Humans as an extended phenotype of their microbiota.

The human gastrointestinal system is colonized by microbiota, consisting of more than 100 trillion cells, which is essential for gastrointestinal development, nutrient absorption, and immune system function. Disruption of the microbiota has been linked to obesity, metabolic syndromes, inflammatory bowel disorders, and cancer. The microbiota has also been linked to nervous system development, and animal studies have suggested that the gut microbiota influences behavior of the host, with significant effects on host fitness. We hypothesize that at least some of these effects on host behavior, in part mediated by factors produced by the microbiota, have adaptive value for the microorganisms. There is evolutionary evidence of host-symbiont co-evolution; the microbiota may have evolved mechanisms to alter host behavior to enhance the fitness of the microbial species living within the host. In this sense, host behavior is an extended phenotype of its microbiota. Therefore, human behavior can also be interpreted as being an extended phenotype of the microbiota, which is a fundamental conclusion of this work. Analyzing the evolutionary tradeoffs with respect to the microbiota and its relationship with the host can inform optimized approaches to enhance host fitness. Such approaches can take advantage of the fitness "needs" of the microbiota as a tool to improve host health and well-being, thus decreasing human morbidity and mortality. We discuss methods for testing this hypothesis and consider its implications.