all inherited it from a shared ancestor, and independently kept it going during millions of years of separate descent, presumably because those diseases, or something like them, have operated throughout that time. In fact, the genes responsible for the A and B blood types probably diverged before our split with the Old World monkeys, around 25 million years ago. This feature of evolution is called trans-specific polymorphism, and it shows conclusively that differences between humans can trace back to before humans themselves existed.
