I’ve just read a fascinating book: Survival of the Sickest: The surprising connections between disease and longevity by Sharon Moalem. 
The question Moalem sets out to answer is: “Hereditary disorders don’t make much evolutionary sense at first glance. Why would genes that make people sick still be in the gene pool after millions of years?” (p. xiii)
At first glance his answer is surprising, but then it makes a lot of sense. The same genes that can produce illness, even on the scale of millions of people dying, can also save the species from being wiped out. Take the second most common genetic disease among Europeans, cystic fibrosis. “New research suggests that carrying a copy of the gene that causes cystic fibrosis seems to offer some protection from tuberculosis. Tuberculosis caused twenty percent of all deaths in Europe between 1600 and 1900. Making anything that helped to protect people from it look pretty attractive.” (p. 21)
If the second most common genetic disease in Europeans is cystic fibrosis, any idea what the first is? Hemochromatosis, a disorder that causes iron to build up in the body eventually overloads organs like the pancreas and liver. Never heard of it? Neither had I. And no doubt nor had anyone in 1347 when the bubonic plague killed between one-third and one-half of the population of Europe in three years. Why didn’t it wipe out everyone? The plague, like other bacterium, multiplies rapidly in an iron-rich environment.  So you would think having the gene that causes hemochromatosis was a death warrant. In a paradoxical twist it turns out that while most cells end up with too much iron, macrophages, a type of white blood cell, end up with less. This makes people with the hemochromatosis mutation especially resistant to the infections like the Black Death.
In an even more controversial theory Moalem suggests diabetes may have helped our European ancestors survive the sudden onset of the last ice age, 13,000 years ago. And when I say ‘sudden’ I mean it. Ice cores from Greenland show that the onset happened in just a decade or so. “Average yearly temperatures plunged nearly thirty degrees. Sea levels dropped by hundreds of feet as water froze and stayed in the ice caps.” (p. 33) Anything that helped people survive that rate of change would have been a bonus.
“Coincidentally people most likely to have a genetic propensity for [diabetes] are people descended from exactly those places ravaged by the sudden onset of [the last] ice age” (p. 44) In another coincidence diabetes is characterised by excessive elimination of water and high levels of blood sugar – exactly the properties that enable living things to deal with extreme cold. Put the pieces together and you have a theory that genetic mutations resulting in diabetes may have helped save the entire North European population from ending up as popsicles.
As fantastic as it sounds, the theory is being taken seriously in some quarters. The Canadian Diabetes Association for one, is funding research into a frog that uses the anti-freezing properties of high blood sugar to survive extreme cold.
Whether all or any of these theories become part of the accepted canon of science (and far stranger things have happened) the idea that “when there is a sudden change in circumstances – a new infectious disease, and new predator, or a new ice age – natural selection will make a beeline for any trait that improves the chances of survival” (p. 47) makes evolutionary sense. At a species level, the more ‘mutants’ in the population the more chance that the gene pool will survive the Black Swan – the unexpected high-impact event that punctuates the equilibrium of normality.
It’s not difficult to map these physical strategies for survival onto the psychological. Is it not a remarkable fact that some children who suffer the most terrible of traumas survive and thrive? Those events may indelibly mark the person for the rest of their lives, but somehow they got through. How do they do that?
In the 1980s David Grove proposed a psychological mechanism by which children (and adults) protected themselves against extreme events. His idea, which he called “T-1, T, T+1”, suggested that the mind used dissociation and fragmentation out into the environment or into the body as ways to immunise a person from the horror of what they were suffering. 
The radical psychiatrist and author of The Divided Self, R. D. Laing, described how he avoided feeling the pain of being beaten by his father by imagining his mind was a tiny point outside his body. This kind if dissociation (and fragmentation) can result in people living in, what David called, “strangely scaled worlds”. 
From this perspective, as Laing and his radical peers proposed, ‘mental illness’ is a sane response to an insane world. From an evolutionary perspective genetic variations that increase a propensity for mental ‘disorders’ may one day end up saving the human race – from itself.
1 Written with Jonathan Prince, and published by Harper Perennial, New York, 2007.
2 Moalem, a PhD in human physiology, specialising in neurogentics and evolutionary medicine, suggests thinking seriously about taking iron supplements or iron-fortified foods while taking antibiotics (even if you have low-iron levels), since they could work against each other.
3 For a brief explanation of T-1, dissociation and fragmentation, see: History of David Grove’s work: 1980-2004. [Link available soon]
4 For an article on the importance of scale to perception, see Penny Tompkins and my article: Big Fish in a Small Pond: The Importance of Scale.