Lewis Wolpert: Over the course of evolution, breasts became permanently enlarged to signal sexual receptivity

Even Darwin found it a very difficult problem: how could the evolution of breasts that give milk to the new-born have come about?

Even Darwin found it a very difficult problem: how could the evolution of breasts that give milk to the new-born have come about? He wrote: "If it could be demonstrated that any complex organ existed, which could not have been formed by numerous, successive, slight, modifications, my theory would absolutely break down." But whatever the small modifications in the lineage of mammalian ancestors that gave rise to lactation and the breasts were, they are far from easy to understand.

One important piece of evidence is the platypus, a monotreme mammal that has a patch on its breast that secretes milk for its infants to suck. This was first described in Australia by Richard Owen in 1832, and while Darwin had seen a platypus he did not know that it laid shelled eggs. Clearly, the platypus was an early stage in breast evolution.

Numerous theories have been proposed for how lactation evolved in the platypus. One theory, more than a hundred years old, suggests that the glands secreting nourishment in the platypus are modified sweat glands, but that the lactating glands in other mammals are modified sebaceous glands, which normally secrete an oily fluid to protect the skin. In the 1960s, JBS Haldane took up the problem and proposed that the ancestors of monotreme mammals might have needed to keep their eggs cool, and so evolved a mechanism for moistening them in their fur, in a manner similar to that used by some Asian birds who moisten their feathers. More recently a theory has emerged in which multiple glands of the skin are involved.

The ancestral line of animals that gave rise to the mammals were lizard-like, go back some 130 million years, and are called synapsids. The evolution of mammals involved many changes in the body, such as the development of a single bone in the lower jaw, diversified teeth, a more upright stance, and a secondary palate. But where in the fossil record is there evidence for lactation? One piece of evidence comes from bones that project forward from the lower waist and are assumed to have evolved to provide support for young in the pouch of marsupials like the kangaroo, as well as in the platypus, which do not have a pouch but whose young need support. The early pouch and breast may have initially provided moisture to the eggs of egg-laying ancestors.

The skin of the ancestral synapsids needed to keep water in at the same time as keeping the skin moist, and so probably contained glands for both watery and oily secretion.

Mammary glands are thought to have evolved from some mosaic of the different glands in our ancestors. The development of the mammary gland in the embryo of monotremes and marsupials has some similarities to that of other mammals, but there are significant differences and the reasons are not understood.

Another aspect of lactation relates to its role in the immune system. Breast milk contains a variety of proteins that are important for the functioning of the immune system, and it is suggested that these factors compensate for the slow development in the developing foetus, and in early postnatal life, of the production of protection against microbial infections. This has important implications for the health of infants.

Finally, why are breasts so important in the sexual life of humans? Why is their size so crucial for sexual self-esteem? It must be related to sexual intercourse changing from the male mating from the rear to front-to-front contact. Some anthropologists have thus proposed that the swell of female breasts is equivalent to the swollen fertile buttocks of our primate ancestors.

Human female breasts may have become permanently enlarged secondary to sexual selection, as they potentially resembled the double ovoid region of the distended buttocks, which in turn are employed to signal appeasement as well as sexuality in primates.

Over the course of evolution, the breasts became permanently enlarged, and thus a permanent sex symbol to signal that the human female was continuously sexually receptive. Evolution seems to have a curious sense of humour.

Professor Wolpert is professor of biology as applied to medicine at University College London