The very best first food for human infants is human milk.
Breast milk supplies the needed nutrients in appropriate amounts and balance.
This is because the milk produced be every mammal is designed to meet the needs of that particular mammal. Human milk contains less protein and minerals than cow's milk because human babies grow more slowly. Further, the mix of amino acids in human milk is different from cow's milk. The fat content in human milk is similar in amount to cow's milk, but it contains different kinds of fatty acids and lipid components. These differences are thought to be important for the developing human baby.