The biological case against a simple notion of individuality is substantial. Traditional views treated an individual as a self-contained organism with a unique genetic identity. Contemporary biology has complicated that picture considerably. Most of the cells in the human body are bacterial cells, and while the exact ratio remains debated, microbes living in close association with their hosts can alter the expression of host genes, shaping the host’s characteristics from within. The concept of the holobiont captures this: a host together with its persistent symbiont population, which can be transmitted vertically during reproduction or acquired horizontally from the surrounding milieu. From a genomic perspective, the hologenome comprises the host genome, the genomes of resident microbiota, and those of transient microbiota, meaning genetic variation can arise through changes in symbionts as well as in the host, through microbial amplification, acquisition of symbionts, or horizontal gene transfer.

The consequences for classical criteria of individuality are far-reaching. As the biologist Scott Gilbert and colleagues have argued, anatomically, PCR data show that our cells and bodies are shared with bacterial species; developmentally, vertebrate and invertebrate development relies upon the presence of symbionts; physiologically, functions such as vitamin synthesis and intestinal permeability are provided to humans by bacteria; and genetically, the one-genome/one-organism doctrine is belied by cases like the mealybug Planacoccus, where two microbes and the host coordinate amino acid synthesis in a nested symbiosis such that neither the microbes nor the host alone has a complete genome. Roughly ninety percent of human body cells are prokaryotic in anatomical and physiological terms, and over thirty-five percent of blood metabolites come from bacteria. On this view, all evolution becomes a matter of co-evolution, and we grow as collections of ecosystems.

Yet the holobiont concept is itself contested. Douglas and Werren have argued against it on the grounds that it relies on overly restrictive assumptions, preferring instead the language of ecological communities. Others, such as the philosopher Ellen Clarke, have reframed the problem entirely: biological individuals are entities possessing mechanisms that increase cooperation and decrease competition among their parts, allowing natural selection to act on the whole rather than on components alone. Individuality, on this account, comes in degrees along a spectrum, and whether a holobiont qualifies depends on the specific mechanisms it possesses. Vulliermet deliberately sidesteps these taxonomic disputes, focusing instead on a different implication of the holobiont literature: the suggestion that we are not autonomous entities distinct from our biophysical environments, and that this alone dissolves the individual.

This is where the social dimension matters. Humans conceive of themselves as physically bounded entities with agency and self-determination, and this phenomenological and social sense of individuality does not obviously conflict with biological interdependence; the two operate at different levels. Indeed, biological and social individuality may be mutually constitutive. The immune system draws boundaries between self and non-self, yet immunological responses are shaped by developmental and environmental factors, including microbial symbiosis and socio-behavioral inputs. Socially, individuality is formed through relationships, cultural norms, and collective experiences. The bioethicist Kris Hens has emphasized a developmental perspective in which identity emerges from the entanglement of biological plasticity and social experience. Moreover, as Gilbert and colleagues observed, the claim that we have never been individuals resonates with a broader historical and political context in which Western societies made the individual, exemplified by the figure of the self-made man, the atom of society and the hero of scientific narratives.

Vulliermet’s move is to argue that thinking of oneself as an individual is already thinking in terms of identity, and that personal identity therefore offers a fruitful foundation for a revised, biologically informed notion of the individual. The paper surveys the main theories. Kurt Lewin’s concept of genidentity, introduced in 1922, defines the identity of an object through time as a continuous series of states, echoing John Locke’s view in the Essay Concerning Human Understanding that the identity of living organisms lies in the participation of the same continued life, a position termed continuism. Against this, Leibniz defended a substantialist approach, and contemporary essentialists hold that a core constituent must remain constant, with genetic essentialism tying identity to possession of the same genome. David Hull, focusing on the continuity of internal organization, argued that the only reliable criterion for biological diachronic identity is the continuity of change itself, since organisms exchange substance and structure while remaining the same individuals, just as a caterpillar and a butterfly retain one identity.

Contemporary philosophy of personal identity splits along different lines: reductionists locate identity in diachronic empirical relations and psychological continuity, drawing on Hume’s bundle theory, under which a person is a series of mental events, a view radicalized by Derek Parfit’s rejection of Cartesian egos. Non-reductionists counter that something is missing from such chains of events, namely who experiences them, and ground identity in the first-person perspective, the Kantian idea that the identity of the ‘I’ is entailed by any statement about personal identity. Vulliermet does not adjudicate between these camps. The crucial point is that whichever conception one adopts, none can forgo the brain, since the capacities underpinning personal identity, consciousness, memory, and self-awareness, are cerebral. Even if personal identity were, as Hume suggested, a fiction, it is a useful one, and the brain is its necessary organ.

Why the brain? Not because it is generally exceptional, but because its specific functions underpin conscious experience, memory, and self-awareness. Neurons are largely non-dividing cells; those established during early life typically persist throughout an organism’s lifetime, and neurons that replicate their DNA are fated to die, although some survive in tetraploid states, a phenomenon documented as an early marker in Alzheimer’s disease. Research on adult neurogenesis in the dentate gyrus and subventricular zone has challenged old dogmas, while studies show significant loss of neurogenesis from adolescence to adulthood. Empirically, people with hippocampal lesions suffer damaged episodic memory and an impaired sense of identity, and patients with Alzheimer’s disease gradually lose memories and personality traits until they no longer recognize themselves or their loved ones, even as vital bodily functions continue. Functional neuroimaging shows heightened activity in midline brain regions during autobiographical memory recall and self-reflection, supporting the view that a sense of self is maintained by identifiable neural networks, including the prefrontal cortex, hippocampus, and parietal lobes.

The final step invokes epigenetics. Waddington originally coined the term to describe the causal interactions between genes and their products that bring the phenotype into being; today it covers how environments influence gene expression, potentially in heritable ways through genetic assimilation. Most environmental factors, toxicants included, cannot alter DNA sequence, but they can alter gene expression through epigenetic processes such as DNA methylation. Neuroepigenetics applies these insights to the nervous system, where neurons’ non-dividing nature makes their epigenetic marks long-lasting and self-regenerating, though not inherited by daughter cells. Because the brain is highly plastic, reorganizing its structure and connections in response to intrinsic and extrinsic stimuli, lived experience becomes biologically embedded. After birth, surplus neurons die unless activated through interaction with the environment; during childhood, synaptic pruning eliminates excess possibilities. Animal studies show epigenetic mechanisms regulating synaptic plasticity and contributing to memory formation, consolidation, and emotional processing. Studies of suicide victims have revealed genome-wide alterations of intragenic DNA hydroxymethylation in the prefrontal cortex and elevated methylation of the hippocampal glucocorticoid receptor promoter in those who suffered childhood abuse, while enriched versus deprived environments alter hippocampal cell survival.

Even the gut microbiome participates, with research suggesting that microbial production of short-chain fatty acids may induce neuroepigenetic modifications, meaning the brain, the condition sine qua non of personal identity, is partly shaped by other organisms. Vulliermet concludes that personal identity is shaped not only by the social environment, as commonly accepted, but by the biological environment as well, and that social and biological individuality therefore intertwine rather than stand apart. The practical implications could be considerable: if the environment is an integral feature of an individual, harming it amounts to endangering conditions that co-construct identity, echoing Glenn Albrecht’s call to re-place our sense of identity in an emerging Symbiocene. Just as the French word terroir ties people to a place, its traditions, and its character, so too might identity be understood as temporally and environmentally situated, plastic rather than fixed. As the paper puts it, saying we are what we live in may be as salient as saying we are what we eat, and if the individual remains a paragon defended by human rights, the environment deserves a ringside seat.

Subject of Research: The role of environment and neuroepigenetics in shaping human personal identity and biological individuality

Article Title: Thorn within: the environment in personal identity

Article References: Vulliermet, F. (2025). Thorn within: the environment in personal identity. Epigenetics Communications, 5(1), Article 3. https://doi.org/10.1186/s43682-025-00036-0

Image Credits: AI Generated

DOI: 10.1186/s43682-025-00036-0

Keywords: personal identity, epigenetics, neuroepigenetics, holobiont, brain plasticity, philosophy of biology, microbiome, individuality, hippocampus, environment, symbiosis, Symbiocene