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A study has found that existing genetic data cannot explain why sections of the world's population can digest milk. The ability to absorb the milk sugar lactose - also known as lactase persistence - emerged about 7,500 years ago in Europe. Once weaned, people in most parts of the world - large parts of Africa, most of Asia and Oceania - cannot digest milk for the rest of their lives. A study led by scientists at University College London shows that the four genetic mutations associated with the ability to digest milk fail to explain why many people can do so. It suggests that other genetic variants leading to lactase persistence exist but have yet to be discovered.