Why do some people have the same last name?

[Shirley T]

When surnames were assigned or taken in Europe during the last millennium it wasn't impossible for legitimate sons of the same man to wind up with a different surname and still each could have shared his with others with no known relationship. The purpose originally was not to identify a man as a member of a family but just to better identify him, frequently for taxation purposes. Too many men with the same given name in the same town or village and they had to have a way of sorting them out. So the root person of someone else with your surname will not necessarily be the root person of your surname.

Now we are probably all related if we go back far enough The law of probability would indicate that whether you accept literally the account of creation in Genesis or not. You have 2 parents, 4 grandparents, 8 great grandparents, 16 great great grandparents and it keeps doubling each generation you go back and begins to pyramid big time. Get back to the first part of the 2nd millennium and most people can by the time they get back to their 20 something great grandparents(can't remember the exact number). That would be 20 something greats in front of grandparents. If each were a different person they would outnumber the population of the world at that time. One of the big mistakes people make when they start their family history is to try and trace their surname. In genealogy we don't trace surnames we trace ancestors who just happened to have the surname.
The spellings of surnames will frequently change as people go back as get back a few centuries and most people were illiterate and how their name was spelled depended a lot on how some official or clerk understood the name and wrote it down on a record. Most in England, for instance, had a surname by the 14th century. Still in many cases it was a couple of more centuries before the same surname was passed down to subsequent generations. Because of DNA studies many biologist believe the whole human race started with one female they call Mitochondrial Eve. So far they haven't discovered our first father with DNA yet.

So you can be related to someone on the other side of the world that has your surname and you might not have any known proof of your relationship to someone who lives down the street from you with your surname.