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Historians believe that some 8,000 years ago a natural genetic mutation occurred in European cattle breeds. Prior to that, cows milk contained only the A2 protein.​ But why it occurred is open to speculation. This mutation has been studied and found to have not occurred in Indian or African cows. This also holds true for other species such a goats, sheep, and buffalo. 

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In some cattle breeds the A1 protein is more prevalent such as Holsteins versus Guernsey, as an example. ​With the majority of the milk consumed in the United States coming from cows, this is why this is a concern...however, genetic repair through selective breeding is possible and has been done. But even with that said, these cows exist today but as is common in milk production cows are mixed in with others thus rendering milk to contain a mixture of A1 and A2 versus being pure A2.

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Of note: Human breast, goat, sheep, and water buffalo milks all contain only A2-like proteins. 

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