA-based phylogeny of animals, and the outgroup keeps changing between sponges and ctenophores.
Why is this an important question? Not just curiosity alone, for its resolution bears on understanding an important fact: like all other animals except sponges, ctenophores have nerves and muscles. This would seem to show that ctenophores are grouped with the other animals, while sponges branches off early, and then nerves and muscles evolved in the ancestor of all other animals. This convinced many that sponges were the outgroup. If ctenophores, on the other hand, were the sister outgroup that branched off first, that would leave us with a puzzle: why are sponges the one exception, lacking nerves and muscles, among all other animals? Here are the two possibilities for the outgroups, with “N&M” showing where nerves and muscles evolved. (I’ve put the dots and “N&M” stuff in myself.)
A. Ctenophore outgroup: The ancestor of ALL animals did have nerves and muscles, but sponges lost them.
B. Sponge outgroup: Nerves and muscles evolved after the ancestor of sponges had branched off from the ancestor of all other animals. (No loss of already-evolved characters required.)
The left side shows “A”, with the common ancestor of all animals having nerves and muscles, but then they were lost in the ancestor of living sponges. The right side shows possibility “B,” with the ancestor of all living animals (red dot) lacking nerves and muscles, which appeared later in the common ancestor of all living animals after that ancestor had branched off from the ancestor of sponges.
As you can see, “A” posits two evolutionary events: the evolution of nerves and muscles in the ancestor of all living animals, and then their loss in the sponge lineage, while “B” posits nerves and muscles evolving evolving just once: in the ancestor of all non-sponge animals.
However, if “A” is the case, you could posit another scenario in which ctenophores independently evolved nerves and muscles from other groups of animals, while the ancestor of all animals lacked them.
The diagram below shows a common ancestor of all animals (red dot) lacking nerves and muscles, and then they evolved twice independently: in ctenophores, and then also in the other groups that branched off later from the common ancestor with sponges. Thus, if A is correct and ctenophores are the outgroup, there are still two explanations for the nerve/muscle presence in animals: either they were in the ancestor of all living animals and then lost in the ancestor of sponges, or they didn’t occur in the animal ancestor but then evolved twice independently (N&M shown where the evolution happened). As you can see, if the red-dot ancestor lacked nerves and muscles, but all modern animals save sponges have them, AND ctenophores were the sister group, then nerves and muscles must have evolved twice OR (as you see above), all early animals had them but the ancestor of sponges lost them. So the left side of the diagram above OR the diagram below show the two evolutionary possibilities for where muscles and nerves occur.
This is why resolving the outgroup is important: it leads to different hypotheses about how evolution worked. Again, the alternatives are A with the ctenophore outgroup, in which cases nerves and muscles were either lost in sponges or evolved twice independently; or B, with the sponge outgroup, in which case nerves and muscles evolved just once—in the common ancestor of all other animals. Because “B” seems more parsimonious to many, that has been the consensus scenario.
But now the consensus seems wrong: new data show pretty convincingly that ctenophores do appear to be the outgroup, and sponges are more closely related to all other living animals than are ctenophores. You can read about this by clicking on the screenshot below, or going to the pdf here (reference at bottom).
The analysis was very clever. Instead of just looking at large amounts of DNA in the animals, they looked at the order of DNA sequences (genes) on the chromosomes. Over the last 800 million years, that DNA has been shuffled around as chromosome fuse or bits of chromosomes come loose and stick to other chromosomes (translocations). In either case, chunks of DNA then get shuffled around among chromosomes and on a given chromosome by inversions.
And that’s what the authors did: they not only sequenced or took sequences from entire genomes of all the animal lineages above (including two species of ctenophores), but ordered genes along chromosomes. (This isn’t hard to do: you get the DNA as a sequence, and the DNA sequence on one chromosome will not run on to the DNA sequence on another chromosome.) They not only looked at all animal lineages, but also single-celled groups that are less closely related to animals, like amoebas and choanoflagellates (these aren’t considered “animals” but whose ancestors are considered outgroups to all living animals).
The results were pretty unequivocal: they found several chunks of DNA that were shared by the single-celled relatives and ctenophores, but also four ordered chunks of genes that were shared by all living multicellular animals except for ctenophores. That is, sponges shared gene chunks with vertebrates, cnidarians, and placozoans, but those chunks were in completely different places in the ctenophores.
The conclusion: the chunks found their shared locations in modern animals after they had already branched off from the ancestor of modern ctenophores. Ctenophores are thus the outgroup, and we’re less closely related to them than to sponges. The scenario in A above is the correct one. (The three groups at the top of the diagram below are single-celled non-animal organisms that are distantly related to animals.) As you see, the ctenophores branched off from all other living animals before any other animal group, making them less closely related to modern animals than are sponges.
Now this analysis may be wrong, but given the irreversibility of moving gene chunks around repeatedly, shared gene chunks on chromosomes almost certainly means shared ancestry. I’m pretty confident, then, that this paper has resolved the long-standing controversy about the “outgroup” of all animals.
But this leaves us, of course, with two questions. Did the ancestor of all living animals have muscles and nerves, and sponges simply lost them, or did nerves and muscles evolve twice independently?
Each of these comes with another puzzle. The first one is this: why did sponges have complex and highly evolved set of features to sense the environment and move about, but then lost it? The second one is even more puzzling: how could such complex features evolve twice independently?
UPDATE: I forgot about this but was reminded. Another trait shared by ctenophores and all other animals save sponges is the gut: a digestive channel formed by “gastrulation”—invagination of the embryo. Thus we have to account for the disappearance of three features in sponges or the independent evolution of guts, nerves, AND muscles.
While we know that the best information we have is scenario “A” above, we don’t know whether sponges lost their gear or that gear evolved twice independently. The authors of the paper don’t discuss this, but in a NYT article on the piece by Carl Zimmer, he finds a hint that nerves and muscles may have evolved independently in ctenophores and in all other animals that have them:
Instead, researchers are looking now to comb jellies to see how similar and different their nervous systems are from those of other animals. Recently, Maike Kittelmann, a cell biologist at Oxford Brookes University, and her colleagues froze comb jelly larvae so that they could get a microscopic look at their nervous system. What they saw left them baffled.
Throughout the animal kingdom, neurons are typically separated from one another by tiny gaps called synapses. They can communicate across the gap by releasing chemicals.
But when Dr. Kittelmann and her colleagues started to inspect the comb jelly neurons, they struggled to find a synapse between the neurons. “At that point, we were like, ‘This is curious,’” she said.
In the end, they failed to find any synapses between them. Instead, the comb jelly nervous system forms one continuous web.
When Dr. Kittelmann and her colleagues reported their findings last month, they speculated yet another possibility for the origin of animals. Comb jellies may have evolved their own weird nervous system independently of other animals, using some of the same building blocks.
Dr. Kittelmann and her colleagues are now inspecting other species of comb jellies to see if that idea holds up. But they won’t be surprised to be surprised again. “You have to assume nothing,” she said.
That is, there are differences between the nerves in ctenophores and in all other nerve-bearing animals: the former appear to lack synapses. This suggests that nerves could have evolved independently, and taken two routes, one route lacking a gap (the synapse) between the nerves. As for the muscles, neither the paper nor Zimmer deals with whether there’s some fundamental differences between how muscles are structured or how they work between ctenophores on the one hand and all other muscle-bearing animals on the other.
As usual, we’ve probably settled one evolutionary question but it’s raised several others. People now will be devoting more attention to nerves and muscles in animals.
As one of my friends, who teaches introductory biology in a major university, said, “Well, I guess I’ll have to revise my lecture notes. For years I’ve been telling students that while the outgroup of all animals isn’t known for sure, it is most likely the sponges.”
Here’s a ctenophore shown on Wikipedia. They are really cool animals, and if you want to see a bunch of them, go to the Monterey Aquarium in California, where they have a mesmerizing display: