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!