Have you ever stopped to appreciate your anus? Laugh all you want, but this bodily orifice is a highly evolved engineering marvel, and it's hard to imagine life without it.
Your anus has a very sensitive job. It's tasked with determining whether solid, liquid or gas pushes up against the sphincter – and then it has to selectively let some or all of it through to the other side. Fortunately, this end of the digestive system is packed with nerve endings to aid in this delicate game of "who goes there?"
Perhaps most importantly, the anus contributes to your ability to voluntarily control your own defecation. To many animals, this means getting to hide your fragrant feces from the detection of roving predators. In fact, in a 2013 paper, Italian bowel experts Gabrio Bassotti and Vincenzo Villanacci went so far as to suggest that our ability to poop our own adventure "may have contributed to the successful march of hominids along the road of evolution."
You should be thankful. But as Joe McCormick and I explore in the Stuff to Blow Your Mind episode Evolution of the Anus, some animals do manage to get by without a proper alimentary portal. Let's meet a few of them.
Making Do Without
Birds, reptiles, amphibians, most fish and the monotremes (think platypus) all boast a cloaca, which combines all excretory and reproductive functions into an all-purpose opening. Sure, "cloaca" is Latin for "sewer," but it may be the next best thing to a dedicated anus.
The jaw worm Haplognathia boasts a temporary or transient anal pore, while sea stars lost their anuses long ago. Meanwhile, most flatworms are anusless, except for species that independently evolved one — or more! Yes, the polyclad flatworm (Thysanozoon nigropapillosum) actually packs multiple anuses, all over its back.
Other creatures may lose or gain their precious anus during their lifetime. Green lacewings live something of a privileged adolescence as they don't develop one until they become adults. Lacewing larva have what's known as a "blind gut." They eat nonstop, but all the waste just piles inside them. When they finally emerge from their cocoons, they take the poo of a lifetime.
Jettisoning the Anus
And be thankful you're not a scorpion from the Ananteris genus. Like a terrified garden lizard, these arachnids can shed their tail in defense against predators. The problem, however, is that the Ananteris scorpion's anus is located on its fifth tail segment. Once they jettison the back door, excrement simply accumulates in their body for the rest of their lives. The internal pressure of accumulating poop can even cause additional tail links to spontaneously pop off, like buttons on an overstuffed vest.
However, the scorpion's plight might not be quite as bad as it sounds, as a 2015 study published in PLOS One explores. The poop-swollen scorps produce little excrement – especially without a tail to aid in the procurement of larger prey – and they can continue to breed.
But it can always be worse. Because there was a time before the anus, and you probably wouldn't be all that happy with the archaic plumbing.
Digestive Tract Divergence
Anal evolution doesn't receive near as much attention as, say, the brain or the heart. But one brave Norwegian molecular biologist named Dr. Andreas Hejnol dares to explore the origins of our ends. In such papers as "The mouth, the anus, and the blastopore—open questions about questionable openings" and "Getting to the bottom of anal evolution," Hejnol and his co-authors consider digestive tract divergence in the animal kingdom.
Because that's what it all comes down to: a tract of gut with a beginning and an end. Some animals, such as sponges and tapeworms, are lacking even this. Other creatures like jellyfish and corals boast a gut with a single opening for both consumption and defecation. Setting aside the fact that this is essentially pooping out the mouth, it's also a very limited system. There's no simultaneous eating and digesting. No snacks until you finish reducing lunch to excrement! U-shaped guts provide the benefit of two mouths for this process, but it still requires a rather simple gut to do everything.
The benefits of a one-way gut like ours include improved digestion and ultimately greater overall size. Because with digestive tract length comes digestive tract specialization. Think of the way that each phase of the human gut tract accomplishes a specific task, from breaking down biomass to absorbing nutrients and removing liquids.
I like to think of it as the difference between a clockmaker's shop and a clock assembly line. In the former, as with one-hole and U-shaped guts, there's a center of activity that does everything. How many clocks do you think that shop can produce versus the assembly line, where specialists assemble the clocks as they roll down the conveyor belt? That assembly line is your fabulous one-way digestive tract, and your anus is the highly efficient shipping department at the end.
So raise a glass to the anus! It does its job quite well, so well that you don't have to think about it most of the time. Even when it's giving you grief, hey, it beats pooping out of your mouth.