On the prospects of basal cognition research becoming fully evolutionary: promising avenues and cautionary notes

The research programme ‘basal cognition’ adopts an evolutionary perspective for studying biological cognition. This entails investigating possible cognitive processes in ‘simple’–often non-neuronal–organisms as a means to discover conserved mechanisms and adaptive capacities underwriting cognition in more complex (neuronal) organisms. However, by pulling in the opposite direction of a tradition that views cognition as something that is unique to neuronal organisms, basal cognition has been met with a fair amount of scepticism by philosophers and scientists. The very idea of approaching cognition by way of investigating the behaviour and underlying mechanisms in, say, bacteria, has been seen as preposterous and harmful to both cognitive science and biology. This paper aims to temper such scepticism to a certain degree by drawing parallels with how the evolution of ‘development,’ another loaded concept that refers to a not-so-easily definable, contested bundle of phenomena, has been fruitfully approached in Evolutionary Developmental Biology (Evo-Devo). Through this comparison, we identify four promising features of the basal cognition approach. These features suggest that sweeping scepticism may be unwarranted. However, each of them comes with important epistemic cautionary notes that should not be disregarded. By presenting these twofold considerations as potential ways to integrate a fully evolutionary perspective into basal cognition, this paper seeks to provide clarity and direction for the advancement of this research programme.

Keywords: Basal cognition, Development, Evo-Devo, Evolution, Cognitive capacities

In the philosophy of cognitive science and cognitive science more generally the following fundamental scope question arises: “where does biological cognition begin?”. By addressing which kinds of organisms exhibit cognition and identifying some shared feature common to such organisms this question is thought to reveal something important about the nature of cognition. There have been two general (yet non-exhaustive) ways that this scope question has been approached. The first is a definition-first approach. It starts off with the assumption that cognition refers to a general process and then proceeds in introducing a criterion (or criteria) that demarcates cognition from other non-cognitive biological processes such as metabolism, physiological regulatory processes and the like. This demarcation criterion, usually in the form of (a) necessary and/or sufficient condition(s), is then used to adjudicate whether or not some specific kind of organismal behaviour is or is not cognitively driven. Such an approach to answering the scope question is usually taken up by philosophers and framed in terms of identifying “the mark of the cognitive” or, similarly, singling out some necessary feature that satisfies the requirements for “minimal cognition.”

There have been numerous suggestions as to what such marks are, ranging from having representational states with intensionality (Adams, 2018) to exhibiting sensorimotor coordination (van Duijn et al., 2006) or future-oriented intentional dynamics (Sims, 2021). Whether or not philosophical debates about the mark of the cognitive or minimal cognition reveal anything about the nature of biological cognition other than one’s pre-theoretical commitments regarding cognition remains to be seen (Facchin, 2023). To date, this manner of addressing the scope question has proven itself to be reminiscent of the many-headed hydra immortalised in Greek myth; any attempt to answer it once and for all has tended to stubbornly give rise to more questions of similar or same form requiring even more criteria and/or more nuancing of old criteria ad infinitum. That said, definition-first approaches can often serve instrumentally as a means to rouse interest in the general notion of cognition, enlisting more soldiers in the battle to slay the many-headed hydra. Needless to say, the hydra is still alive and slithering.

There is another manner of addressing the scope question, however, which purports to be prima facie more promising. Eschewing the search of the mark of the cognitive, it takes an evolutionary approach. Accordingly, a more thorough articulation of the question might be: “how far back can we traverse the phylogenetic tree and identify instances of biological processes that are functionally analogous to and/or in historical continuity with those processes that have been investigated within the domain of traditional cognitive science?”. Central to this approach is an evolutionary assumption; namely, many of the capacities that are found in phyletically younger (more recently evolved) taxa like us have functional predecessors in older taxa from which they have evolved. Some of these older capacities and/or their underlying causal mechanisms, the assumption goes, have been evolutionarily conserved due to their contribution to survival and fecundity (i.e., fitness). With this assumption to hand, evolutionary reasoning-first approaches look to investigate specific instances of putative cognitive capacities in extant organisms that are phylogenetically distant from us (e.g., slime moulds, carnivorous plants, ctenophores, etc.). Working with organisms from early diverging lineages, they aim to identify common principles and conserved cognitive mechanisms that are shared between structurally and functionally simpler organisms and those that are more complex according to certain metrics (e.g., multicellular organisms with a diversity of cell types and cell functions). This latter way of framing the scope question is part of a venerable interdisciplinary tradition, dating at least as far back as to eighteenth- and nineteenth-century evolutionism (Richards, 1987).

In recent years, this approach to investigating cognition has become increasingly more popular (e.g., Barron et al., 2023; Beran et al., 2014; Chittka et al., 2012; Godfrey-Smith, 2020) and has been marshalled in the research programme dubbed “basal cognition” (Levin et al., 2021; Lyon et al., 2021).1 For the purposes of getting empirical investigation off the ground, basal cognition researchers conceptualise biological cognition as a toolkit of adaptive capacities that have been shaped and maintained by commonplace evolutionary processes, including Darwinian evolution by natural selection. Formulated so as to be phyletically neutral, the basal toolkit is based upon various capacities that have previously been observed in bacteria (Lyon et al., 2021).2 These include (but are not limited to) memory, learning, decision-making, perception, anticipation, valence, and behaviour (Lyon et al., 2021). In recognising that each of these capacities have been shaped by natural selection and other evolutionary forces, and are thus fundamentally biological processes, a particular emphasis is placed on taking an evolutionary perspective on cognition. What sets basal cognition apart from a purely conceptual manner of answering the scope question is its focus on theories that generate (in principle) testable hypotheses (Levin, 2022; Sims, 2024a). In this regard, it prioritises experimental results over a priori reasoning about criteria for cognition, evaluating hypotheses based on empirical findings.

This willingness of biologists, cognitive scientists, and philosophers to deploy a bottom-up, explananda-steered approach (see also Waal & Ferrari, 2010) is a significant departure from a long-standing neuro-centric (or more generally zoo-centric) view of cognition, one that often informs or is the basis for the a priori criteria that have sustained the presence of the many-headed hydra. And unlike the mark of the cognitive, definition-first approach to answering the scope question, basal cognition has yielded some promising (yet non-conclusive) results bearing on understanding conserved cognitive capacities and mechanisms (see the 2021 basal cognition double special issue in the Philosophical Transactions of the Royal Society B; for an overview, see Levin et al., 2021; Lyon et al., 2021).

Pulling in the opposite direction of a tradition that views cognition as something that is unique to neuronal organisms, the basal cognition programme has been met with a fair amount of scepticism by some philosophers and biologists (Figdor, 2022, 2024b; Loy et al., 2021; Mallatt et al., 2023). For example, it is argued that there is no irrefutable evidence that what is being called “learning” or “memory” in non-neuronal organisms is remotely similar to what cognitive scientists are interested in when investigating learning and memory (Loy et al., 2021). This form of scepticism aims at driving a wedge between bona fide cognitive capacities as uniformly recognised by cognitive science and other processes that are only superficially similar to those capacities that cognitive science focuses upon. Another form of scepticism draws attention to the abstract nature of the models used by some basal cognition researchers. Figdor (2024b) contends that even when scientists are able to instrumentally employ the same type of (mathematical and computational) models for capturing the adaptive flexibility of non-neuronal organisms’ behaviours (e.g., by modelling them as feedback control mechanisms), the phylogenetic and conceptual relationships of these feedback-controlled behaviours to those of organisms we already accept as cognitive is left out of the models. And yet another form of scepticism rests on the very meaning of “cognition.” If cognition is defined as necessarily involving mental states that have a specific kind of representational format (e.g., exhibiting intensionality), then the very idea of approaching cognition by way of investigating the behaviour and underlying mechanisms in, say, bacteria, is seen as preposterous and harmful to both cognitive science and biology (Adams, 2018).

This paper aims to challenge such scepticism to a certain degree by drawing parallels with how the evolution of development, another loaded concept that refers to a not-so-easily definable bundle of phenomena, has been approached in the now-consolidated, empirically profitable field of evolutionary developmental biology (Evo-Devo). Development, just as cognition, is one of those umbrella scientific concepts whose univocal characterization has proven elusive and disagreement abounds (Pradeu et al., 2016), not to mention that no overarching “theory of development” appears to be in sight (Minelli & Pradeu, 2014). Although the question of establishing a definition-first approach to development seemed to be pressing during the 1950s when embryology transformed into ‘developmental biology’ (Burian & Thieffry, 2000; see also Hopwood, 2019; Pradeu et al., 2016), definitional concerns eventually receded into the background as empirical research took off, and a comprehensive conceptualization of ‘development’ was not seen as a prerequisite for the emergence of Evo-Devo in the 1980s, which was also highly contentious among evolutionary biologists in its beginnings and well into the early 2000s (Amundson, 2005).

The resemblance between the evolutionary and comparative study of development and cognition lies not only in the seemingly insurmountable difficulties of reaching a satisfactory definition that encompasses the varied phenomena to which their extensions purportedly refer (which could then be projected to solve scope problems),3 but also in that many of the same types of questions we can ask about the evolution of development and developmental repertoires can be asked of the evolution of cognition and cognitive capacities. This includes, among others: How many times have they evolved in phylogenetic history? Which cognitive or developmental traits are homologous or homoplastic in which taxa? Could certain cognitive or developmental processes in a particular organism be sui generis due to its prior evolutionary trajectory or are they rather indicative of a larger lineage or clade?

Through our comparison between how ‘development’ has been approached from an evolutionary perspective in Evo-Devo, we identify four specific parallels with the basal cognition approach that bear upon its prospects for becoming a fully evolutionary research field that is able to address the scope problem: First, both fields conduct comparative causal-mechanistic investigations to uncover shared developmental or cognitive toolkits and capacities (Sect. 2). Second, researchers in each area assume panextensionalist positions about the phylogenetic scope of ‘cognition’ and ‘development’ in an attempt to counteract purported evolutionary biases of oligoextentionalist positions that only grant cognition or development to few organisms (i.e., neuronal metazoans and clonal multicellular organisms, respectively) (Sect. 3). Third, when investigating developmental and cognitive traits, sound phylogenetic thinking can aid in distinguishing between homologies and homoplasies, as well as in countenancing convergence in evolutionary scenarios (Sect. 4). Fourth, by adopting a fully evolutionary perspective, the loss, gain, and uniqueness of particular developmental and cognitive traits and capacities should be studied in particular lineages (Sect. 5).

It should be said that these parallels are not categorially equivalent for basal cognition as-it-is-presently-practiced: The first two parallels are descriptive of current work in basal cognition research, while the last two have a normative bent regarding the considerations that scientists could bear in mind when adopting a bona fide evolutionary perspective (e.g., in terms of methodology or inference generation and justification). Importantly, however, is that each parallel signals promising features of the basal cognition approach. As such, these features suggest that sweeping scepticism against it may be unwarranted. This notwithstanding, each of them also comes with important cautionary notes, attention to which we believe will ultimately improve basal cognition’s standing as an evolutionarily grounded approach to investigating cognition. By presenting these twofold considerations as potential ways to integrate a fully evolutionary perspective into basal cognition, we seek to provide clarity and direction for the advancement of this research programme. In this sense, and more generally speaking, we intend to show how evolutionary reasoning-driven approaches to tackle the problem of the scope of cognition can be strengthened and pursed further.

Ever since its inception, one of the goals of Evo-Devo has been to uncover shared genetic-developmental toolkits in phylogenetically distant organisms, and this is now taken to be a rather uncontroversial scientific pursuit. However, in the 1980s, one of the first discoveries of such shared developmental underpinnings took the scientific community by surprise: the finding that Hox genes, first characterized as the genes of the Bithorax and the Antennapedia complexes of Drosophila melanogaster which have homeotic effects when mutated (e.g., fruit flies with an extra pair of wings instead of halteres or with ectopic legs instead of antennae), exist across many animal groups (McGinnis et al., 1984a, 1984b). These are transcription factors, later found in other eukaryotic groups as well, which contain a 180-base pair sequence (the homeobox) that encodes a region called the homeodomain. In a large range of bilaterian metazoans, this region binds to DNA to regulate the expression of downstream genes related to anterior–posterior patterning. Part of the surprise instigated by the discovery of the widespread presence of homeobox-containing genes was due to theoretical expectations from evolutionary biologists and the state of knowledge at the time on what was presumed to be the genetic basis of adaptation. For instance, in a representative statement, the renowned evolutionary biologist Ernst Mayr (1963) stressed: “Much that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives” (p. 609; emphasis added).

In alignment with these conceptual presuppositions, searching for shared genes that partake in the morphogenesis of organic structures was not even contemplated as a worthwhile investigative route. However, just as with the case of the Hox genes and anterior–posterior patterning, soon other orthologous genes involved in other developmental processes, for instance, in eye and appendage formation (Halder et al., 1995; Shubin et al., 1997), were found across invertebrate and vertebrate species in the wake of what we now call modern ‘Evo-Devo’ research. This relatively small set of key regulatory genes (e.g., transcription factors and components of paracrine signalling pathways) related to conserved developmental patterning functions for tissues, organs, and body axes throughout the diverse phyla of bilaterally symmetrical animals came to be known as “the genetic tool-kit” (Wilkins, 2013). Along these lines, the “ancestral complexity” of genomic repertoires came to be hailed as one of the key theoretical tenets of Evo-Devo (Carroll, 2008).

The quest for “master control genes” (Gehring, 1998), which proved to be important for the expansion of concepts such as “deep homology” (Shubin et al., 1997, 2009) and the phylogenetic screening of shared developmental genes, soon was found to be overly simplistic and gave way to a more nuanced approach. This shift involved the characterization and modelling of gene regulatory networks involved in character formation (Davidson, 2006; for discussion, see Morange, 2014) and, in general, more integrative appraisals of the causal-mechanistic basis of development. From its proximal origins to the present day, Evo-Devo has been in the business of providing causal-mechanistic explanations of development and its evolution. Beside gene regulatory networks underlying specific character formation, Evo-Devoists investigate different forms of ‘developmental repatterning’ (Arthur, 2010), generic mesoscale physical forces and dynamical patterning modules (Newman & Bhat, 2009), epigenetic mechanisms (Jablonka & Lamb, 2007), cell and tissue interactions, and other phenotype-generating mechanisms at different levels of biological organization (Salazar-Ciudad & Jernvall, 2013).

Like Evo-Devo’s search for developmental mechanisms, basal cognition research is currently in the business of searching for causal-mechanistic realisers of the various capacities comprising the basal cognitive toolkit. In basal cognition, scientists are investigating intracellular and intercellular mechanisms such as signal transduction (Lyon, 2015), cell–cell signalling (Yang et al., 2020), oscillations (Boussard et al., 2021; Hanson, 2021), cell networks and circuits (Rajan et al., 2023), bioelectric signals (Cervera et al., 2020; Levin et al., 2017), cell–cell adhesion (Dinet et al., 2021; Schaap, 2021), and local, regional, and systemic regulatory responses (Tagkopoulos et al., 2008).

In this sense, basal cognition is similar to comparative animal cognition studies that attempt to explain interesting observed behaviours in terms of the exhibition of particular cognitive capacities and, subsequently, investigate the possible mechanisms underlying those capacities (Shettleworth, 2010). However, in basal cognition research, both cognitive capacities and underlying mechanisms, in their full variety, are sought after also in non-neural organisms outside of the animal kingdom.

It should be noted that Evo-Devo and basal cognition use the notion of ‘toolkit’ differently. In the former, a developmental toolkit describes shared developmental mechanisms such as the various genes and gene regulatory networks causally involved in various ontogenetic processes that may be common to different taxa (e.g., bilaterians). Such processes are taken to be the mechanistic realisers of developmental phenotypes. Conversely, the basal cognition toolkit comprises the various cognitive capacities that may be common to different taxa, the investigation of which is achieved by searching for shared mechanisms that underpin said cognitive capacities. In both research areas, phylogenetic commonalities and evolutionary conservation are thus explanatorily important. However, their epistemic emphasis differs: in Evo-Devo, the focus is on shared explanantia that account for similar or different phenotypes across diverse lineages, while in basal cognition research, the emphasis is on shared explananda—specifically, the particular cognitive capacities that mechanistic-comparative research must investigate across diverse creatures, including both neural and non-neural organisms. For basal cognition research, these commonalities that yield adaptive behaviours across taxa, such as shared mechanisms of information processing, storage, and use, are paramount for understanding the evolution of cognition, regardless of nervous system complexity or absence. Rather than merely cataloguing similarities, basal cognition research could use commonalities to explore how cognitive processes evolve and operate across life forms.

At this point, we must stress an important cautionary note that emerges from this parallel: the search for shared developmental toolkits through causal-mechanistic research did not lead to the revolutionary transformation of evolutionary biology that Evo-Devo researchers were forecasting at the discipline’s onset (see Amundson, 2005). Instead, it became part of standard scientific inquiries, now well-entrenched in the horizon of what counts as respectable ‘evolutionary investigations.’ We anticipate that basal cognition research may follow a similar trajectory: after an initial period of scepticism regarding the fruitfulness of searching for putative cognitive capacities and underlying mechanisms across neuronal and non-neuronal species due to theoretical pre-conceptions, we forecast that this endeavour will become a standard procedure in comparative studies of cognition. And, in this sense, contrary to the pronouncements of some basal cognition proponents (e.g., Lyon et al., 2021), comparative mechanistic research alone is unlikely to bring a ‘paradigm shift’ for cognitive science.

One important reason for thinking along these lines is that finding (causal-mechanistic) commonalities in different organisms is only the first step toward asking interesting and nuanced evolutionary questions. In Evo-Devo, the unearthing of Hox genes and inquiries into the genetic toolkit, branded by some scholars as a period of “Hox mania,” soon led to the “Hox Paradox” or the question of how to explain the incredible phenotypic diversity of animals if they all share the same set of developmental genes (Wray, 2001). Why does a coconut crab look so different than a star-nosed mole or a gossamer worm if they share the same basic genetic-developmental toolkit? Renewed explanations of phenotypic diversity, beyond master regulatory genes à la Gehring (1998), became a pressing need in the explanatory agenda of Evo-Devo. If regulatory genes can be co-opted in evolutionary history and acquire new developmental roles, as biologists had to learn after their wide-ranging explanantia turned out to be makeshift, then their domains of expression cannot be taken at face value as indicating profound anatomical conservation.4 For contemporary Evo-Devoists, it is important to recognize that although developmental regulatory genes are evolutionarily conserved, their interactions might not, and many argue that the evolutionary rewiring of gene regulatory networks has been a pervasive source of morphological change during animal evolution (Kirschner & Gerhart, 2005; Wray, 2001), in addition to other non-genetic, heritable causal processes involved in phenotype production.

Analogously, in basal cognition research, finding common processes and their underlying mechanisms in different organisms, with the goal of understanding their constitution and operation, is only the first step toward asking interesting and nuanced questions about the evolution of such processes and mechanisms. For example, if both communication between individual bacterial cells making up a biofilm and communication between individual neurons in a brain involve electrochemical activity of potassium ion channel gating (Prindle et al., 2015), and such communication in both cases is involved in cognitive capacities such as memory (Yang et al., 2020), then what accounts for the functional variation in the kind of ion channel-mediated memory observed in bacterial biofilms and ion channel-mediated memory associated with neuronal metazoans? In other words, even if mechanisms are conserved, this still leaves us with a central evolutionary question as to why (and how) functional variations in cognitive capacities arose.

This parallel brings us to an important point–a heuristic if you will–that can be mined from the recent history of Evo-Devo and applied to basal cognition research. Although finding taxonomic commonalities in developmental processes or cognitive capacities is of extreme value in furthering our understanding of the evolution of a particular process or capacity, it is just as important to understand differences across taxa given that such differences (i.e., heritable variation) fuel evolutionary processes (e.g., natural selection). Although there has been a tradition of focusing upon organismal differences in both cognitive science and comparative psychology–a spotlight that has unfortunately fed the fire of zoocentric and anthropocentric biases in these fields (Lyon, 2006)–swinging too far in the opposite direction to concentrate primarily on commonalities stands in the way of understanding why the same general cognitive process may have evolved differently, taking different forms in different taxa despite being (partially) underwritten by the same mechanisms. We would thus like to emphasise that adopting a fully evolutionary approach involves taking both commonalities and variations of processes and capacities as the proper targets of investigation. The dialectical emphasis on investigating both conservation and diversification has been referred to by Michael Akam as the “Yin and Yang of Evo-Devo” (Akam, 1998), and we think that basal cognition researchers could do very well in adopting something similar as part of their zetetic orientation. Oscillation between investigating commonalities and differences is a core aspect of becoming a fully evolutionary research field–one that avoids unhelpful philosophical polemics and biases and also avoids glossing over the kinds of conservation and variation that developmental and cognitive phylogeny are dependent upon.

And, in connection to this point, we arrive at the next parallel.

When considering the phylogenetic scope of development, one could adopt a priori, definition-first stances regarding their distribution in the tree of life, similar to the prevailing approach for examining cognition. Scholars that choose this path usually assume that both (bundle of) processes are either highly restrictive in their reach, or that they are ubiquitous (i.e., present in all living forms). We call these oligoextentionalist and panextentionalist positions, respectively.5 Oligoextensionalist views should be familiar to all of us: viz., cognition exists only in metazoans with proper neural systems, while ‘true’ development is limited to five multicellular clades and only to those clades: animals, plants, red algae, brown algae, and fungi. In contrast, panextensionalist views for either cognition or development are less widespread, but they still can be vigorously found in scientific and philosophical scholarship. Under these lights, all living systems are deemed veritably cognitive and with respect to development, all living systems are taken to exhibit and embody this process in their life histories, from extremophile bacteria to blue whales.

For oligoextensionalists about development, the purview of developmental biology only covers the growth, differentiation, pattern formation, and remodelling that gives rise to the ‘typical’ adult forms of clonal multicellular organisms (Conway, 2020). In this sense, development can be investigated as a canonical set of events surrounding the changing structures of an organism en route towards adulthood or, more specifically, for acquiring the capacity to reproduce. For animals, these events ordinarily comprise fertilization, cleavage, gastrulation, organogenesis (including neurulation), and, for some groups, metamorphosis (Love, 2022). Oligoextensionalism about development is a frequent standpoint for developmental biologists and practitioners of Evo-Devo (e.g., Arthur, 2021).

Contrary to this approach, some Evo-Devoists have called for shedding our preconceptions of what ‘development’ is supposed to refer to and being attentive to the kinds of changes and trajectories that organisms from diverse phylogenetic groups undergo throughout their ontogenetic histories (for an overview, see Minelli, 2021). With this evolutionary diverse window frame, development could hardly be seen as the path from unicellularity to multicellularity, or as a process univocally tied to (sexual) reproduction, or something that ceases upon attainment of an adult-typical form. Indeed, if we look at the diversity of extant organisms and their life cycles, few follow the ordered transformation that turns an egg first into an embryo, then into a juvenile, and finally into an adult (Minelli, 2003, 2021). For panextensionalists, then, development refers to the structured sequence of changes any organism experiences throughout its lifespan, which alter its morphology, physiology, and behaviour. These ontogenetic trajectories can diverge significantly based on the individual or its phylogenetic group—be it a unicellular diatom, the gigantic, single-celled bubble algae, a monocarpic tree, or the so-called ‘immortal jellyfish’ that is able to transdifferentiate back into the sexually immature polyp stage.

For the domain of cognition, being a panextensionalist is one way of subscribing a strong life-mind continuity thesis (i.e., the idea that mind is, and always has been, prefigured in and indissociable from all forms of life). In this sense, some defenders of disparate frameworks such as the theory of autopoiesis, enactivism, and the free energy principle can be counted as panextensionalists regarding cognition (see, e.g., Kirchhoff, 2018; Kirchhoff & Froese, 2017; Maturana & Varela, 1980; Thompson, 2007).6 In contrast, oligoextensionalists about cognition differ in which animals, besides vertebrates like mammals and birds, they consider to exhibit cognition (e.g., insects), and they can be quite pluralistic regarding how to construe and assess it (e.g., Bräuer et al., 2020). Nonetheless, they still are to be classified as oligoextensionalists because, across the vast span of evolved lineages constituting the tree of life, they restrict cognition to few clades within the (comparatively speaking) small clade of Metazoa.

Basal cognition is a research program that pushes against oligoextensionalism of cognition, and some of its proponents have explicitly endorsed panextensionalism as an antidote to its biases (e.g., Lyon, 2006). However, we argue that taking a fully evolutionary driven approach means that one cannot subscribe any of the two positions from the get-go, either for development or for cognition. Thinking in evolutionary terms makes us reckon with the fact that we need to situate the evolutionary processes and patterns under study in phylogenetic context. For this, the scope of cognition, in a fully evolutionary view, should be seen as an open question, revisable under the influx of empirical evidence.

In contemporary Evo-Devo, both oligoextensionalism and panextensionalism about development seem to co-exist, but they guide different kinds of empirical investigation. And this is what will help us to bring into focus the upshots and limitations of the second parallel between the study of development and cognition that we have unearthed. Consider for a moment what panextensionalists about development and cognition are actually fighting against. Besides resisting ‘adultocentrism’ (sensu Minelli, 2003), panextensionalists about development have challenged a specific a priori tenet that comes with the traditional oligoextensionalist framing, namely that development is something that only happens to clonal multicellular organisms, ruling out cases of aggregative multicellularity and the changes that single-celled organisms undergo throughout their life cycles. Likewise, basal cognition researchers impugn a particular tenet of oligoextensionalist framings of cognition, namely that cognitive capacities can exclusively be instantiated by neuronal animals. For convenience, let’s refer to the former tenet as ‘strict multicellularism’ and to the latter as ‘strict neuronalism.’

Destabilising these tenets bring some promising investigative avenues. For once, they could give us a more accurate or comprehensive exploration of the bundle of phenomena at stake (i.e., development and cognition). For instance, destabilising strict multicellularism has generated interesting lines of research within Evo-Devo. The idea of development in unicellular eukaryotic organisms may have been completely dismissed not so long ago, but today there is ample evidence for developmental processes in unicellular protists (Bonner, 1967; Gilbert, 2000; Stephenson & Stempen, 1994). Likewise, disavowing strict multicellularism has allowed scientists to investigate bacterial morphogenesis, for example, the mechanisms through which sphere, rod, or spiral shapes are secured (Jiang et al., 2015), or apply the notion of ‘developmental trajectories’ to the formation of biofilms. For instance, Zhang et al. (2021) have investigated the morphogenesis of Vibrio cholerae biofilms under confining environments and Futo et al. (2021) have argued that biofilm formation in Bacillus subtilis is “a bona fide developmental process” as its shifting genetic expression profiles recapitulate central aspects of its phylogeny.