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As weird and wacky as our fellow animals get, there’s one thing
that we pretty much all have in common: we have to eat.
And if you have to eat, you also have to release waste.
So, many animals share a very useful, very familiar feature: a butt.
Technically, an anus, but the word “butt” is hilarious.
And while the function of that rear exit is pretty straightforward,
the evolution of butts is surprisingly complicated and confusing.
When you get right down to it, many of us animals are basically tubes
with an opening at either end: a mouth for taking in food
and an anus for releasing the unwanted byproducts of digestion.
This two-ended tube arrangement is called an alimentary canal
or through-gut, and it’s common among animals that belong to
the vast group of Bilateria.
This includes most animals that are symmetrical down the middle,
such as worms, spiders, fish, and humans.
But not all animals work this way.
Sponges, for example, have no true mouth, gut, or anus at all.
They absorb food particles from the water directly into the cells of their bodies.
Jellyfish and anemones, on the other hand, have a functioning
mouth and anus, but they’re the same opening – a mouth-butt.
In these animals, the mouth leads directly into a sac-like gut,
and then waste is spit back out when digestion is done.
But there are some serious benefits to having a through-gut like we do.
For one thing, the alimentary canal can develop into
specialized regions to serve different purposes.
Your alimentary canal, for example, includes several organs with specific functions.
Your stomach’s job is to start breaking your food down into a liquidy mess,
and your large intestine reabsorbs fluids and prepares waste for expulsion,
and the whole process is helped along by other organs like the liver and pancreas.
The through-gut can also be expanded and adjusted to fit bodies
of all shapes and sizes, from worms to whales.
Just imagine a blue whale with a mouth-butt: its meal would have to make
a ridiculous round trip to drop off nutrients and come back out.
And, very importantly, animals with a through-gut can carry more than one meal at a time.
An anemone with a sac-gut has to finish digesting and
relieving itself before it can take in new food.
But you can happily eat your dinner even while lunch is still working its way along.
Scientists suspect that the familiar through-gut and anus
first evolved in the earliest Bilaterian animals and were later
inherited by their descendants, like us.
But trying to pin down exactly when and how true anuses evolved is tricky,
since the evolution of butts has been a bit wacky.
For example, you’ll find a separate mouth and anus in
starfish and sea urchins, but their close cousins, the brittle stars,
have lost their anus somewhere along the way.
They’ve independently evolved a sac-like gut like an anemone,
using their mouth for both input and output.
Most flatworms have similarly lost their anuses, but one group in particular,
the tapeworms, have gone even further and lost their entire digestive tract!
No gut, no butt.
These parasites don’t digest their own food.
Instead they sit inside their host’s alimentary canal and absorb all the
pre-digested food around them.
And then some animals have evolved entirely new butts.
The flatworm genus Thysanozoon has uniquely evolved
numerous waste-holes across the top of its body.
It has a back covered in butts!
Examples like these suggest that butts have come and
gone many times in animal evolution.
This leaves scientists wondering exactly how an animal goes
from being butt-less to boasting a functional behind.
To find out, they investigate some of our strangest relatives, such as acoel worms.
These microscopic worms can be found in oceans all over the world.
They’re thought to be extremely primitive members of the Bilaterian group,
but unlike us, they don’t have a true respiratory or circulatory system,
and they also don’t have a proper gut or anus.
What they do have on their rear end is a gonopore,
an opening which males use to release sperm.
Research into developmental genes - that is, the genes that control
the growth and organization of the body - has found that the expression of these
genes in the acoel gonopore is similar to what’s seen in other animals’ anuses.
This makes scientists wonder if, somewhere in our early worm-like ancestors,
the gut became linked up with the reproductive opening and voila!
A butt was born.
More specifically, a cloaca, an opening that serves dual purposes
releasing waste and reproductive material.
Cloacas are standard among many modern animals,
including fish, amphibians, birds, and reptiles.
But that’s not the only possible path to a butt.
Another intriguing option is seen in different groups of marine life: comb jellies.
These are another very ancient branch of the animal family tree.
And despite their similar name and lifestyle, comb jellies aren’t very closely related
to jellyfish.
These gelatinous creatures paddle their way around
the oceans with oar-like structures known as combs.
For a long time, scientists thought comb jellies had
a mouth-butt like jellyfish do, but in 2016, researchers caught the jellies
on video pooping out of tiny pores far from their mouths.
This was big news at the time, because it demonstrated
how comb jellies use the full complement of mouth, gut, and butt - a through-gut.
But a 2019 study revealed that the situation is a bit more complicated.
Researchers looked at a species called the warty comb jelly,
placing individuals under the microscope to watch them eat, digest, and poop.
And they found that as much as 90% of the time, warty comb jellies don’t have an anus.
But as they eat and their guts fill with waste, a small hole eventually
opens through the body wall, spits out the poop, and then closes back up.
The scientists described this as a transient anus –
a butt that only appears when it’s needed.
Not only is this amazing and bizarre, it’s another potential clue to the origins of butts.
Perhaps our distant ancestors, before evolving a true anus
like we have now, went though a transition period with a
sometimes-butt like the warty comb jellies.
Or, this might be something unique to that species.
The researchers have investigated other comb jelly species
and so far they’ve only seen permanent anuses.
In the end, there may not be a simple answer to the question
of where our butts came from.
Scientists are hopeful that more detailed genetic and anatomical studies
will help them sort out the relationships between the diversity
of guts and the anuses we see in living species.
What’s certain is that somewhere in your distant ancestry,
some simple creature evolved a simple through-gut,
and it’s thanks to them that you don’t poop out of your mouth.
You may have just learned everything you ever wanted to know about butts,
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