You and I share quite a few common ancestors not that far back in time. OK, you are now asking, what is this guy talking about? You might know a good deal about your own genealogy, and there isn’t anyone named Garte anywhere to be found in it. So I will explain.
Remember that everyone has 2 parents, 4 grandparents, etc. So the number of your ancestors doubles at each generation going back. If we assume every generation takes about 20 years, then 100 years ago you had 32 ancestors. 200 years ago you had about 1000. Of course this can’t keep going on very long because by 800 years ago you should have 1,000,000,000,000 ancestors, and there wasn’t even a fraction of that number of people alive in 1200 AD. This phenomenon is called “pedigree collapse”, and it means that the number of your ancestors (which starts out as = 2^n where n is number of generations) has to flatten out as n gets bigger.
How does that happen? As you go back in time you begin to find that some people show up more than once in your ancestor list. That’s because the same guy who is your mother’s father’s father’s mother’s father is also your father’s father’s mother’s mother’s father. And so on. Since this is true for each of us, it’s only a matter of a few generations (for people who share a European heritage, for example) to find out that my father’s mother’s mother’s mother’s father is the same guy as your etc. etc. The further back you go the more likely this is. When you go back about 800 years, it is almost certain. This is why it is true that we are all descendants of Charlemagne. If the awful book Da Vinci Code was correct in its premise that Jesus had a child, then we would ALL be His direct descendants.
Since most African Americans have some European ancestry, they are included in the close family. For people whose ancestry is completely African or East Asian or Australian, we have to go further back in time to find common ancestors. How far back is back? A recent paper in Nature has calculated that the most recent common ancestor of ALL humans (not an easy calculation to do) probably lived no earlier than 50,000 years ago, and possibly even as recently as 10,000 years ago.
Therefore we are related to some degree. We are all one family in a real sense. So while saying “All men are brothers” is a nice philosophical concept, saying “all men (and women) are cousins” is literally true. Kind of makes you think.