Sunday, June 14, 2020

Why do people touch each other all the time? Sex among holobionts

Nowadays, we are encouraged to exterminate our skin microbiome by means of various poisonous substances. But this is not a good idea. We are holobionts, and our microbiome is part of us. If we kill the microbiome, we kill ourselves. Touching each other is a way to keep our microbiome alive, it is a form of sex ("holosex") intended as a form of communication.  The lady in this picture seems to understand the point, at least judging from her unhappy expression. (see also the "proud holobionts" group on Facebook)

Humans tend to touch each other. They hug, pat, rub, kiss, cuddle, clutch, caress, clasp, embrace, each other a lot. Think of the kissing habits ("la bise") that's typical of the French society, it is done also in Italy and in other Latin countries. In most societies (*), at least some kind of skin contact is supposed to be a sign of reciprocal trust and confidence.

But, today, we are seeing a completely different pattern diffusing all over the world. With the coronavirus epidemic, people are not shaking hands anymore, to say nothing about kissing and hugging each other. Not only people don't want to touch other people, but they are also positively scared of getting close to each other. It is called "social distancing" and it involves a series of ritualized behaviors of dubious efficacy against the epidemic that include wearing face masks, sanitizing one's hands, spraying disinfectants all over people and things, raising plexiglass barriers, and more.

So, what's happening? Is social distancing just a temporary need or something that will last in the future? The answer depends on whether skin contact is useful for something: if it is not, we could as well abandon it, apart from strictly reproductive needs. But why do people touch each other? For one thing, we may be reasonably sure that if touching each other were harmful to us, natural selection would have eliminated this behavior from our genetic pool and from our cultural habits. On the contrary, touching each other has positive advantages. It is because we are all holobionts.

Let me explain: I am a holobiont, you are a holobiont, all the living creatures surrounding you are holobionts. The term is a little abstruse and still scarcely known, but it has been making spectacular inroads in biology from when it was proposed by Lynn Margulis in 1991. You probably heard of Margulis as the co-developer with James Lovelock of the concept of "Gaia" as the control system of Earth's ecosphere. And, yes, Gaia is a holobiont, too!

So, what is a holobiont? It is a community of living beings that share food, shelter, resources, and protect each other. A tree is a holobiont, a forest is a holobiont, a coral reef is a holobiont, your dog is a holobiont. And, as I said, as a human being you are a holobiont. You are an entity formed of a human organism and a large microbiome formed by a complete ecosystem of microorganisms living on your skin, colonizing your gut, helping various hormonal syntheses more or less everywhere in your body. Without a microbiome, you wouldn't survive for long, although you may eke a precarious existence with a reduced set of the full-fledged version.

And, as a holobiont, you are having sex all the time with other holobionts (and, yes, with your dog, too!). That needs to be explained as part of the great fascination with the concept of holobiont. We are starting to develop a definition of "sex" that goes beyond the conventional one. In our case, as humans, we think of sex as the exchange of genetic material between a male and a female of the same species (actually, we also practice non-reproduction oriented varieties of sex, but that's another form of communication). The result of reproductive sex is meiosis and a new individual with a mixed genome. It is called also "vertical sex," meaning that genetic material is transferred from parents to offspring.

In contrast, the horizontal gene transfer is the movement of genetic material from a donor organism to a recipient organism that is not its offspring. Bacteria, by far the most common lifeform on Earth, exchange genetic material simply passing it through their membranes, a mechanism called "conjugation." And viruses are great genetic exchange machines: they are packages of DNA and RNA that move from a host to another.

A holobiont is, as they say, another kettle of bacterial culture. It is formed of an ensemble of organisms, so it doesn't have a proper genome. But it has a hologenome, the ensemble of the genomes of the organisms that compose it. The hologenome has the same meaning of the genome, it is the "blueprint," so to say, of the holobiont. And, since holobionts are living creatures, they are born and die. So, the hologenome must be transmitted from one to the other. It is the transmission of constitutive information. It is a kind of sex that we may call "holosex."

By means of holosex, holobionts transmit hologenetic information from an individual to another. It is in this way that evolution occurs: "bad" holobionts, meaning those which are unstable, or unable to ensure the survival of the organism, are de-selected and disappear. It is a form of natural selection, not exactly in the neo-Darwinian sense, it has a certain degree of "Lamarckian" transmission of information. If a holobiont has developed a capability that other holobionts don't have -- say, resistance to a specific parasite -- it can transmit it directly to others by the exchange of micro-organisms. There is no need to wait for the population to be replaced by a new generation of individuals who have inherited a certain trait.

But then, how is exactly that holosex, sex among holobionts, works? Well, you don't need special organs and, of course, there is no male/female distinction. Hologenetic material is mainly in the form of microbial lifeforms of various kinds. To exchange these tiny critters, holobionts need to be in contact with each other or, at least, close to each other. Then, the passage of microbes mainly occurs by means of skin contact, although there are other possibilities.

That's why holobionts tend to touch each other: they hug each other, they pat each other, they rub each other, and they kiss each other: it is to exchange chunks of their hologenome, performing holosex, if you like to use this term. They tend, as we can imagine, to be cautious in doing that because they could exchange "bad" microbes and be infected with some illness. As we all know, sex is necessary, but nobody ever said that it is not dangerous. Holobionts need holosex in order to transmit and maintain their hologenetic structure and they need to accept that there is some risk involved. No sex, no life. Not for long, at least.

This means that, sooner or later we'll go back to touch each other and, perhaps, in France they will restart with la bise. Now, it looks as obsolete as dancing rituals to the moon Goddess in the night, but that may return, too. And so, onward, fellow holobionts!

(*) it may be possible that the Japanese exchange microbiota by means of their habit of communal hot baths, notoriously a place where bacteria thrive, So, they don't need to touch each other as much as Europeans do.

(**) To know more about holobionts and discuss the subject, see the "Proud Holobionts" group on Facebook


Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)