You’ve probably heard of the concept of “Mitochondrial Eve”. That’s the remarkable discovery by geneticists that every human on Earth has related mitochondrial DNA, all descended from a single individual, a woman who lived in Africa about 200,000 years ago. Unlike the DNA in the cell’s nucleus, which gets a shuffled 50-50 mixture from both parents, mitochondrial DNA is inherited only from your mother.
That means that every human being could trace an unbroken lineage, mother to grandmother and so on, back to that one person. (Other women alive at the time will also have descendants alive today, but only through male offspring at least once in the line of descent.)
On a similar basis, men inherit their Y-chromosome from their father. So-called “Y-chromosome Adam” was also an individual person, where all purely male lineages converge. The time period for this is actually far more recent than the mitochondrial line, at about 100,000 years ago. (Again, contemporaries will have left descendants, but at some point only via a daughter, so the Y-chromosome transmission died out.)
But these are specialised cases: unbroken female-female descent or male-male descent. If you think of your own great-great grandparents, you have eight ancestors in that generation (usually!) where the line of descent could be male-female-male (from your paternal grandmother’s father) or some other combination. (Actually, come to think of it, binary numbers fit perfectly. At generation “n” you have 2n ancestors, and the pattern of inheritance has every binary number represented, say from 000, 001, 010 to 111. Geek break over.)
Of course, it’s impossible that the number of ancestors really increases as a power of two, or at least the number of different ancestors, because you only need to go back a thousand years or so to exceed the entire population of the world at that time. It’s clear that you must have many ancestors who occupy different places simultaneously in the family tree. In some cultures, first cousins are forbidden to marry, but that only covers common ancestry a mere two generations back. Take three, four, ten generations: basically we’re all related.
Oddly enough, that’s literally true. Obviously we’re all related to Mitochondrial Eve and Y-chromosome Adam, but take account of the different mixed male and female lines of descent and you can estimate how far back you would have to go to find a more recent person who is a common ancestor to everyone alive today. The result is astonishing. That person was alive just 2,000 to 3,000 years ago.
That was the conclusion of a famous 2004 paper in Nature by American scientists Rohde, Olson and Chang. To arrive at the estimates, they used both probability calculations and a computer simulation, which tried to model known population movement and human breeding habits (such as the tendency of individuals to choose mates from the same social group, and the relative isolation of geographically separated groups.)
2,000 years ago, each of us would have had a nominal 280 ancestors, or a million billion billion. Estimates of the world population at the time range from 150 to 300 million, giving an “inbreeding factor” of at least a million billion. But for all of us, it’s overwhelmingly likely that there was one ancestor from that time who would occur in all our family trees if we could trace them back that far. That’s amazing.