How did giraffes get such long necks? When kids first learn about natural selection, they often quaintly assume that giraffes must stretch their necks, and somehow all that reaching is passed inherited by longer-necked giraffe babies. But that is wrong, of course.
Natural selection actually works like this: ancient giraffes who just happened be born with longer necks were better at surviving to breed, because they could reach more leaves.
= more survival
= more breeding.
Because they had long necks, so did their offspring. Short-necked giraffes just slowly disappeared, for lack of being born as often.
But can you imagine the cricks that giraffes get in those necks? Wow! This is perhaps the ultimate no-brainer example of evolutionary compromise — of natural imperfection. Ridiculously long necks = orthopedic nightmare! This article is about the evolutionary trade-offs that sacrifice our comfort.
Evolutionary medicine is a (surprisingly new) field of medical science that helps us understand why we get sick. Diseases and physiological imperfections that seem otherwise mysterious can often be understood easily by applying the principles and perspective of natural selection.
Let’s start with a classic example from evolutionary medicine: the appendix.
I think the clearest example of evolutionary medicine at work is the whole appendicitis thing: why do we have an intestinal appendix that is so prone to infection? Tens of thousands of people die every year from untreated appendicitis. Seems like a bit of a design flaw, doesn’t it? How on Earth did we get saddled with a virtually useless little bit of vestigial anatomy that occasionally just ups and kills us?
And why didn’t evolution beat this rap millions of years ago?
The “big idea” of evolutionary theory, confirmed by mountains of archeological and biological evidence (see Was Darwin Wrong? by David Quammen in the November 2000 issue of National Geographic), is usually expressed as “survival of the fittest,” but that can be misleading, especially in this context. Fitness does not guarantee reproductive success, although it certainly helps. It’s more accurate to say that evolution is all about “survival of the best breeders.” You can be as fit as you like — if you don’t breed, your genes stop with you.
And it seems like appendicitis would really tend to get in the way of breeding: “Honey, not tonight, I have appendicitis.” It really seems like people born with smaller appendices would be much more likely to survive and reproduce, and eventually the appendix would just disappear.
But here’s the trouble, and the power of using this perspective: the smaller the appendix, the more prone it is to infection! People with smaller appendices are actually more likely to die horribly, not less.
Yikes. No wonder it’s still there. Now it all makes horrible sense … and it’s clear that we’ll never be rid of the thing.
This simple example of the power of the evolutionary perspective on medicine is taken from the book, Why We Get Sick: The new science of darwinian medicine, by Randolph Nesse and George Williams.1 It’s a fascinating and accessible account of how an understanding of natural selection is changing medical science.
However, it neglects my own field.
What can evolutionary medicine do for orthopedics, for musculoskeletal pathology? How can natural selection illuminate the more ordinary aches and pains of humanity, things like low back pain and neck pain, myofascial pain syndrome (knots in your muscles), or common sports injuries like plantar fasciitis and iliotibial band syndrome, or the vulnerability of shoulders to dislocation, of knees and ankles to sprains?
The evolutionary perspective can also be used to help understand my own medical turf. This has never really been done before, but I’m going to take a stab at it in this article. Please let me make it clear from the start that this is all just “food for thought”: I am simply not expert enough to do anything but speculate. But it’s interesting to speculate!
Evolution is all about compromises and trade-offs. It’s almost impossible to consider a biological feature that’s all good news.
Immunity, for instance, is a delicate balancing act between “too much” and “too little” — if we had an immune system any more sensitive and reactive, we would all be crippled by allergies and worse; any less, we’d be sitting ducks for every wee beastie. And yet it is the very sensitivity of that system that results in the very unpleasant phenomenon of sterile inflammation — incredibly, our over-sensitive immune cells actually attack the contents of our own cells, because they contain symbionts that are mistaken for invaders. This means that internal injuries hurt much more and much longer than they “should” — a fairly incredible example. For much more about that phenomenon, see Why Does Pain Hurt So Much?.
Biological compromise is particularly harsh when it comes to comfort. Comfort has almost no survival value. It just barely gets a vote. A feature that is good for survival is likely to, well, survive — even if it’s annoying.
Consider bipedality. Walking on two legs is a novelty in nature, and it’s got a high price tag, so it must be pretty good. Biologists have spent a lot of time wondering just what it is about being upright that made it worth giving up the speed and stability of being on all fours. One plausible explanation (there are many others) is that standing up freed us to use our hands to make tools. Proto-humans who were inclined to stand up more probably tended to use their hands more — with excellent results.
But what was the cost? We didn’t just give up the speed and stability of four legs. Most people think that the spine is a marvel of biological engineering, and so it is — but it is engineered by natural selection mainly for quadrupeds, not bipeds. We have the same basic spinal anatomy as every other vertebrate on Earth, but we are the only ones using it in an upright position regularly. That’s bound to be uncomfortably awkward — and it is.
Backs may well have some problems with modern lifestyles, but the biomechanical awkwardness of bipedality may be the one factor to rule them all, the most obvious reason by far why we our species appears to be prone to pain in that department — and something we got saddled with a stupendously long time before agriculture, let alone office chairs. If there is any biomechanical factor that does predispose us to back pain at all, bipedality has got to be the big one — and we can’t do anything about it.
Even the design of the visceral anatomy does not suit bipedality. The organs are contained in connective tissue compartments which are ideally hung from a horizontal spine. Those structures didn’t change when humans stood up: we simply started carrying our guts around as though they were in a bag tied to a pole. And as if that weren’t bad enough, imagine pregnancy …
Pregnancy is the ultimate way to demonstrate the evolutionary and comfort problems with walking upright.
Horizontal spines are a good tool for hanging a heavy pregnant belly from, with no obvious compromise to stability, but human mothers have to deal with an awkwardly off-centre weight that absolutely makes staying upright more of an effort. How do women do it?
Well, by leaning back, of course!
We often say the reason people get lower back pain is because we became bipeds and being a biped is a stupid way to use your back. But actually that doesn’t make any sense, because if back pain is so difficult, such a challenge, natural selection surely would have acted to lessen the prevalence and severity of back pain.
Dr. Dan Lieberman, evolutionary biologist, Brains Plus Brawn
Natural selection has “acted to lessen the prevalance and severity of back pain,” and in fact that is shown by some of Lieberman’s own research. It turns out that female backs have larger, stronger supportive structures at the posterior of the spine than men, so they can get knocked up without getting knocked down. In a word, female spines are reinforced. They are flat-out better evolved for a deeper lordosis (“lordosis” is the curvature of the lumbar spine). In 2007 in the journal Nature, Whitcome, Shapiro and Lieberman showed these features in modern women, and also in the fossil record at least as far back as 2 million years.2 The change presumably occurred relatively early in the history of walking upright.
These anatomical differences probably only partially compensate for the stresses of pregnancy — pregnant women still tend to suffer from quite a lot of back pain, as I’m sure they would be quick to agree, while at the same time they suffer from it rather a lot less than many people might suspect would result from such a necessarily extreme lumbar curvature. We men, with our wussy, unreinforced backs, would certainly suffer more — hard evidence that men are less fit to withstand the rigours of pregnancy! As if you needed any.
As a gratuitous side note, I would like to point out that this research also emphasizes — yet again — that excessive concern about lumbar curvature is probably unjustified (like most structuralist theories). This is very common among therapists and doctors, in spite of a pile of contradictory evidence. An entire industry of bad back pain advice is based on the idea that lumbar curvature — either too much or too little — is the important factor in whether or not you get back pain.
But Whitcome’s research shows that women are actually adapted for coping with increased lumbar curvature, demonstrating in one elegant scientific stroke that, even as women evolved a mechanism for coping, it is almost certainly an imperfect mechanism, good enough to keep women from actually breaking their backs during pregnancy, but not good enough to prevent all pain. Such deviations from normal curvature are not critical, or pregnancy would be biomechanically unviable, and bipedality would have resulted in a rather short-lived species of crippled moms. As always in evolution, the result is a compromise: enough adaptation to make breeding possible, not enough to actually make pregnancy comfortable for the back.
Obviously the increased lumbar curvature and heavier loads involved in pregnancy are not easy or risk-free, or there would have been no need to adapt to it. But on the other hand, it is equally obvious that it isn’t a deal-breaker. And plenty of other research angles that support this: for instance, the conspicuous lack of any clear evidence that obesity is a major factor in back pain.3 Dr. Lieberman concludes his comment above with this fascinating observation:
In fact, if you start asking people who work with hunter-gatherers, most people say yes, actually come to think of it, I don’t really recall anybody saying that they had back pain. I’ve never seen anybody have back pain in the hunter-gatherer context.
If true, bipedality is either a relatively minor problem for most backs most of the time, or one that only becomes a problem in the presence of other factors — like a lack of exercise.
There’s some more on this topic in an Appendix to the article.
My clients often ask me, “Why did this happen?” Sometimes the only answer I can give is, “Design flaw.” I suggest that they take it up with The Management, and roll my eyes heavenward. But I’m not giving the divine enough credit: natural selection never creates “flaws,” it just makes deals.
Consider the shoulder, a fabulous joint: so loose and mobile that it would fall apart if it weren’t for a very clever arrangement of muscles, the infamous “rotator cuff,” which is often injured in spite of, or rather because of, its clever arrangement. The shoulder is a perfect compromise between mobility and stability. Any more mobile, it would fall apart. Any more stable, we wouldn’t be able to scratch our backs at all. That compromise comes with a price tag: lots of shoulder injuries! The alternative, however, is to have upper limbs that don’t do nearly as much.
Or consider knees, another spectacular joint, also prone to injury. In the knee, the compromise is between enormous power and vulnerability. The arrangement of the kneecap allows for spectacular leverage on the lower leg. Knee extension is by far the most powerful movement in the human body, and pressures underneath the kneecap can be “greater than three times body weight during stair-climbing and eight times body weight during squatting and deep-knee-bending.”4 This power requires a joint that with more parts than most, mechanically complex, and prone to failure. But the alternative would be to give up most of the lifting power in our knees!
So, design flaws? Or design genius? Your opinion probably depends on whether or not you have a busted knee!
The entire system [of DNA chemistry] was accidental, something frozen into cells billions of years ago and now too ingrained to replace—the QWERTY keyboard of biology. Moreover, RNA employees no fancy anagrams or error correcting algorithms, and it doesn’t strive to maximize storage space, either. Our code is actually choking on wasteful redundancy…
The Violinist's Thumb, by Sam Kean, pl781
The necessity of compromise is not the only limitation of natural selection. Something more like actual “design flaws” occur as a result of the incremental nature of evolution.
Traits evolve in baby steps, small changes to existing features — because the only way for change to occur is through minor random mutations to existing genes. Major mutations are extremely unlikely to be beneficial. However, any slight change that makes you better at breeding is likely to get passed on. But it’s not possible to innovate, to skip steps, or even to get rid of old stuff and replace it with something different and better. An improvement that requires even a single leap of logic is not an option.
This would drive any designer or engineer absolutely crackers. Imagine if you could only improve a car by making one small modification to an existing part — you could never just chuck it out and start over, or even make an improvement that depended on another improvement. Bodies cannot, for instance, develop a trait, no matter how brilliant, that requires a prerequisite modification to another system — mutation would have to provide the prequisite trait by chance, and then the dependent trait by chance. That’s a lot of luck, and it tends not to happen much, even over huge time frames.
If cars were designed the same way that evolution works, they might well still have cranks. Or, imagine if you could only improve a computer operating system by making slight improvements to what’s already there … oops, no need to imagine that one, we all work with the consequences of that every day!If cars were designed the same way that evolution works, they might well still have cranks.
When you look at evolution like this, it’s amazing how marvelously well everything actually works. But, of course, even though natural selection has limitations, it never, ever — by definition — perpetuates a trait that doesn’t make you a better breeder. So, in the big picture, everything always works miraculously well.
But that doesn’t make it comfortable, unfortunately!
John Sarno, author of Mind Over Back Pain, a physician and writer on the topic of back pain in particular, does not think that bipedality is bad for backs. For readers who are keen on this subject, I’ve excerpted his comments here and responded.
The idea of the vulnerability of the back was greatly enhanced a number of years ago when a doctor suggested that the problem of back pain was due to the fact that man was not meant to walk upright. Here is mankind’s imagination at its most fertile. We have become so attached to the belief that back pain is the result of structural abnormalities of the spine that we are willing to foist a totally illogical idea onto the process of evolution. Darwin would have been shocked.
It is my understanding that the process of evolution is characterized by two fundamental principles: (1) There is a powerful tendency for all species to perpetuate themselves, and (2) in order to accomplish this, each species will adapt in a variety of ways, even to the point of changing its basic structures and systems. Adaptation and change are in the services of perpetuation, or to use a term more commonly employed, the survival of the fittest. Species that adapt successfully survive; others, such as the dinosaurs, do not.
Homo sapiens has evolved to be the dominant animal on this planet. To be sure, his brain has a great deal to do with this dominance, but it took millions of years for the brain to develop, and during that time our ancestors had to survive physically. They could not have done so if the spine was maladapted for the upright posture.
From a purely mechanical point of view, the upright posture should be easier on the spine than walking on all fours. Look at swaybacked horses. It is well known in veterinary medicine that dachsunds are quite prone to catastrophic disc herniations with paralysis of the hind legs. This is unusual in human beings.
John Sarno, Mind over Back Pain, p42-3
I sympathize with Dr. Sarno’s intention, but unfortunately I believe that his (justified) bias against back pain of structural origin — the entire point of his book — caused him to become overzealous and careless in this particular matter. The case he makes here was not essential to his thesis, and is incorrect or misleading on at least three important points.
Sarno acknowledges but underestimates the survival advantages conveyed by a larger brain, implying that bipedality preceded it and would never have survived as a trait if it weren’t a robust one. This is undoubtedly true in some senses,5 but misleading in others:
Sarno goes on to argue that “the upright position should be easier on the spine than walking on all fours,” which is unlikely to be true based on his own interpretation of natural selection: if walking on all fours were mechanically unsound, it’s improbable that so many other mammalian species would be using that system! Although it’s inevitable that there are costs to walking on all fours as well as “on all twos,” there are many more successful variations of quadrupedality than bipedality. A bazillion quadrupeds can’t be wrong!
Presenting the dachsund as an example of the relative fragility of quadrupedal spines is irrelevant: the dachsund’s spine has been grossly altered by selective breeding, and can in no way be compared to the effectiveness of quadruped spines in general. As for swaybacked horses, I think that this equine back problem is limited to horses who have carried human riders, have worn poor-fitting saddles, are overweight from feeding, and so on — wild horses don’t have these problems!
Sarno also overlooks a subtlety of spinal function that is widely ignored: namely, that the vertical spine does not function by “stacking” any more than the horizontal spine does, and therefore the stability of neither system can be judged on this basis. Instead, the upright posture is achieved via the principles of tensegrity and hydrostatic pressure,9 which explain why essentially the same spinal structure serves both quadrupeds and bipeds. However, in bipeds, I believe that it takes a little more muscular effort — an acceptable evolutionary cost — to maintain “tensegrity”!
Finally, the whole business of Whitcome’s 2007 research showing that female spines have actually adapted to pregnancy by getting stronger in the posterior joints pretty clearly shows that being upright was so challenging to the spine that adaptation was necessary to make it feasible. At the same time, it supports his other (more important) point that spines are pretty tough!
The point that I have made in this article is consistent with Sarno’s goals: I do not argue that bipedality results in “fragile” backs prone to structural failure. Indeed, I emphatically agree with Sarno that backs are not fragile, and I make my own case for this exhaustively in the article Save Yourself from Low Back Pain! Bipedality does not appear to compromise the structural integrity of spines — but it does seem to predispose the spinal muscles to precisely the benign but uncomfortable condition that both Sarno and I describe as a major cause of low back pain.
Does it hurt to be heavy? It seems possible, and in this study “obese twins were more likely to report low back pain.” But it’s not that simple: there are many variables involved. For instance, several other pain problems were also more likely: “migraine headaches, fibromyalgia, abdominal pain, and chronic widespread pain.” Weight is associated with more pain in general, not low back pain specifically, as you’d expect if the problem were simply due to compression of the spine. Clearly that typical assumption is not a safe one, and indeed the apparent connection between weight and pain weakened when the data were adjusted for common denominators like depression, a strongly confirmed risk factor for low back pain. In other words, if you factored out the depressed cases, the remaining subjects were not all that likely to have back pain. Given such complexity, the researchers made it clear that more and different research is needed to figure out what, exactly, is causing what. No kidding!BACK TO TEXT