My new hobby

They say, if you go back far enough, each one of us has a shared ancestor with every other person on earth. 

Scientists estimate that the most recent common ancestor of all humans, probably lived in either Egypt or Babylonia during the classical period. Evidently, we can all trace our ancestry back to this person.

Assuming an average generation time of 20 years, this means that we are all 120th cousins, descended from someone that was alive when the pyramids were already aging structures. (Many millions of other people living at that time also have living descendants, of course. The last common ancestor is simply the one who is an ancestor to all of us, in addition to our many other ancestors who are not common to everyone.)

Of course, within a given ethnic group, the most recent common ancestor will be much more recent than that, especially so within a limited geographical area with low ethnic diversity. Remember that, randomly, some people leave many descendants and others leave none. If you take a country like Scotland, Sweden, or Poland, you really don’t have to go back very far before you discover someone that is a shared common ancestor to the vast majority of living citizens. For example, in the lands of the former Mongolian empire, around 8% of the population are direct descendants of Genghis Kahn and that goes less than 800 years back. Even as far away as North America, around 0.5% of men carry the Y-chromosome of the great Kahn.

Many millions of Americans of Irish ancestry trace their families back to a specific county in Ireland, but the reality is that, if you’re Irish, you are related to all other Irish people and probably a lot more closely than you think.

In fact, everyone on earth with any trace of European ancestry probably has a shared ancestor who lived in the early Middle Ages. Charlemagne has been proposed as a possible candidate.

Family trees aren’t correct anyway

One thing that Ancestry.com won’t often tell you is that the genealogy that you discover may not be accurate anyway. Inferences have to be made when you are dealing with records that are hundreds of years old. There are many surnames and first names that are quite common. There is no way to be sure that the “Jacob Carter” that turns up in one record is the same “Jacob Carter” that shows up in another from fifteen years later, even in the same general area.

Then, as now, many families were on the move. An isolated census record containing only a name is nothing more than a low-resolution snapshot. There is no way to know how many links and associations that genealogy reveals are simply coincidence.

Furthermore, over the past two centuries, it was not uncommon for last names to be changed, misspelled, or misattributed. Records were frequently lost, recreated, or even forged. We think of our identities as relatively fixed now because we have traceable identification cards, birth certificates, and social security numbers from a very early age. But none of that existed until recently.

There is a long article, with an America slant, you may wish to read on this topic
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Nathan H. Lents, Ph.D., is a professor of molecular biology at John Jay College, of the City University of New York.