Curiously, APOE e4 was not always a bad actor. For millions of years, all our post-primate ancestors were e4/e4. It was the original human allele.35 The e3 mutation showed up about 225,000 years ago, while e2 is a relative latecomer, arriving only in the last 10,000 years. Data from present-day populations with a high prevalence of e4 suggest that it may have been helpful for survival in environments with high levels of infectious disease: children carrying APOE e436 in Brazilian favelas are more resistant to diarrhea and have stronger cognitive development, for example. In environments where
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