Why Indeed? And also,while we're considering the question, why don't chimpanzees get atherosclerosis, rheumatoid arthritis, asthma, endometriosis, myocardial infarction, falciparum malaria, HIV infection, or epithelial cancers? Chimpanzees are, after all, our closest living relatives. We share a common ancestry and we only embarked on separate path some six or seven million years ago --a blink of an eye in evolutionary terms. Why is it that we human beings, so superior, as we like to think, in so many ways from our cousins -- in our big brains and delicate hand manipulations, for example -- are subject to an array of diseases to which our inferior relations are immune. What's the deal?
Here's the current thinking on this important problem (which I draw from Eugene E. Harris's informative and up-to-date monograph Ancestors in our Genome (Oxford, 1914). As I understand it, human beings went through a sort of bottleneck sometime in our recent (say 200,000 years ago) history and our effective breeding population was reduced to a mere 10,000 or so souls. When a population becomes so small, random genetic drift dominates over natural selection, and so moderately deleterious but not incapacitating mutations can survive and even thrive. The defective genes that would be weeded out in larger populations persist in our genome. Hence our many inherited susceptibilities and disorders.
And then yesterday there was a report that the Ashkenazim went through "a severe bottleneck" six or eight hundred years ago which reduced its population to about 350 individuals. O my, what a boon for genetic drift, what a recipe for genetic disaster or genetic success, or more exactly, both success and failure simultaneously.
And so our talents, our deficits, our strengths and weakness, our brains and our bodies, are determined by arbitrary aleatory long-ago events. No surprise, really, but revelatory nevertheless.