Apolipoprotein LI is a serum protein, a component of HDL, sometimes called ‘good cholesterol’. It has another function: it protects people from trypanosomal infections – the kind of protozoa that cause sleeping sickness. That is, it protects against most of them. Two strains can get past this defense: T.b. gambiense and T.b. rhodesiense.
The APOL1 defense is a nonstandard innate immune defense, specific for trypanosomes. It chops holes in their surface membranes. You need this in sub-Saharan Africa, because the adaptive immune system can’t deal with trypanosomes, due to their vast repertoire of coat antigens.
So when you’re bitten by a tsetse fly carrying one of the two strains that have developed resistance to APOL1, you die, without modern (and fairly unpleasant) medical treatment. Unless you’re a mutant.
This used to be a major problem, although the risk is lower nowadays. Here’s a map of the areas subject to sleeping sickness:
Note that cattle (and horses) don’t have APOL1.
It turns out quite a few people who live in or whose ancestors originated in Sub-Saharan Africa have mutated versions of APOL1, changed in ways that protect against one of the dangerous strains. These mutations don’t look all that old, probably less than ten thousand years. I would guess that humans made up an insignificant fraction of potential trypanosomal targets before the development of agriculture, so that there essentially no selection for versions that could get past human defenses. After our numbers increased, strains that could infect humans were favored, and sometime after that, mutations that protected humans against those dangerous strains began to spread.
The problem is that people with two copies of these protective variants are much more likely to experience kidney failure in later life. African Americans are 4-5 five times more likely to end up on dialysis than whites. It used to be that the medicos thought that the kidney risk was caused by increased levels of hypertension – now it looks as if it’s probably the other way around.