Size and Smarts Aren’t Everything

Humans’ Real Best Friends

With all of our intellect, and as the pace of modern technology continues to skyrocket, it can be easy to forget that humans are animals. We have deliberately created ways that remove us from the day to day evolutionary struggles of our ancestors. I do not consider this to be a bad thing, but it can create a blind spot in our perceptions of the natural world as it relates to ourselves.

We consider human existence and human endeavors to be wholly separate from nature. Nature is something to be exploited or overcome. What use do humans have for mutualistic symbiosis? Other organisms exist for our benefit, or so the thinking might go (and sadly, I don’t have to travel far to see evidence of such thinking, the Everglades is a prime example). Of course, it is impossible to fully separate ourselves from the natural world, no matter how hard we try. The connections are too strong and some of them are not so easily severed.

Throughout the 200,000 year history of Homo sapiens, we have cultivated relationships with a variety of species. This has led to the various types of domestic pets (dogs, cats), livestock (cows, pigs, chickens), and crops (corn, wheat) that are such an integral part of our lives. However, I’m not going to focus on any of those relationships for now. It is theoretically possible that a human could become a hermit and distance him or herself from all of those organisms. But no matter how far you go, or how isolated you are, you cannot outrun your symbiotic relationships. Why? Because you are literally carrying them with you wherever you go.

Every hour of every day, there is an entire ecosystem of bacteria living in your digestive tract, sometimes as many as 1,000 different species. The environment of your stomach and intestines is low in oxygen, making it an excellent living and breeding ground for swaths of anaerobic bacteria (those that do not require oxygen). But don’t worry, they aren’t hitching a free ride. You are getting quite a lot of benefit from the colonies of bacteria living inside you. In fact, though you might be able to live without them, it wouldn’t be a particularly pleasant existence.

Much of the food that makes up your diet contains materials that your body cannot digest or absorb on its own. This is especially true for fruits, vegetables, and grains. All of these foods are high in ‘dietary fiber’, a catch-all term for a group of normally indigestible compounds. One of the more common compounds is called cellulose, which is the structural component of plant cell walls.

Cellulose is made from long chains of glucose (sugar) molecules, similar in many ways to starch. In fact, there is only a slight difference in the orientation of chemical groups between them. However, this difference alters the bonds that make up the chain, so that the human gut can break the bonds in starch, but not in cellulose.

This is where the bacteria come in. They possess metabolic processes that can break down those compounds. This sometimes creates byproducts that the human body can absorb. Both the bacteria and the host (i.e. us) get a meal. Each organism has slightly different gut flora, which is why humans can’t just get by on eating grass or leaves. We don’t have the right kinds of bacteria or digestive structure to make use of everything in plant fiber.

Intestinal bacteria can also help synthesize various vitamins and metabolize digestive byproducts, like bile, or outside toxins. All of these functions are vital for humans to flourish, and we wouldn’t be able to do them as well without the bacteria in our digestive tract. That is the reason for my title and subtitle to this story. All of our intelligence, all of our advancements, all of our technology, it is all made possible by the flourishing ecosystem of microscopic organisms that each of us carries around everywhere.

Bacteria often get a bad rap from people, and when we look into parasitism in a few weeks it will be easy to see why. However, eliminating every single bacterium from your body would do much more harm than good. There are beneficial symbiotic bacteria throughout the human body, but the gut has the most by far. Remember this the next time you feel disconnected from the natural world. Everything is connected in some way and just because you can’t always see the connections doesn’t mean they aren’t there.

Next week is the fourth and final mutualism story, after which we’ll move straight ahead into commensalism.