Actually this might an interesting read..I have been obsessed with this for a bit now..re why mammals nourish their young with their bodily secretions..: The Evolution of Milk Casein Genes from Tooth Genes before the Origin of Mammals : https://academic.oup.com/mbe/article/28/7/2053/1047188

And this:

http://www.bbc.com/earth/story/20150725-breastfeeding-has-an... : why did some animals evolve milk and breastfeeding