Milk is everywhere. From the dairy aisle at the grocery store to the explosive cover of the Mother’s Day issue of Time magazine , the ubiquity of milk makes it easy to take for granted. But surprisingly, milk synthesis is evolutionarily older than mammals. Milk is even older than dinosaurs. Moreover, milk contains constituents that infants don’t digest, namely oligosaccharides, which are the preferred diet of the neonate’s intestinal bacteria ( nom nom nom! ) And milk doesn’t just feed the infant, and the infant’s microbiome; the symbiotic bacteria are IN mother’s milk. Evolutionary Origins of Lactation The fossil record, unfortunately, leaves little direct evidence of the soft-tissue structures that first secreted milk. Despite this, paleontologists can scrutinize morphological features of fossils, such as the presence or absence of milk teeth ( diphyodonty ), to infer clues about the emergence of “milk.” Genome-wide surveys of the expression and function of mammary genes across dive
