Where are your boundaries?
“Studies show we think of ourselves now and in the future as different people”
DELINEATING where a person begins and ends used to be quite simple. While philosophers might have tied themselves in knots trying to define the self, and biologists still struggle to locate its steering mechanism (see “Where is your self?”), what it encompassed, at least, was more clear-cut.
Their traditional definition comprises three elements, says at the University of Kiel, Germany: the mind, the genome and the immune system. Each of us is a self-contained organism defined by our mind and genes, with the immune system patrolling our borders and discriminating between self and non-self. Me, myself and I.
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Then we looked more closely, and our relationship status went from “threesome” to “it’s complicated”.
For starters, we are chimeras: some parts of us are human, but genetically not “us”. Most, if not all, of us contain a few cells from our mother, our grandmothers and even elder siblings that infiltrated our bodies in the uterus.
Women who have carried children host such cells too. “Something like 65 per cent of women, even in their 70s, when autopsies were performed, had cells in their brains that were not theirs,” says David Linden at Johns Hopkins University in Maryland. Chimeric cells have been found to contribute to both good and bad health, for example promoting wound healing but also triggering autoimmune disease.
A handful of people even turn out to be true chimeras, created from a merger in the uterus of two non-identical, “fraternal” twins. We don’t know how common this is, because few people undergo the genetic tests that reveal it. It could be you…
You aren’t alone
More profoundly for our definition of self, we are also holobionts: we aren’t individuals, but collectives. Every bit of our body is teeming with microbial life: bacteria, fungi, protists, archaea and viruses. They live on us and in us, on our skin, inside every orifice, and above all in our gut. We are even surrounded by an invisible cloud of them, a bit like Pig-Pen .
These microbes outnumber our own cells, though not by 10:1 as is often claimed. An average human body is made up of about . By mass, we absolutely dwarf our companions: a 70-kilogram human contains just 200 grams of microbe.
But they punch well above their weight. The microbiome is different from parasitic freeloaders like lice and intestinal worms: it is an active and vital participant in our lives. Our gut microbiota, for example, do huge amounts of work digesting food that the products of our human genome can’t break down on their own. They are, in fact, the principal determinant of how we respond to food. Our microbiome influences our health in many other ways, contributing to mental well-being and modulating our emotions and cognition, and helping determine how our immune systems function.
For Bosch, that means we need to develop a that takes account of how some of our most personal traits are actually those of our vast, diverse and ever-shifting microbiomes. “Boundaries, borders, different parts of host and microbe are not so easy to separate any more,” he says. “We are not alone.”
THE ‘YOU’ GENES
For all our diversity, humans are 99.9 per cent identical. Then again, our genome contains 3 billion base pairs, so the 0.1 per cent that varies means that some 3 million components of your genetic blueprint are different from the next person’s. This is where you find the variation that gives you brown eyes rather than blue, makes you tall or fast, or increases your risk of heart disease – although many factors beyond mere genes determine how you turn out (see “How likely are you?”, page 36).
The human genes that vary most, however, are a handful that control how our immune systems detect foreign pathogens. These major histocompatibility complex (MHC) genes code for proteins that present samples of what is being made inside a cell at its surface, allowing immune cells to check that the cell hasn’t been infiltrated by something that shouldn’t be there. Effectively, they are responsible for a system that identifies a cell as “self”, rather than something to be attacked.
These genes differ so much between individuals that they “can almost define your individuality on their own”, says , an immunologist at the University of Manchester, UK. It means that each of us is scouting for and responding to disease in slightly different ways. That helps some of us to fight off diseases that have never existed before, such as covid-19.
But it is also good for the survival of our species, says Davis. “If we all had exactly the same susceptibility, we would have a greater chance of succumbing, as a species, to a particular disease.” For everyone’s sake, you can be glad there’s only one you. Daniel Cossins
All of the “You: Special issue” features
- You are stardust: The long view of when your existence really began
- How nature, nurture and sheer randomness combine to make a unique you
- Think your sense of self is located in your brain? Think again
- You are not one person: Why your sense of self must be an illusion
- Why it’s the aliens living inside you that create your sense of you
- Do we have free will or are all our decisions predetermined?
- If we can’t change the world, does anything we do matter?
- Why we’re in tune with our emotions – but suck at judging our smarts
- If the multiverse exists, are there infinite copies of me?
