a, Number of stem cells and replacement rate of stem cells in normal human colonic crypts, as estimated by approximate Bayesian computation. Each point represents a simulation. Points are coloured according to their similarity to the observed data: the most similar 0.1% are coloured dark red, and so on, until the least similar simulations are blue. b, Approximate Bayesian computation of the rate of crypt fission (fissions per crypt per year) in the human colon. The prior distribution of the crypt fission rate (which was used to simulate many biopsies of the colon) is shown above, and the posterior distribution of the crypt fission rate (estimated by neural network regression on the simulations) is shown below. c, d, Evidence of crypt fusion in human colon. In each case, a phylogeny is shown at the top that depicts the genetic relationships between selected crypts. Dashed blue lines show mutations with a low allele fraction that are shared between crypts in a manner incompatible with the phylogeny dictated by the clonal mutations. Below each crypt in the phylogeny is an image that depicts its position in the section. Sections are labelled according to their z-stacked order. The allele fractions of mutations on each branch of the phylogeny in each crypt are shown at the bottom. The trinucleotide context of the mutations that occurred on each branch is shown on the right. See also Supplementary Information.