The long, slow process of carcinogenesis
Well, this is a surprise:"Using rates of mutation as a clock, some of them appear to go back even to before birth - the key JAK2V617F mutation, long associated with these malignancies, is estimated to have shown up anywhere from the 33rd week of gestation up to the age of 11. The DNMT3 mutation, similarly, seems to have appeared from the 8th week of gestation (!) out to about the age of 8."