From Origins of Breastfeeding to Lactose Intolerance

To say that humans are mammals is not simply an arcane statement about classifying us in the biological scheme of things. The message is far more profound: We have all of the key biological features that set mammals apart from other animals. Two of them, hair and suckling, are plainly evident. What is not so obvious is that hair and suckling are linked.

Let’s take hair first. As a rule, mammals have hair, although it has been reduced or lost in some as a secondary development. Aquatic mammals such as dolphins and manatees, for instance, are often almost bare. Humans also count among the exceptions, as famously proclaimed in the title of Desmond Morris’s 1967 best seller The Naked Ape.

Direct evidence that early mammals had hair comes from rare fossils with impressions of body outlines. We know for sure that mammals had fur coats by 170 million years ago. And indirect evidence suggests that hair evolved even earlier, over 200 million years ago. Before the first mammals appeared in the fossil record, advanced mammal-like reptiles had pits on their snouts for whiskers, which are specialized tactile hairs.

Because living mammals typically have hair, some classifications once used the name Pilosa (from the Latin pilus for hair) for the entire group. But the other striking feature of mammals, suckling, is even more fundamental. Without exception, female mammals produce milk to feed their infants by suckling, and their common name is in fact derived from the Latin mamma for teat. Modern classifications began with Linnaeus, who chose the label Mammalia over Pilosa. Interestingly, science historian Londa Schiebinger discovered that Linnaeus actively campaigned to encourage Swedish women to breastfeed their babies; the emphasis on suckling in his groundbreaking classification was more political than biological.

Both hair and suckling are more basic and ancient than live birth (vivipary). Among modern mammals, a few unusual representatives known as monotremes—platypuses and spiny anteaters—still lay eggs; but they have hair and suckle their infants. As all modern mammals suckle infants, it seems highly likely that their common ancestor already did so. Live birth doubtless evolved later, after the monotremes had branched away, originating somewhere between the origin of all mammals and the common ancestor that gave rise to marsupials and placentals, around 150 million years ago.

But hair and suckling share more than their ancient origin. Different kinds of skin glands evolved along with hair. Biologists recognize three basic types: sweat-producing eccrine glands, scent-producing apocrine glands, and oil-producing sebaceous glands. Milk-producing glands of ancestral mammals most likely evolved from sebaceous glands. Because the oily secretions of these glands help maintain fur condition, they are directly connected to hair follicles. Originally, milk-producing glands were also connected with hair follicles, providing a clue to their origin.

In ancestral mammals, skin glands producing moist secretions were gradually converted to mammary glands yielding milk containing a mixture of nutrients and antibiotics. We tend to think that milk is simply a source of infant nourishment. That can lead to the mistaken belief that artificial milks only need to deliver the appropriate nutrients. In fact, antibiotics contained in a mother’s milk provide the baby’s first line of defense against germs.

However, human children commonly stop producing lactase—an enzyme that helps in digestion of lactose—by about five years of age. Since mammals do not normally consume milk after weaning, it is hardly surprising that many people can’t tolerate lactose as adults. Yet, as the tradition of consuming dairy products became routine in some societies, secondary adaptation occurred, allowing adults to digest lactose. As a result, there is marked variation between human populations. For instance, compared to Asia, where lactose intolerance in adults is predominant, the level in Europe is quite low. Tolerance of lactose arises through evolutionary modification of the control mechanism of the lactase gene so that it remains active after weaning. In fact, this change occurred independently, with different genetic modifications, in central Europe and in northern Africa. Combined archeological and genetic evidence suggests that lactose tolerance in adults originated about 7,500 years ago in dairy-farming communities in central Europe. In Africa, the picture is more complex: Four different genetic modifications permitting lactase persistence have already been identified, and there are probably more. Most of the changes originated separately in Africa thousands of years ago, but the typical European modification is also found there. In any event, in both North Africa and Europe it is undeniable that the evolution of lactose tolerance has yielded distinct social benefits.

His latest book, How We Do It: The Evolution and Future of Human Reproduction, will be published on June 11.

References

Gibbons, A. (2006) There’s more than one way to have your milk and drink it, too. Science 314, 1672.

Itan, Y., Powell, A., Beaumont, M.A., Burger, J. & Thomas, M.G. (2009) The origins of lactase persistence in Europe. PLoS Comput. Biol. 5(8): e1000491. doi:10.1371/journal.pcbi.1000491, 1-13.

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Lönnerdal, B. (2000) Breast milk: a truly functional food. Nutrition 16, 509–511.

Newman, J. (1995) How breast milk protects newborns. Sci. Am. 273(12), 76-79.

Oftedal, O.T. (2002) The mammary gland and its origin during synapsid evolution. J. Mammary Gland Biol. Neoplas. 7, 225-252.

Schiebinger, L. 1993 Why mammals are called mammals: Gender politics in eighteenth-century natural history. Am. Hist. Rev. 98, 382-411.