PRESENCE BEFORE THE BRAIN
- drissboussaoud
- Jul 1
- 5 min read
Updated: Jul 4
The biological roots of sociality…
Has the presence of others influenced living things for longer than the brain itself? From bacteria capable of detecting the density of their neighbors to human brain networks sensitive to social presence, a central hypothesis of the Presence Brain project is emerging: sociality does not begin with interaction, but with the ability to detect that others are there.
Does the presence of others influence living organisms over a longer timescale than the brain itself? From bacteria capable of detecting the density of their neighbors to human brain networks sensitive to social presence, a central hypothesis of the The Brain of Presence project emerges: sociality does not begin with interaction, but with the capacity to detect that others are there.
Most theories of the social brain begin with interaction. Some begin with the brain itself. They seek to understand how organisms perceive their conspecifics, communicate with them, cooperate, or build relationships.
The Brain of Presence project leads to a more radical question: what if sensitivity to the presence of others is older than the brain itself?
We tend to regard presence as a psychological phenomenon, or even as a conscious experience. Yet if we go far enough back in the history of life, another possibility emerges. Before being a lived experience, before being a neural mechanism, presence may be a fundamental biological fact.
Is anyone around?
To explore this idea, imagine a marine bacterium.
It possesses no brain, no nervous system, no consciousness in the sense we ordinarily mean. And yet it is capable of modifying its behavior as a function of the number of other bacteria present in its environment. When the density of neighboring organisms reaches a certain threshold, it changes gene expression, adapts its survival strategies, and behaves differently depending on whether it is alone or surrounded.
This mechanism is known as quorum sensing.
Each bacterium continuously releases small chemical molecules into its environment. As the number of bacteria increases, the concentration of these molecules increases as well — a chemical, continuous estimate of conspecific density, rather than a simple count. When a critical threshold is reached, the entire population modifies its behavior in a coordinated way — but this collective coordination rests on information that has been continuously varying all along. (Miller & Bassler, 2001)
The bacterium does not perceive its neighbors directly. It perceives their trace. But this trace is sufficient to sustain an estimate, stronger or weaker depending on the perceived concentration: is anyone there? And this estimate modifies what the bacterium does.
This is not a metaphor
It would be tempting to regard quorum sensing as a mere analogy for presence. That would be to miss the point entirely.
Quorum sensing is not a metaphor for presence. It is a mechanism for detecting presence. It allows an organism to adjust its behavior depending on whether other organisms are there.
This mechanism has been preserved by evolution presumably because it confers a decisive advantage. In certain situations, survival depends precisely on the capacity to take the presence of others into account.
In very different forms, this principle reappears throughout the history of life. The same idea seems to run through evolution: the presence of others constitutes a biologically relevant information. And this information modifies behavior.
The brain did not invent presence
This perspective leads us to view the brain from a slightly different angle.
We tend to regard the brain as the origin of sociality. Yet it may be more accurate to say that it inherited mechanisms that are far older.
The brain did not invent sensitivity to presence. It transformed it. It amplified it. It integrated it into ever more complex forms of information processing.
In social insects, the presence of conspecifics organizes sophisticated collective behaviors. In fish, it regulates shoal cohesion and predator responses. In rodents, it influences learning, risk-taking, and stress regulation — effects documented across a wide range of social facilitation studies. In primates, it shapes social behaviors, the observation of others, and the acquisition of new skills.
These effects are not incidental. They reveal a remarkable functional continuity: at each level of biological complexity, the presence of others constitutes information that modifies the organism’s behavior. It is this continuity that the Brain of Presence project seeks to formalize, up to the finest levels of human brain organization.
In humans, social presence penetrates to the finest levels of brain organization — from synaptic mechanisms to large-scale networks involved in attention, emotion, and social cognition. “Social” and “asocial” prefrontal neurons have been identified in non-human primates (Demolliens et al., 2017), and effects on large scale brain networks and on synaptic efficacy have been observed in relation to the probabilistic inference of social presence (Esmaeili et al., 2025; Tricoche et al., 2025).
Between the bacterium that chemically detects the density of its neighbors and the human being who instantly perceives that they are not alone in a room, billions of years of evolution have elapsed. But a continuity remains. At every stage, organisms have had to answer the same question: Is anyone there?
An evolving continuity
We tend to regard sociality as a complex property that appeared relatively late in the history of life, through the emergence of intelligence, language, or consciousness. This intuition is understandable. The most elaborate forms of communication and cooperation do seem to depend on sophisticated cognitive capacities.
But this perspective may lead us to overlook something more fundamental.
What if what we today call social cognition, attachment, cooperation, or communication rested on a much older principle?
What if all these capacities derived from a more elementary property: the capacity to detect the presence of other organisms and to adjust behavior accordingly? From this perspective, sociality would not begin with interaction. It would begin with presence.
What this changes
This hypothesis fundamentally transforms the opening question.
If sensitivity to presence is a biological mechanism conserved over billions of years — before the nervous system, before the brain, before consciousness — then it is not a late consequence of social cognition. It may be its substrate.
Communication, cooperation, empathy, and theory of mind then appear as progressive elaborations of a more fundamental property: the capacity to detect that other organisms are there and to adjust behavior accordingly.
Understanding presence is therefore not merely understanding one dimension of the human brain. It may be understanding one of the oldest and most conserved properties of living systems.
Before language. Before thought. Before the brain itself. There was already presence. Perhaps understanding presence is understanding one of the most ancient roots of sociality.
References
Demolliens, M., Isbaine, F., Takerkart, S., Huguet, P., & Boussaoud, D. (2017). Social and asocial prefrontal cortex neurons: A new look at social facilitation and the social brain. Social Cognitive and Affective Neuroscience, 12(8), 1241–1248.
Esmaeili, A., Demolliens, M., Viersen, M., et al. (2025). Probabilistic inference of social presence across brain scales reveals enhanced synaptic efficacy. Communications Biology, 8, 1608.
Tricoche, L., Royer d’Halluin, M., Meunier, M., et al. (2025). Neural bases of social facilitation and inhibition: How peer presence affects elementary eye movements. Soc. Cogn. Affect. Neurosci., 20, nsae079.
Miller, M. B., & Bassler, B. L. (2001). Quorum sensing in bacteria. Annual Review of Microbiology, 55, 165–199.
Articles and links to explore
General presentation of the project and the question that constitutes its starting point.
How social neuroscience has transformed our understanding of the brain, and why the question of presence could complement this theoretical framework.
Why certain ordinary experiences — shared silence, feeling watched, soothing or threatening presence — can help us understand the role of presence in human experience.
In an elevator, a waiting room or a train, the mere presence of another person is often enough to transform our experience.
Videos, reflections and explorations around the neuroscience of presence, attachment, development and psychotherapy.
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