Psychedelics, sociality, and human evolution

Crespi and colleagues situate their argument in the idea that hominin encounters with psilocybin-containing fungi were likely recurring and potentially influential across much of human evolution. They draw on paleodietary and paleoecological reconstructions, primate behaviour, the broad biogeography of psilocybin mushrooms, and contemporary neuropsychopharmacology to contend that incidental ingestion and later ritualised use of psychedelics could have had adaptive consequences. The paper emphasises the pharmacology of classic psychedelics—primarily 5-HT2A receptor partial agonism—and links these actions to increased neural plasticity, altered network dynamics (for example, reduced default-mode network integrity and increased cross-network connectivity), and changes in perception, affect, and social cognition. The central aim is to formulate an evolutionary model of psychedelic instrumentalization: a synthesis that integrates anthropological, archaeological, primatological, and neuroscientific evidence to assess whether psilocybin use could have conferred selective advantages in the emergence of what the authors call the human socio-cognitive niche. Rather than proposing a single-cause origin story, the paper treats psychedelics as an enabling factor that could have entered positive feedback loops with developing cognitive, social, symbolic, and cultural capacities, thereby biasing selection in favour of enhanced sociality, creativity, and cooperative behaviours.

This is an integrative, interdisciplinary review and theoretical model rather than an empirical study. The investigators synthesise evidence from multiple domains—paleoecology and paleodietary inference, primate feeding and self-medication literature, the biogeography of psilocybin fungi, ethnographic and historical reports of psychedelic use, archaeological and paleoethnobotanical data, clinical and experimental neuropsychopharmacology, and comparative neuroscience—to build a coherent evolutionary argument. The extracted text does not report a formal systematic search strategy, databases searched, or explicit inclusion/exclusion criteria for studies; the approach appears to be narrative and theory-driven, drawing selectively on empirical findings and ethnographic cases to support the model. The authors unify these strands via two theoretical frameworks: a homeostatic/drug-instrumentalization perspective (animals and humans select substances to manage physiological and psychological challenges) and niche-construction/gene-culture coevolution (organisms modify their environments and thereby the selective regime experienced by descendants). They use proximate mechanistic accounts (neuropharmacology of 5-HT2A agonists) together with ultimate evolutionary reasoning to generate testable predictions.

The review assembles several lines of evidence that the authors treat as the ‘‘results’’ of their interdisciplinary synthesis. Ecological and palaeodietary evidence is marshalled to show that macroscopic fungi occur predominantly on the forest floor and in habitats frequented by Plio–Pleistocene hominins; 22 primate species are known to consume fungi and African great apes ingest non-nutritional plants for self-medication, supporting the plausibility of early hominin mycophagy. Psilocybin-containing species, including coprophilic taxa such as Psilocybe cubensis, are widespread across most continents and ecological zones, implying availability as Homo dispersed. Neuropharmacologically, psilocybin and related tryptamine psychedelics act primarily via 5-HT2A receptor partial agonism, increasing neocortical pyramidal excitability, augmenting glutamate release in prefrontal cortex, disrupting typical large-scale network organisation (for example, reduced DMN integrity and increased cross-network connectivity), lowering thalamic sensory filtering, and producing a state of enhanced synaptic plasticity and neural entropy. The authors reference the REBUS model (relaxed beliefs under psychedelics) to explain heightened bottom-up information flow and weakened high-level priors, which together create a more flexible cognitive state. Toxicity and safety considerations are summarised: psilocybin is presented as having low physiological toxicity and low addiction potential, with a reported therapeutic index of 641 and an estimated lethal-to-psychoactive ratio around 1000:1. Acute adverse effects (nausea, dizziness, transient anxiety, derealisation) are acknowledged, but prolonged psychosis after a single dose is described as rare in otherwise healthy individuals. Population-level data cited include a large US study of 130,000 adults that found no association between psychedelic use and suicidal behaviour or mental health problems. Ethnographic and archaeological material is reviewed to show ritual, medicinal, divinatory, and social uses of psilocybin mushrooms across many regions (notably Mesoamerica and much of the world except the Insular Pacific). The paper notes that direct archaeological evidence of psilocybin use deep in prehistory is lacking (for example, psilocybin mushroom remains in dental calculus are not reported), but argues that ethnographic continuity and iconographic hints make long-term ritualised use plausible. Behavioural and neuroimaging findings from contemporary experimental and clinical work are synthesised in support of functional claims. Acute and longer-term effects relevant to sociality include increased empathy for positive emotions, reduced recognition/processing of negative emotional faces, decreased amygdala reactivity to negative stimuli, enhanced positive mood, and measurable increases in prosocial behaviour (for example, reduced costly punishment in economic games). Single high-dose experiences are linked to enduring increases in trait openness and socially oriented personality dimensions. Rodent and human studies cited implicate mechanisms such as potentiation of 5-HT2A and AMPA receptor signalling, mTORC1 phosphorylation in medial prefrontal cortex, and increases in striatal dopamine—mechanisms that could underpin euphoria, social reward, and altered affective processing. On cognition and decision-making, the review highlights psychedelic-induced modes of thought described as hyper-associative or ‘‘primary process’’ thinking: increased semantic activation, divergent thinking, imagistic and symbolic mentation, and enhanced associative creativity. Naturalistic and laboratory reports indicate sub-acute enhancements in divergent thinking and creative problem solving following psilocybin. These effects are argued to have potential utility for rapid problem solving, divinatory practices, and leadership-mediated group decision-making in small-scale societies. Synthesising these strands, the paper presents a model of four instrumentalization goals that psilocybin could serve in a socio-cognitive niche: management of psychological distress and health problems; improved social interaction and interpersonal bonding; facilitation of collective ritual and religious activities (including communitas and enculturation); and enhanced group decision-making via divination, creative insight, and dishabituation. The authors further propose that periodic psychedelic use could have produced niche-constructing feedbacks—altering social practices and selective environments such that prosocial traits and cognitive capacities were favoured over generations.

Crespi and colleagues interpret their synthesis as supporting a cautious, non-deterministic claim: psychedelics may have acted as an enabling factor in the construction and reinforcement of the human socio-cognitive niche, but they are not presented as a singular cause of human cognitive evolution. The discussion examines the role of shamanic and ritual leaders who employed psychedelics, noting both their potential to benefit communities (healing, divination, social cohesion) and the possibility of deception; however, the authors argue that social selection mechanisms—reputation, public scrutiny, costly-to-fake skills, and the need to deliver tangible communal benefits—would have limited sustained, Machiavellian abuse by shamans. They emphasise niche-construction and gene–culture coevolutionary dynamics: if psilocybin use increased frequencies of laughter, music-making, ritual synchrony, prosocial leadership, and enculturation practices, these culturally persistent changes could modify selection pressures and promote genetic variants that facilitate sociality and cognitive flexibility (a process the authors liken to a Baldwin effect). The authors also speculate that psychedelics' psychoplastogenic properties—rapid promotion of synaptogenesis and plasticity via 5-HT2A pathways—could have contributed to neural architectures supportive of advanced cognition, perhaps easing energetic constraints on encephalization, though they acknowledge such neuroevolutionary links remain tentative. Key uncertainties and limitations are acknowledged. Direct archaeological evidence for deep, deliberate psilocybin use is lacking; the timing of adoption (early versus late in hominin history) cannot be established from current data; and the review does not rely on a documented systematic search and so remains a theoretical synthesis requiring empirical tests. The authors propose specific, falsifiable predictions and empirical approaches: systematic cross-cultural surveys (to test if psychedelic use is more prevalent among foragers versus agriculturalists and if uses cluster functionally around healing, social bonding, and decision-making), and laboratory experiments manipulating peripheral and central serotonin (via tryptophan depletion/supplementation) in combination with psychedelic administration to test whether psychedelics can substitute for 5-HT in restoring cooperative behaviour. They stress that negative empirical results would undermine the model. The discussion closes by urging the incorporation of evolutionary perspectives into both basic and therapeutic research on psychedelics.

The authors conclude that an adaptive-utilisation model, grounded in homeostatic and drug-instrumentalization frameworks and informed by niche-construction theory, offers a plausible account for how psilocybin could have influenced hominin adaptation. They propose that psilocybin might initially have provided homeostatic utility as a substitute for serotonin under dietary tryptophan scarcity and later became institutionalised in ritual contexts that amplified benefits for stress management, social bonding, collective ritual, and group decision-making. Through cultural and ecological inheritance, such practices could have biased selection toward enhanced social cognition, creativity, and cooperative behaviour. The paper calls for empirical tests of its predictions—cross-cultural analyses and experimental manipulations of serotonergic status combined with psychedelic administration—and suggests further research on potential links between psychedelics' psychoplastogenic actions and human brain evolution.