Humanity's early ancestors had genetic variations of modern diseases. Why?

An article on Discovery News reports on a new study by Omer Gokcumen, assistant professor of biological sciences. The research shows that humanity’s ancestors may have evolved genetic variations associated with certain diseases in order to benefit health. “Our research shows that some genetic features associated with psoriasis, Crohn’s disease and other aspects of human health are ancient,” he said.

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http://news.discovery.com/human/genetics/itchy-skin-diarrhea-evolved-to-promote-good-health-150128.htm