5 min read Feb 14, 2026
We’re Not Just “Us”: A Human as an Ecosystem of Trillions of Cells
When you think of yourself as a person, you picture a single whole: an organism, a personality, a body, a mind. The “I” feels like a single point inside a single shell. And yet, under that everyday certainty, there’s a quiet paradox: what we experience as one is assembled from an unimaginable multitude — processes that don’t “know” the story they are part of. Somehow, out of this grand crowd, something coherent emerges and says: I.
Modern science has pinned down, with surprising precision, how many cells make up the human body. There are over thirty trillion of them in an adult. Skin and liver cells, immune cells and bone marrow, neurons and blood cells, smooth muscle fibers and epithelial tissues. Each lives at a local scale. It divides, works, dies, gets replaced, and reacts to signals that reach it from its immediate surroundings. For a cell, there is no organism as a whole — no biography, no name, no plans. Its world is a tiny neighborhood: chemical gradients, contact signals, hormonal hints.
Calling these “lives” can sound poetic, and it is — but in a specific sense. Not “lives” as in separate little beings with their own destinies, but as autonomous streams of work: metabolism, repair, signaling, division, cleanup. A cell is not a person. It’s a self-sustaining process that only needs its nearest context to keep going.
And there are other inhabitants inside us too — cells that are not human. Around thirty-eight trillion microorganisms: bacteria living on the skin, in the airways, in the mouth, and above all in the gut. For all their numbers, they weigh very little — just a few hundred grams. Yet in count they are comparable to our own cells, and in diversity and function they behave like something close to an organ with its own jobs. They help digest food, train and tune immunity, protect us from pathogens, influence metabolism, and — through chemistry and nerves — can shift mood and mental state.
You’ll still hear the old myth that there are ten times more bacteria, archaea, and fungi in us than human cells. By modern recounts, that isn’t true. The ratio is closer to one-to-one, with a slight edge to our microscopic companions. Human cells are larger, more complex, and overwhelmingly dominate the body by mass. But by sheer number, we are literally half human and half microbial community.
That thought is unsettling, because the body that feels unified, gathered, personal is, in fact, an alliance of two worlds: human cells carrying our DNA and a vast number of “passengers,” foreign organisms living inside us without our awareness. They don’t know they are part of a human — just as the cells of our gut or liver don’t know they labor inside a person with a name, a history, a character. Each exists at its own level of complexity, responding only to the nearest cues. And yet this unimaginable multicellular democracy — this quiet microscopic city with trillions of residents — gives rise to a stable being that can think about stars, generate ideas, and reason about life and death.
And here is what we usually miss. This city isn’t separated from the outside world by a wall. It lives behind membranes — boundaries that do not merely block, but regulate exchange. A human being is not a sealed container but a living system with ports, filters, gates, and checkpoints through which the biosphere continuously flows. Air, water, food, dust, spores, microbes, molecules, ions, chemical messages — everything comes in, gets processed, temporarily becomes part of us, and flows back out again. The biosphere runs through a person like a living reactor: some things are retained and built in, some are broken down and excreted, some become raw material for new structures.
In that sense the microbiome is not simply “a set of bacteria inside.” It’s a symbiosis between the cells of the human body and microorganisms arriving from the environment in a continuous stream. A child is born with minimal maternal microbial company and is then rapidly colonized by the surrounding world — through contact with air, water, food, objects, and other people. After that, the microbial makeup doesn’t just “live in us.” It is dynamically maintained. Some species disappear, others arrive, others shift, because the environment shifts, diet shifts, immune tuning shifts, lifestyle shifts.
The “human” part is just as fluid. It feels as if the body is permanence, but the organism holds itself together at the price of constant replacement. Some cells die and are carried away; others are born. And that birth is always tied to an influx from outside. The building material for new cells comes with air, water, and food — with the chemical elements the biosphere feeds into our metabolism. Cell turnover is also a flow. We don’t simply consist of matter. We hold a form while matter passes through it, as long as there is energy and supply.
Sometimes you want to ask it bluntly. Is there at least one cell in a human being that lives a whole lifetime, from birth to death? There isn’t a neat universal yes-or-no, because different tissues renew themselves differently. Much in us truly changes constantly — blood, epithelium, and more. But there are also long-lived cells that can persist for a very long time. The important point is different: even where a cell lives for decades, it still exists in the stream. It constantly renews molecules, repairs structures, replaces components. A “permanent cell” does not mean an “unchanging” one.
So where, then, is the “center”? The tempting picture is a commander, a single control room. But living systems rarely work that way. Coherence is built from networks: signals, feedback loops, constraints, local rules that add up to global order. No individual cell “understands” the organism. Yet the organism holds, like a melody emerging from an orchestra — no single instrument contains the whole piece, but together they produce something you can recognize, follow, remember.
The human organism is not a body in the usual sense but a community, an equilibrium, a dialogue between biological worlds. We are a biosphere in miniature, an ecosystem built from our own cells and those that arrived from outside. And only in the interaction of these two parts does a person arise — a biological organism that is at once a whole and a multitude.
From this follows a natural, almost inevitable conclusion: a person cannot exist without the biosphere. The biosphere assembles us and supports us every minute. That’s why any “closed biosphere,” if you mean it as an attempt to cut a human off from the living world once and for all — to stop or severely limit exchange and flow — will sooner or later run into limits. Cycles will falter, fine interdependencies will be depleted, and errors will accumulate — errors that in an open biosphere are smoothed out by the larger circulation of matter and life.
And now, let’s try not to think about it. Let’s get up — this whole busy, anxious crowd — and head… fishing.
Well… just to have some time alone… somehow. Or…?
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