A genealogy researcher has come to the startling conclusion that humans - all 7 billion+ of us - come from one big family tree, and furthermore that its roots are located in Egypt.
In "Common Ancestors of all Humans", Humphrys makes a strenuous case that we're all a lot more interrelated than we thought. It isn't even anything remarkable to say that we're all related - this has been claimed by anthropologists before. But this site goes the step further and concludes that the Ancient Egyptian Pharaoh Khufu, circa early 2500 B.C., is the great grand-daddy of us all.
One of the effects that goes into this is the phenomenon of pedigree collapse. This states that if you must insist that none of your ancestors were related, then you have a binary family tree that can only go back 30 generations before maxing out at a billion people - greater than the population of the world at the time! Thus, it is logical to conclude that in a world where the human population is always increasing, we would necessarily have had to have gotten those offspring from a smaller pool of ancestors.
This quote from the Wiki sums up the situation nicely:
"Small, isolated populations such as those of remote islands represent extreme examples of pedigree collapse, but the common historical tendency to marry those within walking distance, due to the relative immobility of the population before modern transport, meant that most marriage partners were at least distantly related. Even in America around the 19th century, the tendency of immigrants to marry among their ethnic, language or cultural group produced many cousin marriages."
If you're not already feeling a little queasy from the incestuous implications, consider that most religions have a creation myth involving an Adam and Eve figure, so they've been going around saying as much for years.