Why did 40% of the human population evolve to tolerate lactose?

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Explanation:

Primary lactose intolerance is the most common. It is caused by a decrease in lactase production with age, so that lactose becomes poorly absorbed ( 5 ). This form of lactose intolerance may be partially caused by genes, because it's more common in some populations than others

Explanation:

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colors do exist. the color of the sky is actually violet; humans percieve the color of the sky as blue.