A question often asked is: If every person has 2 parents, 4 grandparents, 8 great-grandparents and so on, why are there more people alive today than in the past? The Encyclopaedia Britannica (2003 edition) estimates that 10,000 years ago the world population was 5 million (5,000,000); and by 1990 it had risen to 5 billion (5,000,000,000). If we allow 30 years per generation, although it may have been less in the past, then each person in 1990 could have 2333 possible ancestors in 8,000 BC. Multiply this by 5 billion people and we get a figure that is so large it is beyond the scope of a pocket calculator. Even 1,000 years ago each person could theoretically have over 8½ billion ancestors.
There are two separate questions to be asked in connection with this paradox, although they are related: 1. Why does each person have fewer ancestors than the number suggested above? and, 2. Why is the world population larger today than it was 10,000 years ago?
The answer to the first question is that it is because of inbreeding; which need be no closer than a first cousin marriage. e.g. A person whose parents are first cousins will have 2 parents, and 4 grandparents, but only 6 great-grandparents and 12 great-great-grandparents. The diagram figure 4 shows that individual W1, whose parents are first cousins, has lost 1/4 of his potential ancestors right back into antiquity. Thus, inbreeding causes many ancestors to be repeated in several lines of descent.
Ancestors who occur more than once are printed in red.
In a large random mating population, where every single male has an equal chance of mating with every female, there will be very little inbreeding. In real life, however, large populations tend to be split into several smaller semi-isolated groups. In the past, transport was limited and most people found their partners in the same or a neighbouring village. Other reasons for reproductive isolation were the existence of religious and ethnic ghettos, and small island populations like Tristan da Cunha where only a few surnames were shared. Class and caste systems and apartheid also subdivided large populations.
Reproductive isolation is exacerbated by "bottleneck situations". As a result of major disease outbreaks such as the Black Death, Influenza, Typhus and Cholera, and more recently Aids in Africa, these small groups become even smaller. Major wars and ethnic cleansing also contribute. This means that the only people of the opposite sex available for marriage in these situations will eventually be cousins. When the groups expand again, first cousin couplings will be supplemented by 2nd, 3rd, 4th and more distant cousin marriages. Most people are unaware of cousins beyond the second degree, so they will continue to occur and will have a cumulative effect on the loss of possible ancestors.
The answer to the second question; is that in addition to inbreeding reducing the number of ancestors for each individual, the fact that each pair of parents often has more than two offspring causes the population size to expand. Although some people do not have children, this is more than compensated for by many of the rest having large families. Full sibs share all their ancestors, including their parents and grandparents, while half sibs share one half. Similarly, starting back from their grandparents, double first cousins have all their ancestors in common, whereas single first cousins share a half. Sharing of ancestors continues with more distant relatives; counting back from their great-grandparents, double and single second cousins share one half and one quarter of their ancestors respectively. Therefore, as we go back through the generations, an increasing number of the present generation will have common ancestors. Any inbreeding would increase this sharing still further.
If we go back 10,000 years, using the figures quoted in the first paragraph of this essay, each person living in 8,000 BC is a potential ancestor of at least 1,000 people alive today. In fact, recent research using Y chromosomes and mitochondria as markers, has shown that only a few males and females from the 5 million potential ancestors are actually common ancestors of today's world population. The majority left no long-term descendants, largely because of recurring 'bottleneck situations', when the sizes of the breeding populations were very much reduced. This means that practically everyone alive today is related to everyone else with respect to the 8,000 BC population.
The true picture about ancestors, therefore, is that their numbers do not increase as we go back in time, but actually decrease because of marriage between relatives. The world population has increased in size because of the reproductive success of survivors from each generation. This has led to some inbreeding and the large scale sharing of distant ancestors.