In 2016, a room of scientists gasped  while watching a video of a comb jelly.

Known to science as Ctenophora, comb jellies are  translucent, gelatinous creatures of the sea.

And they’re so ancient that they’re probably  the oldest animal lineage out there.

They   were likely drifting around the world’s  oceans by at least 600 million years ago.

And researchers have studied the ins and outs  of comb jellies for centuries — literally.

Like other animals with ancient  roots, from sponges to anemones,   they were long thought to use  just one hole to eat and poop.

Their gut was thought to be like  a cul-de-sac, where food goes in,   gets digested, and then is shot back out.

But the video showed fish that had  been gene-edited to glow in the   dark moving through the jelly’s digestive system.

And once the jelly was done breaking  down its luminous food, two holes in   its bottom suddenly appeared.

And it pooped… Out of a pair of…mysterious holes.

Now, while watching a see-through jelly poop might  not strike you as that exciting, it upended much   of what scientists thought they knew about how and  when animals evolved anuses in the first place.

And it turns out that, while  the evolution of the butthole   was a major breakthrough in animal development,   its story might actually end with redefining  what it means to have a butthole at all.

Now, there are a couple of different ways  that animals can eat food and then excrete it.

And a butthole — an opening at  the end of the digestive tract   where waste is pushed out of the  body — is only one of those ways.

Some creatures completely lack digestive  tracts, like adult giant tube worms at the   bottom of the ocean, who get their nutrients  from bacteria living inside their bodies.

But most ancient and simple organisms — those  that originated over 500 million years ago,   like jellyfish, sea anemones, and  some flatworms — have sack-like   digestive systems, with literal potty mouths.

Stuff comes out the same way  it came in, from the same hole.

Now, this simple plan can have some disadvantages.

Like, every meal needs to be completely  digested before new food can come in.

So it’s pretty limiting if you need a lot  of energy to grow, develop, and do stuff.

And since these sacs have dead-ends,   they can’t really have different segments and  compartments doing slightly different jobs… Which means they miss out on the benefits  of having different microbes and enzymes   breaking down different components  of the meal at the same time.

Later in the evolutionary history of animals,   it’s thought, some creatures solved  this problem by evolving through-guts,   which are more tube-like digestive systems where  nutrients come in one way and go out another way.

Ta-da!

The birth of the butthole.

This, it’s long been thought, was a necessary  step toward animals becoming more complex,   because they could eat more without  waiting to have digested everything else.

Now, exactly when and how this happened is a  mystery scientists are still trying to solve.

The first attempt to explain the  through-gut came in the late 1800s,   when a German zoologist applied his  theory of recapitulation to the problem.

Recapitulation was the idea that you could tell  how animals had evolved by watching how they   developed through their embryonic stages...but  no one could replicate his drawings of embryos.

Later, in another attempt to explain how anuses  evolved, scientists started using the velvet worm   as a template.

This led them to claim that anuses  originated from the same opening as the mouth.

See, in velvet worms, the embryo is a  spherical blob that develops a first hole   in the middle called the blastopore.

Although  not everybody agrees it’s a true blastopore.

Over time, the circular-like organism becomes more   elongated and the blastopore  becomes more furrow-like.

The furrow slowly starts to divide its labor,   sucking water and microscopic nutrients in from  one side, and then spitting them out the other.

Eventually, the groove pinches in the  middle to create two separate holes.

And as the holes drift farther and farther  apart, the sac-like gut folds inside where the   pinch is and slowly turns into a tube.

This tube, then, is the through-gut.

Maybe this journey that velvet worms go on  as they develop shows us how the evolutionary   process of the anus unfolded – and this became  the longest-standing hypothesis about anuses.

According to this school of thought,  the first anuses appeared in this manner   sometime between 600 and 540 million  years ago — before animals became   more sophisticated “bilaterians” with a  right side, left side, bottom, and top.

And in 1908, an Austrian zoologist  studying the embryonic development   of other animals noticed that they didn’t go  through the same journey as the velvet worm.

Instead of becoming a groove  that formed two holes,   the blastopore hole stays a single  hole through embryonic development.

And in some animals, that hole  develops into a mouth.

In others,   that same hole develops into an anus.

Then, the hole grows into a tube that  becomes elongated through the body,   until it meets the other side, forming  an anus, or a mouth, respectively.

According to this theory, the butthole is   basically the bottom of the gut that  slowly poked its way out of the body.

And scientists actually started using this process  to categorize animals into two different groups.

When the blastopore first forms the  mouth, the animal is a ‘protostome’,   meaning ‘first-mouth.’ Earthworms  are protostomes, for example.

And when the blastopore becomes the anus  first, the animal is a ‘deuterostome’,   a ‘second-mouth.’ Starfish are  deuterostomes, and so are we.

This school of thought suggested that the butthole  was basically just turned on during evolution   when organisms evolved in directions where a  butthole would be helpful to their success… And that it could sometimes be lost  again when it’s not helpful anymore.

If buttholes can break out of the body  during the evolution of an animal to   fit its lifestyle – and not through  the complicated tubing of a single   hole – this helps explain anomalies that the  theory of recapitulation couldn’t explain.

Like, for example, the marine  worm Ramisyllis multicaudata   evolved a branched gut with hundreds of  different buttholes along its entire body,   likely because that’s what  worked for its life strategy.

Or the acoel flatworm, which doesn’t have an anus  at all, even though some of its ancestors did.

Building off of this, a slew  of scientists came up with new   ideas for how, exactly, the breakthrough happened.

In the 1940s, for instance, one German  scientist theorized that the digestive   system poked its way out of the body  by connecting to the genital opening,   which was already there as  another hole before the anus.

While another Austrian zoologist  suggested animals first had a   sac-like gut they stored their ovaries  in, too – and the through-gut developed   together with the genital tract when the  ovaries started getting more complex.

Hypotheses that bring the genital  cavity into the picture help us   understand why birds and reptiles today  have cloacas — not exactly buttholes,   but instead one hole for  reproducing and excreting waste.

So this growing body of research has  built on top of itself throughout the   decades — sometimes dismantling  or upsetting older ideas.

And there are still some big unanswered questions  that keep researchers on the fence.

Like,   is it one hole splitting into two, or  one hole that poked out after the other?

But most hypotheses tend to  agree on a similar trend.

In all of these ideas, the  scientists have painted a linear,   progressive picture of increasing complexity.

Animals were simple, with sac-like guts,   and then they evolved through-guts when  they became more complex and sophisticated.

And that’s why that 2016 video  of the comb jelly pooping was   so shocking.

It completely upends this picture.

Because these seemingly simple animals apparently   didn’t have the seemingly simple gut that  scientists had assumed simple animals had.

This initial bias is probably why it was so  hard to find those anuses on comb jellies,   even though they’ve been studied for so long.

Back in 1850, a biologist had even speculated  the jellies probably had a through-gut.

But because he never really observed any  excrement coming from anus-like pores,   his idea quickly fell flat.

So when scientists saw that their comb jellies  were expelling waste from the same hole the   food had entered their body, they assumed it  was the way their digestive system worked.

But it turns out, the jellies were  probably just puking this whole time,   maybe because they were fed too  much during the experiments,   or fed something they didn’t like.

They hadn’t been pooping at all.

If one of the most ancient organisms on  the planet already had a through-gut,   this invalidates the core belief that  the butthole evolved when organisms   started getting bigger and more sophisticated.

Or even that buttholes allowed for animals to  start to grow and develop and diversify so much.

See, back when scientists began  asking questions about the anus,   they were assuming that evolution was about  progress and that complex was just, better.

But maybe through-guts were instead   the most simple solution for the  digestive system to start off as.

And maybe we need to rethink what we  assume is ‘simple’ and what is ‘complex’   in the evolution of life.

Once we rethink  these categories, new possibilities emerge.

Like maybe ancient sea anemones and jellyfish  initially already had through-guts with bumholes,   but closed them up over time because they  weren’t all that useful to their success.

Or maybe buttholes evolved independently,  in unique ways, at different times   throughout history.

And maybe there are some  radically different ways to be a butthole.

After all, if some animals evolve their  mouth and butthole at the same time,   some evolve mouths first and  others evolve buttholes first,   some animals have two buttholes and others  have hundreds — What exactly is a butthole?

We just don’t know yet which hypothesis is correct  – with some scientists maintaining aspects of the   original sole-hole concept probably ring  true, while others favor other ideas.

It’s hard to track this down through  prehistory because there aren’t many   fossils to work from — jellies  don’t exactly fossilize well.

And up to the modern day, we didn’t really have   the technology to fill some of  these holes in our knowledge.

It’s only now that scientists can study  the cells and molecules that make up these   different holes in different animals that we  can start to figure out what buttholes are,   molecularly, and what cells they came from.

For instance, scientists don’t yet know whether   comb jellies activate the same genes  other animals do when growing an anus.

Maybe they’re triggering something completely   different with no affinity to  what we now call a butthole.

Maybe it’s something completely  different and new!

Butthole 2.0!

But what we do know, is that evolution is  about finding solutions that work for all   different animals with all their  different lifestyles — not about   following a linear path from sac to  tube, and from simple to complex.

And with modern advancements in  technology, and maybe a few more