The human infant gut is relatively sterile up until birth, where it takes up bacteria from its surrounding environment and its mother. The microbiota that makes up the infant gut differs from the adult gut. An infant reaches the adult stage of their microbiome at around three years of age, when their microbiome diversity increases, stabilizes, and the infant switches over to solid foods. Breast-fed infants are colonized earlier by Bifidobacterium when compared to babies that are primarily formula-fed. Bifidobacterium is the most common bacteria in the infant gut microbiome. There is more variability in genotypes over time in infants, making them less stable compared to the adult Bifidobacterium. Infants and children under three years old show low diversity in microbiome bacteria, but more diversity between individuals when compared to adults. Reduction of Bifidobacterium and increase in diversity of the infant gut microbiome occurs with less breast-milk intake and increase of solid food intake. Mammalian milk all contain oligosaccharides showing natural selection. Human milk oligosaccharides are not digested by enzymes and remain whole through the digestive tract before being broken down in the colon by microbiota. Bifidobacterium species genomes of B. longum, B. bifidum, B. breve contain genes that can hydrolyze some of the human milk oligosaccharides and these are found in higher numbers in infants that are breast-fed. Glycans that are produced by the humans are converted into food and energy for the B. bifidum. showing an example of coevolution.