The mammary gland has been a pivotal feature in the evolution and taxonomic classification of animal species, and it has even had a role in the acceptance of evolutionary theory. The presence and secretory capacity of the mammary gland provided the basis for the taxonomic grouping of species into the class Mammalia more than two centuries ago; and Darwin's explanation of how lactation may have evolved satisfied an early challenge to his theory of evolution by natural selection [1]. The challenge was that evolution of lactation was not feasible, because a neonate could not obtain a survival benefit from consuming the chance secretion of a rudimentary cutaneous gland. In response, Darwin hypothesized that mammary glands evolved from cutaneous glands that were contained within the brood pouches in which some fish and other marine species keep their eggs, and provided nourishment and thus a survival advantage to eggs of ancestral species. Two hundred years after Darwin's birth, the theory of evolution by natural selection remains a cornerstone of biology, as it has withstood this and other challenges. However, it is now clear that the mammary gland did not evolve from a brood pouch [1].

Milk nourishes the neonate and helps to establish immunological and endocrine competence in the offspring. The nutrient composition of milk varies dramatically across species, and it can also be strongly influenced by the stage of lactation. For example, the fat content of milk may be as high as 60% in seals and negligible during early lactation in wallabies [2, 3]. Furthermore, milk in the tammar wallaby (Macropus eugenii) changes from a very dilute secretion containing primarily carbohydrate during early lactation to a more energy-dense milk that contains substantial quantities of protein and fat during later phases of lactation. Thus, the details of lactation have evolved to meet the diverse reproductive and environmental demands of different species. About 10,000 years ago, the domestication of plant and animal species led to the Neolithic Revolution, with its changes in societal interactions and the evolution of civilization. Milk and dairy products were tightly coupled to this cultural evolution, and dairying (then and now) provides an important source of food and fiber throughout the world.

Sequencing and assembly of the bovine genome, establishment of mammary transcriptome and proteome libraries, the discovery of single nucleotide polymorphisms [4], and discoveries and developments to come, are providing important tools for agricultural scientists to investigate the biology of lactation and to adopt genotype-based breeding schemes to select for desired traits. Moreover, comparative genomic studies enable evaluation of lactation across numerous and diverse mammalian species. Regardless of the perceived target species of this research, such knowledge improves our understanding of mammary gland biology and is applicable to normal and pathological states. Danielle Lemay and colleagues, in a recent report in Genome Biology [5], have taken this important step towards greater understanding through comparative genomics.