In a recent lancet article, Rook et al examine our place within the microbial world. They conclude that it amounts to a tug-of-war between managing beneficial microbes, excluding harmful ones, and diverting as much energy as available into other essential functions. This tug-of-war shapes our life story - how fast we grow, when we mature, and how long we live.
Since the evolution of life, the biosphere has been dominated by Bacteria. Much of the human genome originated in microbes. Today, all vertebrates harbour large communities of microbes, and at least 20% of small molecules in human blood are products of this microbiota. Changing human lifestyles and medical practices are disturbing the content and diversity of the microbiota, and simultaneously reducing our exposures to organisms from the natural environment with which human beings coevolved.
Meanwhile, population growth and crowding is increasing our exposure to novel pathogens, particularly the “crowd infections” that were not part of our evolutionary history. Thus some microbes have co-evolved with human beings and play crucial roles in our physiology and metabolism, whereas others are entirely intrusive.
Rook G, Bäckhed F, Levin BR, McFall-Ngai MJ, McLean AR. Evolution, human-microbe interactions, and life history plasticity. The Lancet. 2017 Jul 29;390(10093):521-30.