Viviparity is widespread across the animal kingdom, and many groups evolved a means of nurturing the young via a placenta that forms an intimate contact with the uterus of the mother. This required many adaptations to allow the genetically foreign offspring remain in the uterus for the duration of pregnancy. However, only mammals extended the period of maternal protection longer, dependent on the amazing secretion that is milk. In eutherian mammals, this ranges from days and months to several years (elephant lactation 3-5years, !Kung Hunter Gatherers of the Kalahari, 4 years, in whom it acts as “Nature’s contraceptive”, and even up to 3-4 years in breastfeeding mothers in Melbourne. In contrast, marsupials deliver highly altricial young that complete their development during an extended lactation, producing the most complex and changing milk composition known for any mammal. In most mammals, apart from the immediate post-natal colostrum, the mature milk does not change in composition. In marsupials and also in monotremes, milk composition is an ever-changing process, and the milk is tailor made for each particular stages of the development of the young. Growth of the young is under maternal control via the milk, much as the placenta does in eutherians. Mismatching the stage of lactation with the age of the young can growth accelerate development, and maturation - and cause obesity. We now know that a number of genes expressed in the mammary glands in the tammar are the same as those that are functionally important in the placenta in eutherians. In marsupials lactation therefore controls post-natal development of the exteriorised fetus until it reaches independence with at least some of the same genes and molecular pathways as the eutherian placenta. Thus marsupials have truly exchanged the umbilical cord for the teat.