Dear Dr. Boli: I have to do a research project on ancient Egypt for Mrs. Hurdle’s tenth-grade social-studies class, and there’s one question I can’t find the answer to. What was the total population of the earth about five thousand years ago, when Egyptian civilization was ramping up? It wasn’t in my history book. —Sincerely, Maximilian F. O. P. Pond IV, Mrs. Hurdle’s Tenth-Grade Social-Studies Class, Point Breeze Preparatory Academy for the Idle Rich.

Dear Sir: Let us see if we can solve this problem mathematically. You have two parents, and your parents each had two parents, and so on back through the generations. Estimating about three generations per century, we have 150 generations back to the time five thousand years ago for which you are looking for a population estimate. Therefore, we simply calculate 2 to the 150th power, and we find that there were about 1,427,­247,­700,­000,­000,­000,­000,­000,­000,­000,­000,­000,­000,­000,­000 of your ancestors alive five thousand years ago. Now, at the present moment there are roughly seven billion people on earth, and each of them has a similar ancestral tree. Therefore, we need only multiply our result by 7,000,000,000, and we shall have a very good estimate of the population of the earth five thousand years ago.

1. RepubAnon says:

Ah, yes, the perils of the population implosion… if this trend continues, before long all human life will disappear!

2. Most of those umpteen million ancestors of yours were the same few people being counted multiple times. The same guy was probably your great-great-great-great-great-great-great-great-great-great-great-grandfather several times over, and a few times plus or minus a great or two as well.

A thousand years ago, all of us had at least one ancestor living in a small rural town someplace. A thousand years ago, people in small rural towns were even less likely to leave town than our ancestors were to leave their small towns a century ago.

Now think about your ancestors from a century or so ago, living in a small rural town, where everybody knew everybody else, and everybody had half a dozen or more kids with suitably foot-long Victorian names. It doesn’t take many generations of everyone having half a dozen kids and most of them staying in the same area marrying their neighbors for everyone in the town and its surrounding farms to be related to everyone else, at least by marriage.

Now think about our ancestors from a thousand years ago, with a few centuries of their family living in the same rural area, having lots of kids and marrying them off to the neighbors who also had lots of kids. Pretty soon they’re marrying their second and third and fourth cousins over and over again for generation upon generation, and the family tree ceases to fork and just starts folding back and forth over itself like braiding a ponytail.

This is how you get people who look Swedish or Irish or Italian, dozens of generations of inbreeding within the same confined gene pool, century after century, with only the occasional random mutation or invading horde of barbarian rapists to enliven the genome.

3. What Martin said, plus I’m my own grandpa, so that’s 2 the 148 less right there.

Ray might be too