We may not like worrying, but, so the theory goes, there must be some survival advantage in it.

As the mutation confers high or almost complete resistance to kuru, carriers have a survival advantage and have had more descendants.

That lightning-fast evolutionary change suggests that direct milk consumption must have provided a serious survival advantage over peoples who had to ferment dairy into yogurt or cheese.

In the distant past, such an abrupt change may have had a survival advantage.

Eventually, should plenty become the norm, these genes will cease to provide any survival advantage and will become less common.