Take two cells in the present and follow their lineages back through time. There's a 100% chance that the lineages eventually meet at a common ancestor. How long ago does that happen?

In the continuum limit, each lineage is a Brownian motion with variance 1/4 per unit time. The lineages stay independent until they meet, so their difference is a Brownian motion with variance 1/2 per unit time. We’ve turned our question about common ancestors into a question about random walks: if a Brownian motion with variance 1/2 per unit time starts a distance \(r\) from zero, how long does it take to hit zero? The probability density of first hitting zero at time \(t\) turns out to be \[ \rho^\text{hit}_r(t) = \frac{r}{\sqrt{\pi}}\,t^{-3/2}\,e^{-r^2/t}. \]