All Humans are Siblings?!

Today`s topic will be elementary mathematics. No matter who you are, you have a mother and a father. Consequently, one generation back, you have two ancestors. The mother and father each have their own mother and father, respectively. So you have two grandpas and two grandmas, giving you 4 people when going 2 generations back. Taking that in, it is apparent that for every generation back you go, the number is multiplied by 2. By this math, going 3 generations back gives you 8 more people, 4 back gives you 16, and 5 back gives you 32. Assuming that each generation is about 25 years, going 5 generations back puts us bout 125 years ago, somewhere around 1900, right around the time of the Sino-Japanese and Russo-Japanese war. Jumping a little further back ten generations (250 years about, or 1780, the middle of the Edo period) would give us 1024 ancestors.

Let`s take a break here. Even now, people who boast that “My ancestors were great samurai!” are still sometimes around. However, if during the Edo period you had 1000 ancestors, you may have some great people within that number, but you also probably have people from many walks of life. Taking this into perspective, is it not best to put any bragging about your ancestors on hold?

Now, if you will, let`s get back into making some calculations. 20 generations back (500 years ago or 1520, during the Sengoku (Warring States) period) gives us about one million ancestors. 30 generations back (750 years ago or 1270, the Kamakura period) gives us around 1 billion ancestors. The population in Japan at this time is estimated at around a few million people, so clearly, something is off here. What`s happening is that we are counting common ancestors if they are unique people. This would become an even bigger contradiction if we tried to assume people did not have common ancestors. If you go back more than 20 generations, at the very least, if you look at a small village, town, or city with much interaction among the inhabitants, you will find many populations of relatives.

In the past, unlike today, transportation was not as well developed; if we are talking about the Edo period, contact between different clans was limited, not to mention contact between different territories. However, according to the results of DNA analysis, we Homo sapiens all come from a single woman in Botswana located in Southern Africa, about 200,000 years ago. What would she think if she saw her descendants fighting, or in the worst case, killing one another? All humans are siblings.

[1] Nature 575 (2019) 185-189