Every person experiences roughly one million mutations throughout the body per second, and in a rapidly proliferating organ like the intestinal epithelium, nearly every single letter of the genome will have been mutated at least once in at least one cell by the time an individual turns sixty. This mutational process begins from the earliest moment of fertilization, and as the single-cell zygote goes on to divide into two cells, then four, then eight cells of the growing embryo, the new mutations it has acquired
