If all humans descended from one man and one woman in the last six to ten thousand years, what kinds of things would we expect to see? Blood donations would be simple—no need for cross-matching. Organ transplants? Piece of cake. No need to match donors with recipients, and no need for anti-rejection drugs. If all humans descend from a population of two, humans would be so genetically identical any sort of transplant would be a breeze. But that is not what we see. The broad genetic diversity in modern humans could only arise from a large founding population.
