Perhaps but that makes a rather odd assumption that every pair of adults would be allowed to select one mate with which to have one child. Supposing everyone o

Perhaps but that makes a rather odd assumption that every pair of adults would be allowed to select one mate with which to have one child. Supposing everyone obeyed that, some number of generations in the future, which would be a blink of an eye compared to eternal immortality, you would end up with a strange selection problem of there being very few people in the most recent generations, and most of them having a large overlap in ancestors.