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There have been a lot of significant moments in evolutionary history.
There’s the emergence of unicellular life, the endosymbiotic origin of chloroplasts and
mitochondria, the transition from life in water to life on land, and all the mass extinctions
along the way.
More than 600 million years ago, one particular group of single-celled eukaryotes faced their
own very important fork in the road.
Today, we call that group and its descendents the Choanozoa.
But at that point in time, they were just some single cells staring down two futures.
In one future, they would continue life as single cells.
In the other, they would become something entirely different, something that we have
to be extremely grateful for.
Because down that other fork came the metazoans, more commonly known as animals.
So...us.
For the Choanozoa that went down that route, cells would divide and develop and bind together,
creating biological links and relationships that we see as a whole animal instead of just
individual cellular selves.
And while we could spend this episode marveling at the complexity of animal life, you might
also be wondering…what about that first group of eukaryotes, the ones who didn’t
follow the animal path?
What happened to them?
What do they even look like?
Well, they look kind of like this.
No, not the big gold things, those are synura.
And not the big clear thing in the middle either, that’s an empty frustule of a diatom.
No, we’re talking about that small, balloon-looking thing hanging out on top of the diatom.
So, what is that?
It is a choanoflagellate, but you can call it a sibling.
A very, very, very distant sibling of sorts, but a very important one.
Found in both marine and freshwater, we’ve only identified around 150 extant species
of choanoflagellates.
But this single-celled flagellate is the closest ancestor that we know of to animals.
The word "choano" translates to “funnel” in Ancient Greek, a reference to the collar
that wraps around one end of the choanoflagellate.
That collar is made up of microvilli, a series of thin projections.
But under most microbes, it’s difficult to make out the individual microvilli, and
instead they blend together into one big cone.
Encased in that collar is the organism’s single flagella, which it uses to create currents
in the water to shake up and draw in their favorite food: bacteria.
Some choanoflagellates are non-motile, meaning they spend most of their life not moving around.
Instead, they just stick themselves to a nice, sturdy surface and grab whatever food passes by.
There are also motile choanoflagellates, which move around with their flagella, but in kind
of a weird way.
You see, other flagellates—like this euglena—use the twisting rotor movement of their flagella
to pull them around the microcosmos.
But choanoflagellates use their flagellum to push themselves through the water.
All of these things make choanoflagellates interesting to watch, but if you’re looking
at them and wondering how they could possibly be the closest relatives to animals, we don’t
blame you.
So what made biologists look at them and see any kind of kinship to animals?
Why not some other protist?
To see the resemblance, you have to go to something a bit more fundamental when it comes
to animals: the sponge.
Yes, the sponges you find in the ocean (and also occasionally in freshwater) are animals,
very primitive ones.
And in 1867, when naturalist Henry James-Clarke published an article documenting the choanoflagellates,
he did more than just describe their appearance.
He compared them to the flagellated cells that live inside the chamber of sponges, a
resemblance that would be further emphasized when those sponge cells were named choanocytes.
James-Clarke wrote of their similarities in a paper titled “Conclusive proofs of the
animality of the ciliate sponges, and of their affinities with the Infusoria flagellata”.
And we have to say that “conclusive proofs” is a bold statement coming from a biologist,
especially given the number of occasions during our journey through the microcosmos where
we have seen morphological similarities between organisms suggesting a close kinship that
genetic research eventually undermines.
But on the other hand, maybe we have a bit of bias towards those kinds of stories because
a shift in understanding is part of what makes a story compelling.
But what makes James-Clarkes’ observation fascinating is that a century later, his observations
and conclusions would be supported by electron microscopy—which deepened our understanding
of the structures shared by choanoflagellates and choanocytes.
And those observations would later be validated by molecular phylogeny, which not only established
the genetic resemblance, but named choanoflagellates as the nearest relative to animals.
Knowing this fact about choanoflagellates does more than just illuminate our evolutionary
lineage, it helps shed light on what hundreds of millions of years of evolution might have
looked like.
From the differences and similarities that exist between choanoflagellates and sponges,
we can start to make predictions about our last common ancestor, that unknown organism
that came before the fork in the evolutionary road.
What we have in common suggests traits that may have existed in that ancestor, like the
flagellum and collar that we see in both the choanoflagellate and the sponge choanocyte.
And the similarities go beyond the morphological.
While investigating the choanoflagellate genome, scientists found a number of genes encoding
proteins that help cells stick to each other.
These particular proteins were thought to be an animal thing, acting like glue that
keeps our cells bound together.
So it’s not completely clear why choanoflagellates have this molecule.
But the fact that they do—along with the observation that some species group together
into colonies—suggests that whatever our mysterious shared ancestor was, it was likely
capable of some basic form of multicellularity.
Metazoans just ran with it.
And of course, the work continues, with scientists looking to choanoflagellates for whatever
kinship can give us insight into our past.
This is why we look to family and genealogy, isn’t it?
We go to our past, to our ancestry, to understand more about ourselves.
We seek answers in our family resemblances, and find questions in the differences.
And so, while choanoflagellates may seem an unlikely sibling, they are also, it seems,
a generous one.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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