The tighter the integration, the more closely intertwined the fates of host and microbe become. For such holobionts, Bordenstein says, you can’t understand the evolution of either the host genome or the microbial genomes in isolation because the community of organisms as a whole shapes the traits of the individual. “We need to understand what the microbes make, what the host makes and potentially how those products work together,” he said. The holobiont, he argues, adds up to more than the sum of the host and microbes. Out of their interaction emerges a coherent entity that natural selection might act on alongside other units of selection, like the individual or a gene.

~ Jonathan Lambert from, https://www.quantamagazine.org/should-evolution-treat-our-microbes-as-part-of-us-20181120/

I’m am fascinated by the reality that there are countless microbes which coinhabit our body.