The human body is a hodgepodge of imperfections and leftovers, the result of evolution doing the best it can with limited materials.
The human body still has a few skeletons in the evolutionary closet. Lucy Bridges/Flickr (CC BY-NC-SA 2.0)
History is written all over us.
That’s what Richard Dawkins said in his book The Greatest Show on Earth. Every characteristic of our bodies is the result of millions of years of evolution. Every trait that has, or once had, a purpose provides a rich map of the history of our evolution.
Unfortunately, evolution sometimes takes an unexpected turn, and the evidence for this is written all over us, too. We’re just drivers on this evolution highway, and our vehicle is natural selection. Here, the road is one-way and there are no U-turns, hook turns or roundabouts. The traffic is crawling, and there are only two options: keep on trucking, or get off the road.
The human body is riddled with imperfections and vestigial traits. We are, after all, one long string of compromises. Traits that may once have given a distant ancestor the edge it needed to survive have now become impractical and, at times, detrimental.
Natural selection is brutal. To develop a superior trait within a species, all inferior models must be destroyed, erasing undesirable characteristics from the gene pool. Traits that give an animal any advantage over its rivals are fiercely supported. An antelope that’s a fraction faster than its fellows is more likely to survive and carry its genes into the next generation. The flipside of natural selection is that, while eliminating undesirable traits keeps evolution moving, it also means there are no take-backs and definitely no clean slates. Evolution has to work with what it's got, even when that has become impractical.
The evolutionary changes that led to modern humans were not necessarily logical nor straightforward. T.H. Huxley/Wikimedia Commons (public domain)
Richard Dawkins uses this analogy: Imagine you’re an engineer who has to build a plane from scratch… whilst in mid-air. Each part that you add must improve the plane’s performance in some way. You can’t remove a part or start over, but you can alter and repurpose pre-existing characteristics to make them more suitable for the environment in which they must operate. After a few ‘generations,’ you’re going to end up with quite a few unnecessary parts, and quite a few recycled ones, too. Now the human body is your plane, and natural selection is your engineer. With that in mind, you can understand the evolutionary flotsam gathered around the human body.
When we are cold or frightened, we get goose bumps. Why? Think of a bird fluffing its feathers in a cool breeze, or a dog puffing up its fur when challenged. Our ancestors were once covered in fur, and they, too, may have puffed up their fur to look bigger or protect against the cold. Today, what we call goose bumps are just a marker of our past, and serve very little purpose to our primarily hairless bodies.
Another prime example of evolution doing its best with the materials provided is the detour of the recurrent laryngeal nerve. In humans, this nerve supplies all the muscles to the larynx, opening and closing the vocal cords and allowing us to talk. In our fish ancestors, the nerve led directly from the brain to the gills. But as the necks of mammals lengthened, the nerve was forced to compensate, taking a much more meandering path through the body.
It would have made sense to start over, with the nerve running a direct route to the larynx, but the ‘no take-backs’ rule of evolution meant the nerve was instead forced to stretch. It now takes in the sights, detouring down the neck and looping under the aorta, before heading back up to the larynx. The human race should think itself lucky, however, as the ancestor that provided this nerve to humans is a common ancestor of all mammals. While in humans the detour is just a couple of inches, for a giraffe it’s as much as five metres.
The path of the recurrent laryngeal nerve, as shown in the neck of a giraffe, takes an extensive and unnecessary detour. Vladimir V. Medeyko/Wikimedia Commons (CC BY-SA 2.0)
Similarly, the pathway of male reproductive organs in humans takes quite the detour. Early in our history, the testes were located inside the body, but over the course of evolution, many mammalian testes moved down and out of the body. What possible benefit could have warranted such a migration? There are many theories, but the classic explanation involves temperature. Since the optimal temperature for sperm production is a few degrees below body temperature, it is only logical that the testes be located somewhere outside the body where it is cooler, much to the chagrin of vulnerable males everywhere.
As the testes moved outside the body, the vas deferens – the duct that carries sperm from the testes to the penis – should have headed south, too, but an unfortunate stroke of bad luck saw it getting hooked up and over the ureter. Rather than undoing this obvious mistake, evolution was forced to just roll with it: the vas deferens continued to lengthen until finally the testes found their way outside of the body.
Beyond serving as an anchor point for several muscles and ligaments, the coccyx – or tailbone – has lost much of its functionality, but it remains a testament to a time when our ancestors had tails. Interestingly, during the formation of the human fetus, embryos do still develop a tail for a period of time before it’s reabsorbed into the growing fetus. Abnormalities in this absorption mean that, on rare occasions, a baby is born with an intact tail.
While many vestigial traits are benign, there are some that are detrimental to us. Take the human spine, for example. Our ancestors walked on four legs, but, when walking upright proved beneficial, natural selection pushed us towards bipedalism. Unfortunately, turning the spine of a quadruped, designed to spread body weight horizontally across four legs, into a biped is not a simple task. Evolution simply took a quadruped’s spine and cranked it up into a standing position. As a result, our body weight is now precariously balanced vertically down the length of a spine that was never designed to support it, and humanity has suffered spinal issues ever since.
Similarly, because our two legs now take on weight that’s designed to be distributed across four legs, our knees are under immense pressure. It’s no surprise that knee and back pain are some of the most common complaints faced by ageing humans; it’s simply a relic of our quadrupedal ancestry.
One of the more recognisably problematic vestigial traits is the appendix. Once involved in digesting plant matter, the human appendix no longer serves that purpose; instead, it may help protect our beneficial gut bacteria when faced with a serious infection. But many would say that its true purpose these days is to line the pockets of joyous surgeons everywhere. The appendix is prone to infection and, should it burst, can send deadly bacteria rushing through your abdomen. Though not all people require an appendectomy – removal of an infected appendix – the surgery is more common than, say, if the human race had evolved without a volatile, poison-filled time bomb growing in their bodies.
The human appendix often causes enough trouble that it needs to be removed. Ralf Roletschek/Wikimedia Commons (GFDL 1.2)
Another vestigial trait known to plague the human species is the third molar, or wisdom tooth, which often develops in early adulthood. When wisdom teeth become impacted, growing in the wrong direction or being blocked as they grow, many people require an extraction. These teeth are a gift from our longer-jawed ancestors. The much shorter jawbones of modern humans mean that, quite often, there’s simply not enough room for wisdom teeth to grow.
Another mark of times gone by when we were required to move about in trees is the human foot. Our feet, comprising 26 bones and 33 joints, were once well suited for grasping branches, but are now unnecessarily complex for our modern lifestyle. We could easily get about with our ankle and lower leg bones fused into a single strong structure, preventing common injuries like sprains and shin splints. Unfortunately, such traits were not on the genetic cards for our species, and we have been doomed to collapsed arches and rolled ankles ever since.
Evolution doesn’t make mistakes, it makes compromises. But when you’re attending your 50th physio appointment for crippling back problems, or forking out the dough for your new orthotics to save you from painful collapsing arches, the difference between a compromise and a mistake is negligible.
"History is written all over us," said Richard Dawkins. While evolution can't start over or undo an evolutionary trait, it can improve an animal by rejigging and repurposing, or simply decommissioning characteristics. Natural selection’s slap-dash, MacGyver-esque methods make for some pretty choppy evolution at times. But at least next time you’re lying prone on the floor, paralysed by back pain and waiting for someone to find you, you can take the time to say "Thank you, evolution, for making me a better me".
Edited by Andrew Katsis and Ellie Michaelides
