To know the self is impossible as the self is constantly changing, growing, and adapting. To paraphrase Heisenberg, we cannot simultaneously know our position and direction. Perhaps that is the paradox in the command from the Oracle at Delphi. To know thyself is to know that the self is unknowable. The self doesn’t really exist.
We think that we exist as individuals. Singular beings. A drop in the ocean. A pixel on a screen. A speck of dust suspended in a sunbeam. But we are the ocean. We are the screen. We are the personification of the universe perceiving itself. The focus on knowing the individual as an entity is entirely misplaced. Even without knowing ourselves, we have continually demonstrated the capacity for progress since the inception of our species.
Contrary to Darwin’s assertion that evolution only occurs by natural selection, populations do indeed evolve even during the course of their lifetimes, adding a Lamarckian twist to our current understanding. To isolate the genes of an individual as the sole drivers of evolution is to ignore the contributions of billions of years of symbiotic co-existence, at a dangerous cost. Forging ties with our microbial symbionts have allowed us to achieve what we never could in isolation.
Our human chromosomes provide an incomplete picture of the genetic repository we possess. To only consider the aforementioned would be to neglect the bacterial, archaeal, fungal, protistan, and viral genetic information that is equally integral to human identity, if not more so. If we accept and acknowledge that we are not individuals but living ecosystems, comprised of hundreds of trillions of microbes, it is not difficult to envision how we can evolve leaps and bounds faster than if we really were the individuals we perceive ourselves to be. Enter the hologenome.
The hologenome accounts for the genetic material of a host, or holobiont, plus all its symbiont microbiota. Many bacteria that live in and on our bodies multiply rapidly enough to go through several generations of offspring within the span of 24 hours, accounting for why antibiotic resistance evolves so rapidly. Bacteria reproduce, mutate, and therefore evolve very quickly. In this manner, the hologenome can evolve at a much more rapid pace than the human genome. As science writer Ed Yong expounds in I Contain Multitudes, “By partnering with microbes, we can quicken the slow, deliberate adagio of our evolutionary music to the brisk, lively allegro of theirs.”
Microbial symbiosis provides the crucial complement to features missing in the host’s core genome. Consider the case of lactose intolerance. Tolerance to lactose is usually assessed based on the activity of the human lactase enzyme. The lactase enzyme breaks down lactose into its monosaccharide constituents, glucose and galactose. In the majority of humans, the gene that encodes for the lactase enzyme shuts off shortly after weaning, making lactase persistence rare and lactose intolerance the default status. However, many bacteria belonging to the genus Lactobacilli also have genes that encode for the lactase enzyme. Therefore, if an individual did not have a functioning human lactase gene but nonetheless carried enough lactose-degrading symbionts, he would nonetheless be able to tolerate some amount of lactose, depending upon the activity of his fellow inhabitants’ lactase enzymes.
Critics of the hologenome theory question whether the microbiome can be transmitted with sufficient fidelity across generations to be considered a unit of selection in evolution. However, another school of thought argues that transmission of specific species is irrelevant as long as the metabolic processes enabled by the microbiome are conserved. In other words, while humans harbor taxonomically unique microbial communities, the functions those communities carry out do not exhibit much variability. Evolutionary biologist W. Ford Doolittle expressed this idea in a 2017 article entitled, “It’s the song, not the singer,” an inversion of a popular Rolling Stones song.
Revisiting the question of who is really in charge, researchers at Cornell University tracked the changes in the gut microbiome of a newborn for over two years and made a tantalizing discovery. Metagenomic analyses revealed that polysaccharide-digesting functional genes appeared in the gut microbiome while the baby was still exclusively consuming breast milk, several weeks prior to the introduction of polysaccharide-containing solid food in the diet, suggesting a microbial priming of the infant gut in preparation for an adult diet. This metabolic preprogramming implies that perhaps our microbes are the ones pulling the puppet strings, a humbling insight in the context of our anthropocentric view of the world.
Through our partnerships with our resident microbes, we have transformed so quickly as to become unrecognizable to our former selves. Though we may have spent most of our lives blind to the microscopic bonds we have formed, the interactions have nevertheless left indelible imprints on our lineage. While we may not have always been aware of the existence of our co-evolutionary comrades, we still had a gut instinct that our species would find ways to not merely survive but thrive. If the perilous infancy of our species is any indication, proper perception of the self need not be prerequisite for progress.