Common Alzheimer's Disease Gene May Have Helped Our Ancestors Have More Kids

upstart writes:

Study of traditional society in Amazon suggests why evolution hasn't purged harmful variant:

Roughly one in five people are born with at least one copy of a gene variant called APOE4 that makes them more prone to heart disease and Alzheimer's disease in old age. That the variant is so common poses an evolutionary mystery: If it decreases our fitness, why hasn't APOE4 been purged from the human population over time?

Now, a study of nearly 800 women in a traditional society in the Amazon finds that those with the disease-promoting variant had slightly more children. Such a fertility benefit may have allowed the gene to persist during human evolution despite its harmful effects for older people today.

[...] The APOE gene encodes apolipoprotein E, a molecule that helps the body transport cholesterol in the blood. There are three main variants and people can inherit a mix from their parents, with the one called APOE3 being much more common than APOE4. In populations of European ancestry, having one copy of APOE4 raises a person's risk of cardiovascular disease and triples their odds of developing Alzheimer's disease; those with two copies face a 12-fold or higher risk of the brain condition.

[...] University of Copenhagen epidemiologist Rudolf Westendorp notes that his team, which saw a similar result in a Ghanaian population, has also observed such a trade-off in families with another cholesterol-related gene variant that raises heart disease risks: In the 19th century when many people died from infections, carriers actually lived longer. "In the past, carriers of that gene had a survival benefit, which explains why the variants are present nowadays," he says.

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