Every mammal produces milk only for its own young. The nutrients in the milk of every animal are suitable for the growth of its own species. Milks of animals that grow quickly have higher levels of protein. The human child grows till 18 years, so the rate of growth of a human being is much slower than that of most other animals. This is why human milk contains less protein than cow’s milk. We don’t need the extra protein nor the extra growth hormones which come to us through cow’s milk, even when we are children.