More than 500 million years ago, the three living groups of chordates — the ones with backbonish structures — went our separate ways. These are the cephalochordates (lancelets), the tunicates (sea squirts, salps, and some others), and the vertebrates.
The cephalochordates are still with us today as a small group of only about 25 species which are also collectively known as lancelets or amphioxi. They seem fishlike until you realize a couple of things about them. While they do have a nerve cord supported by cartilage running down their backs, they don’t have any true vertebrae. They also don’t have organized heads with brains, just a filter-feeding apparatus.
Because cephalochordates do have a backbonish thing, while many adult tunicates don’t, it was thought for a long time that cephalochordates were the last group that vertebrates diverged from. But a 2006 Université de Montréal study using genome sequencing showed that cephalochordates actually diverged from both us and tunicates before the two of us split up, which leaves tunicates as the evolutionary sister group of vertebrates. They’re our closest living relative that is not a vertebrate.
So a simplified family tree for all of us vertebrates is shown below. If you want one with more detail, you can click here.
Up until now, no one had found any convincing tunicate fossils from anywhere near the time they diverged from vertebrates, even though, for example, the primitive cephalochordate Pikaia gracilens has over 100 well-preserved specimens, from as early as 505 million years ago. There’s only one known tunicate-like fossil, described in 2003 (actually a set of eight specimens), but not everyone is convinced they are even true tunicates, as they don’t match some key features of modern ones.
Well, now this gap in the fossil record has finally been unambiguously filled. Another bad day for evolution deniers. Karma Nanglu and other Harvard researchers describe a 500-million-year-old fossil from the Marjum Formation in Utah that strikingly resembles a modern tunicate in the July 6 issue of Nature Communications.
Tunicates are still with us today as an amazing array of about 3000 species of sea creatures. Two classes of tunicates (Appendicularia and Thaliacea) swim freely their entire lives, while the other three (the ones ending in “-branchia”, collectively called the ascidiaceans) swim freely when they are larvae but then attach to something like a rock or pier permanently as an adult.
I’ll give one visual example of each of the five current classes of Tunicata below, but if you want to see a huge and awesome gallery for any of them, go to iNaturalist.org, hit “EXPLORE”, and search for Aplousobranchia, etc. You’ll be amazed.
Ascidaceans — the anchored ones — have a backbonish structure made of cartilage called a notochord when they are free-swimming larvae, but then they lose it as anchored adults. Appendicularians never mature into an adult form at all, and they keep their notochords throughout their free-swimming lives. Thaliaceans also remain free-swimming, but they don’t have any notochord. Apparently they lost that when they diverged from Phlebobranchia and Aplousobranchia. I mean, look at the salp in the above picture. He’s just a squishball, but he came up as a chordate, so I guess he still gets to be a chordate.
All this ambiguity around backbonishness, though, got resolved once and for all by us vertebrates. We went all in on the backbone plan somewhere around 550 million years ago. (Some current vertebrates, though, seem to be reconsidering).
One more-visible feature that distinguishes tunicates is that they all have two siphons, one which takes water in and the other which lets it out, and the idea is to filter some microscopic food out of the water that is processed this way. Part of what makes the current fossil so compelling is the obviousness of these two siphons. The dark strands leading up to the siphons in the fossil also closely resemble muscle fibers associated with siphons in today’s tunicates.
Besides the coolness of seeing a 500-million-year-old sea squirt fossil, this will help scientifically as well because it gives us some more clues about how the big family tree came to be. What specific differences can we see in today’s genomes that might give us clues about what made it possible for vertebrates to diverge from other chordates?
One of the questions about tunicates themselves that has gone unresolved is: What were tunicates like when they diverged from vertebrates? Were they free-swimmers or adult attachers?
We now have a fossil from not too long after tunicates had distinguished themselves as tunicates and diverged from vertebrates, and that fossil suggests an animal that looked a lot like the seafloor attachers of today.
That means the whole larva-swims/adult-attaches lifestyle had already developed by this time, and it lends support to the idea that an “attacher” like this is the ancestor of all tunicates. That would also mean that we vertebrates went our own way from something like this “attacher” state. So that thing you see above is your closest relative that isn’t a vertebrate. Beats some of my relatives.
If that’s what we diverged from, what changed in our genomes that let us do that? How did we get a hard backbone? How did we become free swimmers? How is that different from the way some tunicates like salps became free swimmers but remained tunicates? The more information we can get about relationships at this early stage, the better we can understand our own evolution.
It also might be tempting to say that if ascidiaceans (the anchored ones) were already totally committed to being ascidiacean-like 500 million years ago (the age of this fossil), they might have already split off from appendicularians (the floaties) by then. That’s 50 million years earlier than estimated by the genomic “molecular-clock” approach. It might make us scootch back the whole tree by about 50 million years, which would put the divergence between tunicates and vertebrates not at roughly 550 million but at 600 million years ago.
Of course, a single fossil doesn’t resolve these questions with a magic wand, but it does start narrowing the possibilities and sets us up to be more informed if and when more tunicate fossils from this period are found.
We still don’t know why tunicate fossils, among the chordates, are so rare, but the unambiguous identification of this one gives us motivation to complete the story of where we came from by looking for more. Now we know for sure they’re out there.