The Earth is approximately 4.54 billion years old. Evidence from microfossils suggests life has existed on Earth for at least 3.8 billion years, and possibly longer. During that time life has spread to every crack and corner of the planet, creating a “biosphere” that extends 47 miles up into the atmosphere, and at least 12 miles below the surface.

For the vast majority of those 3.8 billion years, microbes were the only living things. These tiny factories of life were so ubiquitous that over time they sculpted the atmosphere and terrain that Homo Sapiens was born into 300,000 years ago. Even today, the vast majority of life forms on Earth are microbes. They invisibly permeate the fabric of our existence. That interleaving raises a question biologists still struggle to answer: what is an individual?

Microbes are single celled organisms, distinguished by their diminutive size. They were life’s pioneers, adapting over millions of years to every habitat on the planet. Microbes exist in all three domains (bacteria, archaea, eukaryota) within the “tree of life” described by microbiologist Carl Woese et al circa 1990. The eukaryota domain contains all animals, plants, fungi and alga. The visible organisms that we are most familiar with - reptiles, birds, fish and mammals - are actually recent additions to life on Earth and microbes are their progenitors.

Microbes have also colonized every creature on the planet. Like every animal and plant, each of us hosts a menagerie of microscopic communities. We collect microbes continually as we go about our business. Communities of them - microbiota - live on our skin, in our ears, our eyes, mouth, hair and gut. Each of these multi-species communities is an ecosystem unto itself, a microbiome. Just as ecosystems on Earth each attract particular species, so too does each host-based ecosystem have its own characteristic community of microbes.

The relationship between microbes and humans is fraught with misinformation. Some microbes are intrinsically parasitic (the microbe benefits from the relationship but the host is harmed), some are mutualistic (both the microbe and the host benefit from the relationship) and most are commensal (the microbe benefits; the host is neither helped nor harmed). Pathogens - microbes that cause diseases in humans - dominate the headlines. Media scares have convinced many people that all microbes are germs, but the truth is that the majority of them are not. There are fewer than 100 species of microbes that cause infectious diseases in people, thousands that simply hitchhike unnoticeably within us, and several dozen that perform services that we could not live without.

Microbiome health influences host health. Beneficial microbes help to digest our food. They produce vitamins and minerals that are missing from our diet. They break down toxins and hazardous chemicals. They outcompete and even attack more dangerous microbes.

As microbiome conditions fluctuate, population shifts occur in the community, and those changes can profoundly impact host health. Powerful links between the gut microbiome and depression have been discovered. That microbiome has been shown to exert such a strong influence on the mind that scientists now metaphorically refer to it as the “gut brain”.

Biologists and philosophers have long struggled to define what “individual” means in biological terms. Some argue that it is crucial to have a working definition because Darwin’s theory of evolution by natural selection depends on the differential in survival and reproduction between individuals. Is a worker ant, incapable of reproducing on its own, an individual? Or does the colony qualify as an individual because it functions as a superorganism? What about our friend the slime mold, which lives part time as a collective creature akin to a slug and the rest of its life as single celled amoeba?

Based on mass, the largest living creature on Earth is a stand of aspen in Utah’s Fishlake National Forest. The grove weighs an estimated 13 million pounds and is roughly 80,000 years old. Every “tree” is actually a clonal eruption from the root system - genetically identical to its neighbors - so the entire grove is actually an individual.

Humans exist in symbiosis, every one of us a walking zoo. In TED talks it is said that the average person contains 10 microbial cells for every human one - making us a minority in our own bodies. Recent research has refined these estimates: adults have roughly 30 trillion human cells and 39 trillion microbial ones. Regardless of which ratio you use, none of us is biologically singular. Perhaps like ants, bees and termites it would be more accurate to describe human beings as superorganisms. In light of the pronoun changes proposed by transgender advocates, one can make a case for the use of “We” instead of “I” to properly describe the multitude that resides within each of us.