GERSHOM MUGIRA COMES from a long line of cattle-keepers. His people, the Bahima, are thought to have migrated into the hilly grasslands of western Uganda more than a thousand years ago, alongside a hardy breed of longhorns known as the Ankole. For centuries, man and beast subsisted there in a tight symbiotic embrace.

In recent decades, global trade, sophisticated marketing, artificial insemination and the demands of agricultural economics have transformed the Holstein into the world’s predominant dairy breed. Indigenous animals like East Africa’s sinewy Ankole, the product of centuries of selection for traits adapted to harsh conditions, are struggling to compete with foreign imports bred for maximal production. This worries some scientists. The world’s food supply is increasingly dependent on a small and narrowing list of highly engineered breeds: the Holstein, the Large White pig and the Rhode Island Red and Leghorn chickens.