We are all split. From the moment a haploid sperm cell meets a haploid egg, the resulting diploid’s very existence depends on its being able to divide. For the first act of creation, a rift is formed in cellular DNA: the double helix is ripped apart to be copied by protein machinery, mended, only to be ripped apart again and again and again. This tearing asunder produces cell after cell, and a few days into gestation, structures in the growing number of cells begin to emerge: brain, heart, lungs. But what forms first and foremost is the structure for that hollow tube which perpetually splits us from mouth to anus; our internal negative space that serves as a vital mediator to our world: the gut.

This seemingly simple tube (site of some of our more embarrassing moments) is the ground zero of most of our interactions with the outside world. As such, it is organized in complex structures that rival the landscape of a river-carved canyon. These structures facilitate interactions not just between us and the food we eat but between us and other species. The gut is the first outcome in multicellular evolution. It is our center. Before animals get brains and hearts, they get guts. And with guts comes an entire ecosystem.

We do not simply exist in an ecosystem. We are an ecosystem. A breathing landscape inhabited and shaped by both microbial and human cells as well as viruses. We actually have more minuscule inhabitants than our own cells: ten microbial cells and at least 100 viruses for every human cell–roughly we are ten percent “human.” Our microbial and viral associates shape us: they help us digest our food, educate our immune system, and regulate hormones and brain activity (to name a few). And at the end of our life, these microbes will happily at us from the inside out.

We are walking worlds, teeming with diverse life.

When talking about the uniqueness of an individual, it is the inside that counts. We all have a microbial fingerprint composed of specific species that is unique to you and you alone–even identical twins don’t share microbial fingerprints.

Think about it this way: everyone has a box of crayons (these are your microbial species). In everyone’s box, there are about 160 different colors (types of species). And 57 of those colors are shared by all of us here. However, the number of crayons of the shared colors differs among individuals; while we all have the color red in our boxes. I might have three red crayons and you might have 300. The colors of the crayons and the amounts those colors (variations in microbial populations on the species level) is my microbial fingerprint.

Exactly how ecosystems in the larger world rely on a vast reservoir of organisms to enable that system to endure despite any fluctuations or stress, the health of our human ecosystem also hinges on the variety of organisms (which for microbes can be reduced to genetic diversity), filling a myriad of ecological niches in our guts.

Scientists speculate that for many reasons (coughantibioticscough), we are rapidly losing diversity with each successive generation. So, the humans who painted the cave pictures we saw today most likely housed a greater variety of microbes in their guts than we do, and ours will house many more than our great-grandchildren.

However, “diversity” is a tricky term. It sounds great, but we need to be careful in focusing on a specific microbial profile as an across the board indicator of human health (another tricky term). Remember, we each possess a unique microbial fingerprint, and that uniqueness needs to be taken into account. Noted plant ecologist, Charles Peters* reminds us that “maintaining biodiversity is less important than maintaining a functioning ecosystem.” A functioning human ecosystem should be our goal.

Understanding the human species as an ecosystem and how diversity plays into that system is vital to the future of human and global health. We need to keep Peters’ words in mind as we return to our gut instincts.

This return requires four calls to action. First, we need to revise our view of what it means to be human to include our microbial inhabitants–we are complex and unique ecosystems. Second, we need to understand our source code–our DNA as well as our microbial and viral DNA–and how that code is read and modified. Third, we need to consider how what we eat contributes to changes in our source code and thus ourselves. Finally, we need to empower ourselves and others to become citizen scientists who take control of their personal health.

*Quote taken from Alan Weisman’s The World Without Us