WHAT ARE THE ODDS OF YOUR BEING HERE? Some time when I was about ten I read a popular book about genetics. It included a chapter about human genetics. I accepted the idea of a sperm matching up with an egg, although on an abstract level. (I had no idea of the physical process involved in getting that sperm together with that egg). The book emphasized the low probability of any one person being born to one set of parents. The scale of the numbers made a big impression on me.

This post by Dan Falk on a PLOS blog elaborates on those numbers. Falk links to a probability chart posted by Ali Binazir. (Here is the chart.) Benazir works out the odds “that the right sperm from your father hooked up with the right egg produced by your mother – by his estimate, it’s about one chance in 400 quadrillion (that number seems only slightly more tame in scientific notation: 4 x 10^17).” This is the calculation that made the impression on me when I was ten. But I had missed the point that these odds also apply to all of my ancestors (my great grandfather would have had to have the same luck with his parents, for example. Including this in the calculation gives the odds of your existence as 10^2,685.000 (that’s ten with 2,685,000 zeros after it). And then, as Falk notes, there is the separate calculation for the probability of the appearance of intelligent life….