Survival of the Sickest

The key question Dr. Sharon Moelem asks is: Why do hereditary disorders exist? After all, evolution likes genetic traits that help us survive and reproduce.  According to natural selection, if a gene produces a trait that makes an organism less likely to survive, carriers will be less likely to reproduce and eventually the gene won’t get passed on, at least for very long.  

And yet, hereditary diseases perpetuate.  Why?

The short answer is that some conditions once thought to be exclusively detrimental—like hemochromatosis, diabetes, cystic fibrosis—may in fact have evolved as defences against even more devastating illnesses, and thus far from being what kills us are actually a part of what has enabled humans to survive.

Take hemochromatosis.  Normally, one’s body knows when it has sufficient iron – which is necessary for life – and subsequently causes excess to pass through the body rather than be absorbed.  With hemochromatosis, though, one’s body doesn’t metabolize iron in the ‘correct’ way, and that excess iron becomes locked in.  Unable to escape, it ‘rusts’ the person from the inside, causing major organ damage.  (That is why one of the most effective ways of treating this condition is through donating blood to reduce iron levels; historically, bloodletting fulfilled this role, providing relief to sufferers who didn’t understand why this odd practice felt so good). 

While all this sounds terrible, Moalem suggests a silver lining: Bacteria, cancer cells, and other parasites also need iron to survive – and in a normal body, they will find (and use) this iron in our macrophages.  However, in a person with hemochromatosis, the iron was locked away before ever reaching the macrophages, thus depriving the deadly invader what it needs to grow.  So, for example, when people with hemochromatosis faced something like the Black Plague, they would have a significant advantage of surviving thanks to having all their iron locked away.  Yes, this condition could one day kill them, but that would be in five to ten years (and only if left untreated).  Without it, though, their chances of dying immediately from the Plague increased dramatically.

Another fascinating example is diabetes.  Why would evolution allow a disease that harms us so terribly by allowing sugar to overrun our systems continue to promulgate all these years? Shouldn’t natural selection have proscribed it from being passed down from generation to generation? Here, too, he suggests a long-term danger is mitigated by a short-term salvation.  Consider this: pouring sugar into water lowers the freezing temperature of a liquid (conjure up a Slurpee from 7-11 for example; the reason why the drink can have a below-freezing temperature but not actually freeze is because of the addition of sugar).  Dessert wine from Italy – which is sweeter than regular wine – was discovered because particularly cold temperatures threatened to destroy a grape harvest one year by freezing (and ruining) all the impacted vineyards.  To everyone’s surprise, the grapes survived thanks to a natural phenomenon where the grapes released sugar into their veins.  This, in turn, lowered their freezing temperature, simultaneously saving them as well as sweetening them. 

Which brings us back to diabetes.  Moalem suggests that yes, diabetes can kill a human, but again, only after decades.  For those confronted with the temperatures of the last ice age, though, increasing sugar in one’s extremities (via the blood) would provide protection from freezing to death in the same way the grapes protected themselves.  Better to have humans survive an immediate global catastrophe than avoid a long term local disease.

More counterintuitive thinking about other deadly diseases follow, all making the same point and turning on its head our belief that evolutionary survival is reserved for the strongest alone, noting that sometimes weaknesses can be strengths.  While the book is primarily a serious, albeit enjoyable, exploration of scientific knowledge, I found this last point inspiring: Our flaws are not always bad, danger can threaten but also open new windows for survival, and difficulties contain seeds of growth.