Human Origins Icon Human Origins

Doctor’s Diary: No “Butts” About It

Geoffrey Simmons
The Three Graces, by Raphael
The Three Graces, by Raphael / Public domain.

Editor’s note: Dr. Simmons is a physician and the author most recently of Are We Here to Re-Create Ourselves? He is a Fellow with Discovery Institute’s Center for Science & Culture.

How did human butts evolve to look that way?” So asks Darcy Shapiro, a PhD in evolutionary anthropology from Rutgers University, writing at the site Massive Science.

Shapiro’s article is intriguing, entertaining, yet falls short. She writes that the evolution of bipedal-walking primates was primarily caused by the shifting of select bones and muscles in the pelvis. Why these bones shifted and changed shape is not clearly stated. But that is just the question. And it’s made more interesting by the fact that fossils do not document the gradual evolution of the human pelvis. Instead its appearance seems to have been “very rapid.” Whatever the case, the process resulted in the larger, more desirable human buttocks (I mean, desirable in a strictly scientific sense, of course).

Humans and Apes

Consider the striking and symmetric increase in adipose tissue in each cheek, which is unique to us. Apes barely have this. Their butts are mostly flat. Other differences include the following: many primate butts (not ours) can change at mating time to a violet red color and swell to several times their normal size. I’ve not seen this phenomenon in humans, not during my four years in medical school, my rotations during specialty training which included OB, nor during my more than 44 years of practice. Some primates can also get markedly enlarged perianal glands, bulging from the rectum and giving the buttocks, I’m sorry to say, a rather unappealing appearance. Fortunately, we didn’t inherit that trait, either. 

Shapiro comments that our large brains and the use of language with symbols are distinguishing human features, but not nearly as distinguishing as our butt. Honestly? Seriously? Perhaps that is tongue in cheek, but one can’t be sure. I agree all butts are important, and, as I age, maybe sitting down has become more important to me, but I’m inclined to think that our intelligence remains the most important quality.

The author offers a nice, smooth story, but one that is fitted to selected facts. It’s so smooth that you can easily envision a documentary film showing primates achieving increasingly erect bipedal walking as butts change into their present shape. Perhaps evolution occurred to permit reaching higher and higher hanging fruit, doing pull-ups on chinning bars, or eventually playing basketball. We do know that basketball players were under six feet tall a hundred years ago and now many are well over seven feet. Proof of evolution? 

Another change involves the shape and size of the gluteus maximus, the major muscle in our backside. There are two of them. They are the ones we sit on. Was foresight at work? One aimed at our eventually fitting into lounge chairs and car seats? Maybe we lost our tails for the same reason. We also lost prehensile toes, which makes hanging in trees more treacherous. These days I wouldn’t even consider it.

Perilous Research

Dr. Shapiro states that the great apes do not have butts that are proportionally as big as humans. Indeed, that seems true, but I’m not aware of research where someone has actually gone out and measured butts on a variety of primates. Or sought a comparative, random cross-section of human butts measured when the subjects least expect it. Maybe researcher safety was an issue. We have a lot of fossilized pelvic bones from the past, but no soft tissues to compare. Perhaps the famed Lucy had a wonderfully shaped buttocks. We don’t know. One might ask why primate arms shortened while the legs lengthened. Perhaps, knuckle walking resulted in too many splinters or the powers of natural selection knew there would be an upcoming need to do desk work. 

According to research from the University of Leiden, chimps recognize each other by their butts. We are inclined to use faces. Good thing, I think. But why did the pelvis really need to change shape at all? Monkeys and apes can walk upright, albeit in a bowlegged fashion. Perhaps it had to do with birthing. Pelvic outlets, where the baby’s head comes out first, are strikingly different between the great apes and humans. Might it be that human babies are often double the size of gorilla babies (5-10 pounds compared to 3-4 pounds) and evolution knew this would eventually be a problem? Of note, our babies have to look sideways to get through and then look downward.

A Classic Error

In seriousness, Shapiro commits a classic error: she identifies a set of features needed for some function, but conflates that with evidence for evolution, constructing a just-so story to account for its origin when that could be equally well explained by intelligent design. 

After all, designers use foresight and goal-oriented thinking to piece together multiple features and components that work in coordination to perform a function. As Shapiro explains, this coordination is exactly what we see in the carefully crafted shape of the bones, muscles, and fat tissue of our butts to allow us to efficiently walk upright, and also to do something that evolution could never have anticipated: sit for long periods of time and do intelligent work at our desks. 

This coordination is not evidence of blind and unguided evolution but rather of purposeful action by an intelligent agent. Design is implied, no “butts” about it.