Milk is a product of evolution designed specifically for the nutrition of infant mammals. It bridges the nutritional gap between intrauterine dependence and extrauterine independence. The same nutrients are present in the milk of all species, although in different proportions. Such quantitative differences appear to be an adaptation to the nutritive requirements of the young of each species.
At the beginning of this century, Bunge1 postulated that the chemical composition of the milk of each species was correlated with the composition of the newborn. Such a correlation, if any, is not apparent. At about the same time, Abderhalden2 suggested a parallel between milk composition and maturation rate of the infant mammal. However, Brody3 noted that the relationship "is complicated by many factors, evolutionary and physiologic. The situation is too complex for a simple generalization."
In spite of marked quantitative differences in the milk composition of different species,
Williams HH. Differences Between Cow's and Human Milk. JAMA. 1961;175(2):104–107. doi:10.1001/jama.1961.63040020005006a