Who Are You? (And What Defines Your Boundaries, Exactly)?

"Abundant taxa in the human microbiome that have been metagenomically and taxonomically well defined in the HMP population." From the Nature article  Structure, Function and Diversity of the Healthy Human Microbiome.

Scientific American (in an article getting some attention online) links to and discusses several articles in Nature and PLoS One regarding the diversity of the humane biome. The articles are reports of ongoing studies of the Human Microbiome Project ("HMP") to catalog and assess the affect of the numerous microbes that live on and in our bodies. There are -- as should come as no surprise to anyone who paying attention -- trillions of microbes living on us and in us at any time. What is surprising is the diversity of microbes from person to person. There is no doubt, however, that we need many of them to survive (and share those species in common, though lifestyle and genetic factors may affect their prevalence and our health).
The amount and diversity of the genetic material of microbes that live on or in us far exceeds that in the human chromosomes -- "our" DNA. But what constitutes "our" DNA? Since many microbiotic cultures spend their entire existence within us -- arising early in our lives and dying when we die -- and since some are essential for us to live, are they not part of "us"? This is part of a larger question as to how well defined our physical boundaries are by natural processes; the question as to where the edge of each of us stops and the rest of the world begins.

Of course, some (but not all) microbes live independently from us and traffic into and out of us. Human cells can also be kept alive outside of the body; of course, that takes (from what we currently know) artificial conditions to maintain their life. The independence of some microbes from "us" suggests that are not "us." Microbes also contain separate DNA from that of "our" cells, but that is a circular reference; the tautological assumption is that "their" DNA is not in fact a complement of and part of a larger full set of "our" DNA (and so that "they" are not a part of "us"). Referring to different DNA does not necessarily answer the question as to what is human.

A more pressing distinction is that we are not conceived with our complement of microbes. They are acquired in a variety of ways during our lives. At least most are. But then there's mitochondria.

Mitochondria are now generally accepted to be captured symbiotic bacteria which are essential for us to live. They live in each of our cells and have their own DNA. They are part of us from conception, passing from mother to child as part of the ova. There is thus one form of bacteria -- one that is no longer independent -- that is arguably part of "us." It's a stretch to call mitochondria "microbes"; they are organelles within our cells.

So, too, it is current believed about 8% of our DNA comes from retroviruses. Indeed, the action of viruses may be a substantial source of mutation leading to evolution.

The historical roots or mitochondria and our DNA, however, does not mean that our bodies are not well bounded, that what is a part of "us" and what not is not well defined by nature, we simply need to recognize that we are not a tight collection of unique cells that has always been distinct from the world.

But in that lies, as well, the seeds of why we cannot understand ourselves as physically distinct. The movement of bacteria, viruses, fungi, protozoa, and on and on, into and out of us, and the intimate interchange of these organisms with our cells, prevents us from saying their is any definitive line between ourselves and the rest of the world. Where we end and the rest of the living world begins cannot be known at any moment.

This is true, as well, for our interaction with the inanimate world. We eat food and that becomes part of our living selves. Even if alive when consumed, we break it into its chemical constituents before incorporating it back into us. Likewise we breathe air -- an of incorporating inanimate matter into ourselves and making it part of ourselves. Likewise, we drink. And, likewise, we excrete and shed and perspire and exhale and give back an amount not too distant from what we taken in.

The constant interchange between our bodies and the world is not just true at the levels of organisms and compounds, but at the atomic and subatomic scales as well. We are, as all things are, engaged in a constant dance with the rest of the world trading electrons, photons, gluons ....

Two hundred and fifty years ago it might have been possible for even the very well educated to reasonably believe that humans stood apart from other animals and that each human was a unique entity standing apart from his or her peers and the physical world. This is no longer the case.

But why should it matter? It matters because any view of the world that depends on the physical distinctness of humanity or of any human is wrong. And yet those distinctions, unfortunately, are the foundations of many human beliefs.