ABSTRACT
Has depression survived as an evolutionary inheritance because it protects us against something worse? Disease mutations persist in the population instead of being removed by natural selection when they are useful in particular genetic combinations (at the same or different loci) and/or under particular environmental conditions. All adaptations come with a cost, and it is only when the cost outweighs the benefit that the trait will be selected out of the population. Every environment carries a different cost-benefit ratio. Thus humans with dark skin were protected against the cancerous equatorial sunshine in our original African home, but as we migrated north, pale skin carried a selective advantage because it absorbs sunshine more efficiently and permits us to synthesise adequate amounts of vitamin D under the weaker light conditions. Likewise, a genetic make-up leading to susceptibility to type 2 diabetes in one dietary environment (rich and plentiful) enhances survival in another (the periodic semi-starvation conditions that characterised our evolutionary past and are still the case for millions of people today) (Diamond, 2012).
