xkcd

Most of your body’s mass does not have a human genome, it represents other living things existing in symbiosis with your body. And your digestive tract is nearly 100% reliant on these microbiota to break down food and provide it to the small intestine. If you don’t have the right mix/balance or you have too many of the wrong species, you can suffer extremely deleterious health effects. If you have none at all, you starve pretty quickly regardless of how much food you eat.

Fun facts:

1. Almost all of your excrement that isn’t visible remnants of unchewed food are the remains of gut bacteria that died.
2. Scientists have recently confirmed that your appendix acts as a “safe room” for your good, beneficial gut biome to retreat to when the rest of the intestinal tract is suffering from catastrophic environmental issues or another bug is running rampant and dominating in a destructive manner. Once things calm down, the intestines are re-colonized by good bacteria from the appendix.