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The Human Biome

Humans like to think of themselves as self-determined individuals who make their own decisions on their fate. Even when humans lose control over their minds — in anesthesia, or under the influence of hallucinogenic or psychotic drugs — humans think of it as a consequence of their own choosing.

But the idea of human self-determination is grossly overstretching scientific data.

Jean-Paul Sartre, a famous humanist, reasoned that you can always make something out of what you are made into.

Just don’t be overly optimistic about the extent to which you are the master of your destiny.

The competing forces that pull you in every direction, every second of your life, are multiple: your genes and your upbringing, the Zeitgeist and your peers, and the condition of the biosphere at your geography.

But wait a moment — we’re not yet done curtailing your freedom of choice.

Because there isn’t just one human biosphere — the external one. Humans, each of us, are also a biosphere in ourselves. This environment is then called “biome”.

Each of us is host to a greater number of viruses and bacteria than there are humans on the planet.

And it’s not that our symbiotic bacteria are there to help us digest our food. They are there for their own agenda.

Sure, there are also fungi, protozoa, and other organisms commonly called parasites. But let’s just focus on viruses first.

Viruses have a very fast evolutionary pace indeed. They can go through many generations in a matter of hours. They multiply like crazy. Viruses go viral, so to speak.

And they adapt to us, their host, faster than we adapt to them. “HIV-1... has a very high virion clearance rate and short generation time,” writes Perelson et al. (1996). Meaning: they outmaneuver us inside our own bloodstream.

It’s generally not the intention of viruses to kill their hosts. “A virus that kills its host too quickly is like a parasite that burns its only bridge,” says evolutionary biologist Paul Ewald (2000).

Do we adapt to viruses or do viruses adapt to us? Fact is, old viruses — like herpes — often don’t kill, and can accommodate themselves in a human host for decades.

Newer viruses, like HIV, are often more virulent. They haven’t learned to live with us — yet.

Viruses optimize to exist in the largest possible number, and for as long as possible.

For this purpose, viruses modify a large range of behaviors of their hosts.

It’s no accidental coincidence that rabies-infected dogs salivate excessively and bite randomly. These are symptoms and behaviors induced by the virus — evolutionary strategies to ensure its spread.

It’s not our immune system that makes us sneeze when carrying an influenza virus. It’s a cascade of reactions, orchestrated by the virus itself, through sure-fire physiological modifications in the host.

And how about herpes viruses that spread by genital contact? Such viruses have a vital interest in a host’s sexual promiscuity, even into old age.

So — a good herpes strain as a cure for low libido and erectile dysfunction?

You can bet on it. It may sound wild, but “the impact of the gut microbiota on brain and behaviour” has already been demonstrated (Cryan & Dinan, 2012). If gut bugs can tweak your mood, viruses might one day tweak your libido.

And it won’t stop there.

Humans will farm — or was it “pharm”? — viruses as treatments for all kinds of maladies. And if there will ever be medications that expand the human lifespan to hundreds of years, viruses, and viral vectors, will have a crucial role to play.

The human biome isn’t just a support system. It’s part of the control room. And we’re only just beginning to understand who — or what — is really running the show.