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The Beginning and End of It All

As objects of anatomical study, butts and anuses usually sit at the bottom of the totem pole. Opposable thumbs, bipedalism, the abdominal cavity, thoracic rotation, our big brains: these are surely more worthy of study, right?


But as Katherine Wu points out in a recent article in The Atlantic, for certain scientists, anuses are a big question mark, shrouded in mystery. For while anuses may be the end of a journey of sorts, they may have been the start of our evolutionary story as a species.


Let’s start with the mystery: there is no “first anus.” Like the rest of the soft tissues in our bodies, anuses do not show up in the fossil record. To boot, anuses do not form around a bone, so scientists are unable to study any imprints anuses would, in theory, leave behind. (By comparison, scientists infer brain size/composition of human ancestors by taking “endocasts,” images of imprints left behind on the cranium.)


Theories abound as to the origin of the anus. One states that, in a primordial dance of symmetry, mouth and anus came into existence simultaneously, divided by a “pinched middle.”


A sponge, with multiple posterior ends in action. Photo from the Zoological Journal of the Linnean Society.


But the one-hole theory itself has holes, as Wu mentions. If it were true, scientists could expect to find similar genetic expression in mouth and anus. In fact, most animals have different genetic expression in these areas of the body –suggesting that the two features evolved at different times.


Another theory, in our early days of sac-organisms, is that, over time, the mouth “caved through” before bursting out at the other end of the organism. If you think this sounds fishy, there are certain creatures (worms, we’re looking at you) that have branched digestive systems that lead to multiple anuses. Generating an anus, it turns out, isn’t rocket science.


Of course, primitive life forms, those basic, multicellular “blobby” organisms likely floating around in water, lacked an anus. Or rather, mouth and anus were one: a single, multipurpose hole from which organisms ingested and later ejected food. A single hole, as Wu mentions, is not quite advantageous when it comes to evolution: “You can’t eat lunch while digesting breakfast.”


The development of a two-hole system (mouth and anus) empowered ancestors to get more out of our food. Suddenly, we had the capability to ingest more calories, which made us longer, affording us the space in our bodies to develop complex digestive systems… which in turn helped increase our intake of more calories, leading to the development of bigger brains: on and on goes the positive feedback loop.


Yet human anuses, it turns out, are “pretty boring,” Wu states. One-dimensional. Nothing like the swiss-army-knife anus of a sea cucumber. A sea cucumber’s anus can a) become a second mouth, converting into a respiratory tube through which the animal exchanges gas with surrounding water; b) turn into a home for a pearlfish and c) a survival mechanism. When a sea cucumber is disturbed, it shoots internal organs via the anus to quell the threat.


Our butts, on the other hand, are quite interesting. Those two round, fleshy parts that form the lower area of the human trunk—how did they get there? And why are they so, well, big?

Reporter Heather Radke, author of a forthcoming book on butts, says that the answer probably has to do with running.


In our early days, when we spent considerably less than 90% of our time indoors, our ancestors stalked the savannah in search of food. Trailing an animal over long distances was a task that not only required commitment, but also “long” bones coupled with muscles that contracted slowly.


A deposit of fat, a source of energy for a long journey, wouldn’t hurt either. Humans have among the highest percentage of body fat in the animal kingdom, a trait Radke attributes to the demands of a big brain. Most of that body fat is stored in the lower rear.


As for the location: why don’t those two round, fleshy parts hang, say, from our elbows? It may have to do with our center of gravity around the waist, says Radke. The location on the lower rear is such that it does not mess up our locomotion. After all, playing basketball or practicing the violin with fatty elbows may be a bit difficult.


The examples of butts and anuses offers a reminder that anatomy and human evolution take on a variety of forms that are never set in stone. With regards to theories on origin of anuses, Wu says, “both are probably true” and that it is quite possible that different kinds of anuses evolved independently over time. And as Radke details in her work, there are explanations not only evolutionary, but also historical, societal, and cultural that shaped our unique behinds.


To listen to Wu and Radke discuss their work and more, check out “Getting to the Bottom of Butts” from Unexplainable, wherever you get your podcasts.

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