The curd sets only when we add a little curd to the milk , then how was curd found for the first time?

There are several stories regarding discovery of curd but the fact is that curd was prepared for the first time by natural curding of milk. Milk was left in warm climatic condition for whole night. Next morning the milk turned into thick, tangy, lumpy matter called curd. Due to warm climate and proper food source (milk) bacteria grew up in the milk in large amount and converted milk into curd.