Figure 1 |. The effect of maternal status on the resident microbiota of the next generation.

We propose that, since the earliest days of the evolution of mammals, there has been major maternal transmission of microbiota to their offspring (vertical transmission). However, loss of the conserved microbiota in one generation leads to its loss in the next. For humans, until recently, horizontal microbial transmission also occurred and could compensate for the loss of vertical transmission. Members of the microbiota were horizontally transmitted through faecally contaminated drinking and bathing water, and high physical contact as a result of social crowding and large families; in many modern societies, these routes have diminished. The progressive loss of vertically transmitted microorganisms without horizontal replacement represents a cumulative birth cohort phenomenon.