So lately, I’ve been thinking about evolution a bit, just another Saturday.
And I’m thinking, there have to be some basic body
parts that animals would absolutely never lose, once they evolved them?
We know that there are plenty of animals that are kind of basic,
but that’s because they never gained functions.
They started off simple, and stuck to it, because it was working,
and they don’t need any fancy bells or whistles.
And we know that sometimes, evolution giveth and evolution taketh away.
Like how we have tailbones, but no tails. What gives?
But there must be some anatomical features that are just so great
that you’d never lose them after all that hard work to gain them in the first place!
Like, there can’t be any animals that have lost their heads after they evolved them?
[Kallie]: Actually, there totally are!
[Stefan]: Oh, oh! It’s Kallie, from our sister channel Eons! But wait, really?
That seems kinda crazy!
But, okay so maybe heads can come and go, but like, every animal needs a body, right?
[Deboki]: Not necessarily, Stefan!
Deboki, from our channel Journey To The Microcosmos is here as well,
I guess? And also, what??
Ok fine, maybe you can lose a head and a body,
but there’s no way that any animal could possibly lose their butt.
[Sarah]: I’ve got some bad news for you, Stefan.
COME ON. Now Sarah from Bizarre Beasts is also correcting me?
Well ok, fine.
Well, if you three are so smart, well, why don’t you just host the entire episode.
For humans, our head is the home of our brain, so it truly runs the show.
That’s not the case for bivalves though, who actually do just fine with no head at all.
They’re part of the Bilaterian group.
Bilateria are bilateral animals, which means we’ve got one plane of symmetry.
This basically means that you can picture folding them in half along the midline,
and the two halves you make are mirror images of each other.
And within this massive group we find the bivalves,
a class of molluscs which includes clams, oysters, mussels, and scallops.
Most molluscs do have heads, but bivalves break the mold. See,
they evolved from ancestors with heads, but then lost them somewhere along the way.
Now for this video, we’re defining a head as a nervous
system command center at one end of the body.
It doesn’t have to be as complex as our heads,
so long as it’s a centralized location for most of the nervous system’s processing power.
And having at least a simple head is pretty much the standard across bilateral animals.
This concentrated nervous system hub plays a role in information processing,
and usually coincides with the development of other key features, like eyes and mouths.
But bivalves don’t have concentrated nervous systems.
Instead of a true head, they have several sets of nerve clusters,
spread across their whole nerve network.
And this could seem like a big evolutionary mistake.
Having a head is really useful, so in order to evolve it and then later lose it,
there must be a pretty big benefit.
It turns out that benefit comes in the form of saving energy.
See, bivalves have it pretty good in the evolutionary scheme of things.
They’re living the easy life, so there’s not a lot of complex processing that has to happen.
For one thing, they don’t move around a lot.
Many of them spend a lot of their time buried in the muck or literally
glued in place, like mussels or oysters.
This means they don't have to spend mental energy on navigating their surroundings or
building the neural circuits to control movement.
Plus they rely on nice cozy shells for protection from predators, which means they don’t need to
spend energy on staying alert for threats or learning what clues indicate danger.
A head isn't just good for brains though.
There's usually important sensory activity going on there too.
For example, a head usually has eyes.
But without a head, these bivalves found new places to put their eyes.
And they have a lot of them.
Their eyes are more diverse than the head-based eyes of a lot of
their relatives, and can number in the thousands.
But there was one more anatomical problem that came along with losing their heads.
Bivalves are also the only group of molluscs that are missing a key feeding feature called a radula.
This organ is comparable to a tongue, which is used to scrape away at their food sources.
But bivalves lost their radulas along with their heads.
So in order to find food, they became filter feeders.
Instead of needing to probe out into the water for food, they let it drift right to them.
And because they have a constant source of food that's always just floating by,
there's no need to spend time or energy thinking about their next meal.
So given the choice between use it or lose it, bivalves picked the latter.
And with over 9000 species alive today, they seem to be doing just fine.
Honestly, being a bivalve sounds pretty chill. I guess life without a head, not too bad.
But now, let’s check in with Deboki from the channel Journey To The
Microcosmos and she will explain how anything can be alive without a body?
The tardigrade is a fan favorite organism of the microcosmos.
They have nicknames as cute as they deserve, like water bears or moss piglets.
But the story of the tardigrade isn’t one without loss.
Hundreds of millions of years ago,
tardigrades went through a major reduction in their midsections.
Now you might look at these tardigrades and think their body is plain to see.
But tardigrades are part of a superphylum called Panarthropoda,
which is a huge and diverse grouping of organisms, most of which are far larger.
And compared to their cousins, tardigrades have lost a lot of what we’d call their body.
Alongside our friends the tardigrades, the Panarthropod group contains the many-legged
velvet worms, as well as the Arthropods, which include animals like insects, spiders, and crabs.
So it’s a diverse group that’s found in a wide
range of environments expressing a wide range of specialized segments.
But although this supergroup varies a lot,
one thing they have in common is a segmented body lined with many paired appendages.
And at first glance, tardigrades appear to fit this mold, too. After all,
they have an obvious head and a long body with four pairs of legs.
But these moss piglets are not as they appear.
Compared to their relatives, tardigrades are not only very little, as in one of
the smallest multicellular animals on Earth, but their bodies are just… short.
At least, molecularly speaking.
Tardigrades have what's called a miniaturized body plan,
and that miniaturization came about thanks to a loss of major gene groups.
The genes for their last set of legs are associated with a specific posterior segment.
But the trunk region, where you’d expect the rest
of the legs to be coming from, is completely missing.
Basically the section of genes that corresponds to a thorax and abdomen are just… gone.
That means the first three sets of legs are attached to
segments that would evolutionarily be considered an extension of the head.
But that doesn’t mean tardigrades are less evolved simply because they’re missing their middle.
There are over 1300 described tardigrade species,
and we find them just about everywhere, from the deep sea to the soil in your backyard.
So it’s clear that tardigrades are doing just fine after cutting out the middleman.
I never would have guessed that such a cute,
teeny little animal had lost so much – genetically speaking, anyway.
But speaking of weird losses,
let’s go over to Sarah now to talk about one of the animal kingdom’s biggest losers.
Tardigrades missing a body might seem about as extreme as it gets,
but sea stars have a leg up on the competition.
Or, what looks like quite a few of them.
But they aren’t just body-less, but butt-less too. They are basically just a head.
This may come as a surprise given that sea stars,
AKA starfish, really look like anything but a head.
Sea stars are part of a superphylum called the Deuterostomes,
so named because animals in this group form an anus before forming a mouth as an embryo.
That includes you, too, along with the rest of the chordates, which is anything with a true spine.
This supergroup also includes the kinda-spined hemichordates and the
echinoderms, which is the group in which you’ll find the sea stars.
Everyone in this big happy deuterostome family
is bilaterally symmetrical, which Kallie told us about.
That means you can fold them down the middle hot-dog style,
and the sides are basically mirror images, minus a few shifted organs here and there.
But sea stars have bent that rule.
Even though their ancestors were bilaterally symmetrical,
sea stars have what’s called radial symmetry, meaning that their axis
of symmetry starts in the middle at a single point, instead of along a line.
And despite growing up to be radial adults, sea star babies begin development as bilateral larvae!
Sure, you could fold them down the middle too, but the difference is that for anything with
bilateral symmetry, down the middle is the only way to fold it and get mirrored images.
So recently, researchers looked at sea star genetics to unlock the
evolutionary history of how sea stars landed at the form they have today.
Their research subject was the bat star, which is found along the Pacific coast of North America.
They get their names from the webbing between their arms, which resemble bat wings.
This study, published in Nature in 2023, set out to determine how
they got from a bilateral ancestor to being five-limbed with radial symmetry.
We know they share a number of patterning genes across their bilateral body plans,
thanks to previous studies comparing the genetic development of hemichordates and chordates.
And there are a lot of genes shared between the embryos of these groups.
In chordates, we typically clump them together into the forebrain, the midbrain, and the trunk.
But clearly, sea stars are doing something really different, developmentally speaking.
And for a while, researchers have had
a whole bunch of hypotheses as to exactly how sea stars do it.
Maybe they ended up with five copies of those patterning genes,
allowing them to essentially make five mini-bodies all connected at the head.
Or maybe one arm actually started as the head, but then the head anatomy stuff migrated to the middle
of the whole body during development. Yet another hypothesis was that their
various body parts are simply stacked and flattened, meaning that they never really
lost their bilateral symmetry - we were just looking at them from the wrong side.
So to get to the bottom of it, this new study looked at gene patterning in sea stars to see
which body segment genes were where.
And they found that the forebrain and midbrain genes were present in each arm,
meaning that the hypothesis about one arm originally being a head was out.
But perhaps the most interesting aspect is not what genes they found, but the ones they didn’t.
The genes for the whole trunk section were missing.
While the sea stars maintained the rest of the patterning genes
of their fellow deuterostomes, the trunk genes didn’t make the cut.
But of course, they moved the all-important anus to the top of their dorsal surface,
since it can be hard to go without one of those.
Somewhere along the way to evolving this flattened body plan, sea stars lost every
trace of what had existed from the neck down, making them essentially ambulatory heads.
But really, who needs the rest when you can have a multi-armed head?
Well, this has definitely been illuminating. And thanks to our friends over at Eons,
Journey To The Microcosmos, and Bizarre Beasts for correcting my silly ideas.
I now have a nagging sense that I’m about to be visited by the Ghost of Christmas Past,
but I guess it was worth it for all this evolutionary knowledge.
But the main message here is that more complex
anatomy doesn't necessarily make an organism any better.
Simply put, evolution isn't a one-way street and loss isn't always a bad thing.
So to each their own, I guess, butt or no butt! Because sometimes a hard
earned body part just isn’t worth keeping around.
Even if it means losing your head over it.
And if you liked this video, be sure to check out all of our sister channels for
more about the old, the small, and the weird in the world of biology.
Links are all down below, and thank you for watching!