ABSTRACT
All eukaryotes harbor a diverse, complex ecosystem of viruses, archaea, bacteria, unicellular eukaryotes, fungi, and invertebrates called the microbiome. Every eukaryote species needs its microbiome for optimal health and function. The microbiome needs its host to survive. The microbes of the microbiome are almost exclusively symbiotic and commensal with each other and with the host. The microbiome is indispensable for digestion and absorption. The microbiome breaks down plant compounds, and, in ruminants, cellulose. It aids in the absorption of minerals, synthesizes vitamins, and produces short-chain fatty acids. It is essential for the development of the nervous system and brain function. Digestive tract microbes communicate with the host’s brain via the vagus nerve. The microbiome is essential for the development of its host’s immune system. The microbiome can pass genes to its host. The appendix evolved to provide a refuge for microbes. The microbiome was involved in the speciation of a wasp species. The holobiont, not the individual, is the unit of selection.
