The Body’s Most Embarrassing Organ Is an Evolutionary Marvel

I hadn't really thought about it before, but it actually is a bit of a mystery how assholes came about.

The Body’s Most Embarrassing Organ Is an Evolutionary Marvel

Link won't let me in.

I posted a long time ago about Mary Roach's book "Gulp," which talks about the alimentary canal. One of the most interesting chapters -- and maybe this Atlantic article draws on her text? -- was the chapter on the anus. The sensations around the anus have to know if the pressure you feel is gas, liquid or solid. When we make mistakes, it can be very embarrassing.

Weird science, but I've retained that nugget of truth ever since as "something important you never give much thought to."

The other big discussion point about the anus is the placement. Robin Williams I believe did a bit about placing the playground right next to the garbage dump.

God is obviously not an engineer.

Some quotes to give the flavor.

The Body’s Most Embarrassing Organ Is an Evolutionary Marvel

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The appearance of the anus was momentous in animal evolution, turning a one-hole digestive sac into an open-ended tunnel. Creatures with an anus could physically segregate the acts of eating and defecating, reducing the risk of sullying a snack with scat; they no longer had to finish processing one meal before ingesting another, allowing their tubelike body to harvest more energy and balloon in size. Nowadays, anuses take many forms. Several animals, such as the sea cucumber, have morphed their out-hole into a Swiss Army knife of versatility; others thought that gastrointestinal back doors were so nice, they sprouted them at least twice. “There’s been a lot of evolutionary freedom to play around with that part of the body plan,” Armita Manafzadeh, a vertebrate morphology expert at Brown University, told me.

But anuses are also shrouded in scientific intrigue, and a fair bit of squabbling. Researchers still hotly debate how and when exactly the anus first arose, and the number of times the orifice was acquired or lost across different species. To tap into our origins, we’ll need to take a squarer look at our ends.

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One of the oldest hypotheses holds that the anus and the mouth originated from the same solo opening, which elongated, then caved in at the center and split itself in two. The newly formed anus then moseyed to the animal’s posterior. Claus Nielsen, a developmental biologist at the Natural History Museum of Denmark, is a fan of this theory. It’s both reasonably parsimonious and evolutionarily equitable: In this scenario, neither the mouth nor the anus technically arose first; they emerged as perfect developmental twins.

Hejnol and others favor a different idea, in which the mouth formally preceded the anus, which spontaneously burst through the other end of the body. “It’s a secondary breakthrough,” Hejnol said. “The gut forms, then [makes] a connection to the outer world.” Punching an extra hole in the body is not so difficult: Some worms have managed the feat dozens of times over.

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Many of the animals that have managed to keep some version of the anus embellished upon it, and now harbor an organ of immense extravagance. Turtles, like sea cucumbers, breathe through their butt. Young dragonflies suck water into theirs, then spew it out to propel themselves forward. Scorpions jettison their posterior when attacked from behind, evading capture but tragically losing their ability to poop (and eventually dying with their abdomen full of excrement). Lacewing larvae incapacitate termite prey with the toxic flatulence they emit from their end—“they literally KO their enemies with death farts,” Ainsley Seago, an entomologist at the Carnegie Museum of Natural History, told me.

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The only redeeming quality of humans’ humdrum posterior hole is the feature we evolved to cushion it: our infamous buttocks, the most voluminous one documented to date, thanks to our bizarre tendency to strut around on our two primate legs. “Our bipedalism is obligate; it’s special; it’s the only way we get around,” Darcy Shapiro, an anthropologist, told me. That pattern of locomotion reshaped the pelvis, which in turn reoriented our muscles. The gluteus maximus—the hefty muscle that powers our ability to run and climb—swelled in lockstep, and blanketed itself in a cozy layer of fat that some scientists think serves as an energy reserve. Anuses aside, “our buttocks are the real innovation here,” Manafzadeh said.

Evolution blew the human butt out of proportion; our cultural norms quickly followed suit. We regard one another’s bums with lust, disgust, and guilty fascination. We shrink them, we sculpt them; we sexualize them. We rap about them with abandon. They, in return, make it much easier to sprint, but much harder to keep our rear ends clean. Our anus is a sheep dressed in a very fabulous wolf’s clothing, and we simply cannot deal.

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Researchers still hotly debate how and when exactly the anus first arose,

That's not true. It was 2016.

"Ontogeny recapitulates phylogeny" and the tube-like gut structure develops during the 5th week of gestation, very early.  The only critters without the tube are ones who are not bilaterally symmetrical, like starfish and sponges and jellyfish, who are genetically very far removed from vertebrates.

Most (all?) vertebrates excrete liquid and solids through different orifices, but birds do not.  Does that mean dinosaurs didn't?  There is substantial dinosaur scat (coprolites) in the fossil record, but they're so dried out that I don't suppose anyone knows.  Interesting idea though.

Ah, of course: an article.
https://www.smithsonianmag.com/smart-news/we-know-dinosaurs-pooped-now-we-know-they-may-have-peed-too-180949763/

Quote :

Creatures with an anus could physically segregate the acts of eating and defecating, reducing the risk of sullying a snack with scat

I don't remember now where I read it, but many years ago there was a poem or a short story about how we're all built on the floorplan of a tube. We put pieces of the world in one end, and push them out the other. We are all worms, processing our environment by consuming it and making it into our bodies.

Soil is literally mostly worm excrement; without worms the ground would be nothing but rocks and dead leaves. Plants rely on worm excrement to root themselves and intake nutrients through their roots. Everything recycles the environment, running it through their gut. It's like one giant relay race, where we derive our bodies from what we eat, and then other things eat us.

Life on Mars could not exist without a whole ecosystem to process the raw materials.

Not unless Martians can survive on weak sunlight and iron-infused rock.

The point being made was that at the very beginning, little multicellular things tended to develop a sack-like structure, with just one opening where food came in and waste was subsequently expelled. Developing a tube-like arrangement is obviously better, but it isn't exactly clear how that came about. Things like anuses just don't fossilize very well.

The idea of one opening splitting into two, and one of them migrating around the body to eventually form a tube, seems kind of bonkers to me, but apparently was formulated a long time ago and still has some adherents. On the other hand, just randomly punching out a new hole to make an anus doesn't seem very plausible either. And as you mentioned, some animals have a cloaca, where most everything (shit, eggs, sperm, whatever) all get expelled out of the same hole, which again seems bonkers to me. An interesting mention was that the genes for developing a mouth and an anus were quite different, which suggests that the anus did have a genesis independent of the mouth, perhaps connected to reproduction in some way. Which begs the question of how did a reproductive-type hole get linked with digestion, and why don't we all have a cloaca instead of an anus?

Some animals grow multiple anuses. Maybe at some point this happened, and then there was some simplification/specialization where one of the anuses became dedicated to shit and the others went to reproduction?

As the article started out by saying, it's something of a mystery.

NoCoPilot wrote:

Life on Mars could not exist without a whole ecosystem to process the raw materials.

Not unless Martians can survive on weak sunlight and iron-infused rock.

I think you are mistaking what we are looking for.

Earth was formed about 4.5 billion years ago, more or less. Mars is distinctly older than Earth. It is possible that 4 or 5 billion years ago, before Mars was stripped of its atmosphere, life started on Mars, even if only microbial. It is even possible that such microbes were transferred from Mars to Earth by meteorites, billions of years ago, and we are all the descendants of Martian immigrants. We are looking for evidence of such life. But that life would have to have survived on the Mars of 4 or 5 billion years ago, not today.

I'm not optimistic about the possibility of finding such evidence. But I don't think it's crazy to look. I don't think we'll find living microbes: the only realistic possibility would be if they are well below the surface, pitiful survivors of the original ecosystem. But maybe there will be fossils from 3 or 4 billion years ago (again, as I said, not at all likely, but not impossible).

richard09 wrote:

Some animals grow multiple anuses.

We voted him off the island.

richard09 wrote:

here one of the anuses became dedicated to shit and the others went to reproduction?

Male and female genitalia develop from the same tissue, depending on the hormones present.  That's why men have nipples, that's why a small percentage of people have ambiguous genitalia.  You can't really equate the anus -- which goes all the way through you -- to the vagina, which only goes, what, a couple of feet? (I've never been to the end of that road...)  

It *is* interesting that urine and sperm share a duct, but the urethra in women is not inside the vagina.  The vagina is basically the same tissue as the penis turned inside out (as they do when doing sexual reassignment surgery).  That's a bit of biology that wouldn't be guessable from ontology.