Scott Hershberger writes:

The late esteemed English actor Christopher Lee traced his ancestry directly to Charlemagne. In 2010 Lee released a symphonic metal album paying homage to the first Holy Roman emperor—but his enthusiasm may have been a tad excessive. After all, says geneticist Adam Rutherford, “literally everyone” with European ancestry is directly descended from Charlemagne.

The family tree of humanity is much more interconnected than we tend to think. “We’re culturally bound and psychologically conditioned to not think about ancestry in very broad terms,” Rutherford says. Genealogists can only focus on one branch of a family tree at a time, making it easy to forget how many forebears each of us has.

Imagine counting all your ancestors as you trace your family tree back in time. In the nth generation before the present, your family tree has 2n slots: two for parents, four for grandparents, eight for great-grandparents, and so on. The number of slots grows exponentially. By the 33rd generation—about 800 to 1,000 years ago—you have more than eight billion of them. That is more than the number of people alive today, and it is certainly a much larger figure than the world population a millennium ago.

This seeming paradox has a simple resolution: “Branches of your family tree don’t consistently diverge,” Rutherford says. Instead “they begin to loop back into each other.” As a result, many of your ancestors occupy multiple slots in your family tree. For example, “your great-great-great-great-great-grandmother might have also been your great-great-great-great-aunt,” he explains.

The consequence of humanity being “incredibly inbred” is that we are all related much more closely than our intuition suggests, Rutherford says. Take, for instance, the last person from whom everyone on the planet today is descended. In 2004 mathematical modeling and computer simulations by a group of statisticians led by Douglas Rohde, then at the Massachusetts Institute of Technology, indicated that our most recent common ancestor probably lived no earlier than 1400 B.C. and possibly as recently as A.D. 55. In the time of Egypt’s Queen Nefertiti, someone from whom we are all descended was likely alive somewhere in the world.

Go back a bit further, and you reach a date when our family trees share not just one ancestor in common but every ancestor in common. [Continue reading…]