Human milk contains an evolutionary puzzle: complex sugars that babies largely cannot digest. Why would a mother spend resources manufacturing food her infant cannot directly use?
The answer involves another hungry population—the microbes living inside the baby’s gut.
These compounds, called human milk oligosaccharides, nourish particular bacteria. Maternal investment may therefore reach a child indirectly, by supporting a microbial community that helps protect the developing intestine.
The strongest evidence comes from experiments revealing how bacteria use these sugars. A 2008 genome study identified specialized machinery in Bifidobacterium infantis for importing and breaking down milk oligosaccharides. A 2016 study went further in another species, Bifidobacterium breve, identifying the enzymes and metabolic pathways that allow it to consume two specific milk sugars. Genome study, Experimental study
Well supported: particular gut bacteria can consume milk sugars that infants cannot readily digest themselves. These sugars also have functions beyond feeding bacteria; some can interfere with pathogens attaching to intestinal surfaces. Scientific review
Plausible but still debated: precisely how natural selection shaped this partnership, and whether nourishing beneficial bacteria was the original advantage or one of several benefits. Demonstrating bacterial sugar consumption does not establish that every sugar improves infant survival.
The broader evolutionary idea is striking: parental care can extend beyond feeding a child’s own cells. A mother’s milk may also help cultivate the microscopic community sharing her child’s body.
