The genetic code as an operational a priori: xenobiology and the rewriting of life’s grammar

1 Introduction - the code as biological a priori

Every organism enters the world already equipped with a pre-installed operational framework: the genetic code (Nirenberg, 1963). The cell does not discover or negotiate the rule that UUU encodes phenylalanine; this mapping is inherited (Fox, 1974). In this precise sense, the genetic code functions as a biological a priori: a condition that precedes experience and constrains all subsequent biological organization. A crucial distinction underlies this argument. The genetic code is the nearly universal rule system mapping nucleotide triplets to amino acids (Kubyshkin et al., 2018a). Genomes are species-specific sequences interpreted by this rule system. Genomes vary; the code is overwhelmingly conserved. The code thus functions as a lingua franca of terrestrial life, a universal grammar shared across the biosphere (Kubyshkin and Budisa, 2017). Following Konrad Lorenz (Lorenz, 1941; Lorenz, 1982), such structures constitute a phylogenetically acquired a priori: contingent in evolutionary origin, but absolute for every organism’s lifespan. The code is not a blueprint; it is the grammar that makes blueprints readable. Synthetic biology has traditionally edited sentences written in this grammar. Xenobiology now attempts to alter the grammar itself (Luu et al., 2025).

Throughout this study, the term “a priori” is used in a naturalized, evolutionary sense, following Lorenz: a phylogenetically installed operational constraint on biological interactions, not a Kantian transcendental necessity (Malabou, 2014). Unlike the cognitive a priori, which is timeless and universal, the biological a priori arises through contingent historical events yet becomes functionally non-negotiable for any organism that inherits it. It does not precede matter or chemistry but constrains the space of viable interactions once particular molecular organizations have stabilized. This tension between contingent emergence and obligatory operation echoes Monod’s distinction between “chance and necessity,” (Monod, 1974) where stochastic origins give rise to molecular systems whose subsequent functioning is strictly constrained by physico-chemical law. A similar pattern appears at macroevolutionary scales: as Vermeij (Vermeij and Dietl, 2006) has shown, innovations that appear singular in origin often recur independently, indicating convergence toward stable functional solutions. Together, these cases support a view in which biological systems impose strong, historically derived operational constraints without invoking metaphysical necessity, preserving the explanatory force of the a priori analogy while avoiding ontological reification. Related structuralist approaches (Lévi-Strauss, 2008) anticipated rule-based constraints in living systems (Chaabani, 2024) but lacked an experimental framework capable of modifying the underlying biological grammar itself (Hénaff, 1998).

Philosophical claims about the genetic code as a biological a priori - a universal framework enabling and constraining life’s possibilities - rest squarely on precise biochemical facts. Therefore, prevalent misconceptions must first be dispelled to clarify the argument’s foundation. Contrary to suggestions that the code has been “expanded” by selenocysteine or pyrrolysine (Zhang et al., 2005) or is “still evolving” (Osawa et al., 1992), these phenomena do not alter the core decoding rules. Selenocysteine and pyrrolysine, often called the “21st and 22nd amino acids,” are incorporated via context-dependent recoding mechanisms, semantic overrides rather than revisions of the standard genetic dictionary (Baranov et al., 2002). Similarly, post-transcriptional and post-translational processes like RNA editing or protein modification (Alqinyah and Hooks, 2018) operate outside or above the translation layer and must not be conflated with the decoding mechanism (i.e., code) itself.

Since its deciphering in the 1960s (Crick, 1967), the Standard Genetic Code has proven a remarkably stable framework for nearly all life. While modern discoveries have identified alternative codes - most notably in mitochondria and chloroplasts (Jukes and Osawa, 1990) - these represent local “dialects.” They involve reassignments among the same 20 canonical amino acids rather than radical departures from the framework. This stability is functionally profound. The code’s near-universality acts as the Biosphere’s shared lingua franca, enabling horizontal gene transfer and constraining large-scale divergence (Gogarten and Townsend, 2005). It allows genetic innovations, such as antibiotic resistance (Sun et al., 2019), to propagate across life’s domains. Claims of the code “still evolving” in isolated lineages (Kivenson et al., 2025) are often overstated; where gene flow is restricted (e.g., in organelles or certain extremophiles), minor deviations emerge as species-specific adaptive tweaks (Santos et al., 1997), not fundamental revisions.

Thus, the genetic code stands as a universal and stable rule system - a prerequisite for the unity of life and a foundational constraint on its evolution. Existing reviews provide a valuable descriptive overview of xenobiology’s conceptual breadth, yet do not engage with the genetic code as an operational a priori governing translation itself, rather than merely one component among others (Wicaksono and Cristy, 2021). The present work addresses this gap.

While the genetic code has often been described as fundamental or a priori in a loose philosophical sense, this work advances a distinct claim: that the genetic code functions as an operational biological a priori - a material constraint that can now be experimentally modified. Xenobiology transforms the a priori from a descriptive philosophical category into an experimentally accessible and rewriteable condition of biological possibility.

Throughout this work, terms such as “essence,” “grammar,” and “transcendental” are used in a strictly operational and biological sense, referring to materially instantiated constraints rather than to autonomous metaphysical entities. The manuscript makes no ontological claims about the essence of life, nor does it advance metaphysical realism. The argument is explicitly epistemological and operational, concerning experimentally accessible constraints on biological construction.

2 The central dogma as the operational form of the biological a priori - and its xenobiological rewriting

The genetic code functions as a biological a priori only because the Central Dogma (Crick, 1970) provides the operational machinery through which its formal rules become causally effective. DNA → RNA → protein is not merely an information flow, but the material process that translates abstract coding constraints into concrete biological form. Without this translational pipeline, the code would remain a mere mapping; with it, the code shapes every phenotype (Scheme 1).

This pipeline embodies a layered architecture of constraint - precisely what characterizes an a priori system. Beyond the canonical “first genetic code” of codon–anticodon pairing, a “second genetic code” (De Duve, 1988) (governing aminoacyl-tRNA synthetase identity) and multiple translational proofreading mechanisms together form a hierarchical grammar inherited wholesale by all organisms (Hopfield, 1974). Genomes vary; this interpretive machinery does not. That is why the code operates as a universal rule-system: a stable, evolutionarily installed logic that every cell must obey from its first moment of existence.

Xenobiology (Budisa et al., 2020) intervenes exactly here. While classical synthetic biology (Endy, 2008) rearranges genomic content within the inherited grammar, xenobiology alters the grammar itself. Genomically integrated orthogonal translation systems (Karbalaei-Heidari and Budisa, 2024), recoded genomes (Wannier et al., 2018), expanded amino acid alphabets (Koch and Budisa, 2024), and xeno-nucleic acids (XNAs) (Handal-Marquez et al., 2020) all modify the operational rules of the Central Dogma (Agostini et al., 2017). These constructs are not just new messages in the old language; they are experimentally instantiated alternative a priori’s - new pathways from information to function (Luu et al., 2025).

Thus, the Central Dogma is both the executor of life’s inherited logic and the point at which that logic can be rewritten. Terrestrial life has, until now, been bound to one translational grammar (Levin and Budisa, 2023). By engineering new versions of the Dogma (Liu et al, 2018), we shift from studying life as it is to exploring life as it could be, moving from a single biological a priori to a plurality of possible ones. Xenobiology transforms the Central Dogma from a fixed operating system into a design space for biological possibility (Budisa, 2014).

Minor deviations in the genetic code (Osawa et al., 1992) - found in mitochondria, some ciliates, and endosymbionts (Tourancheau et al., 1995) - do not undermine this universality; they highlight it. These are not independent inventions, but lineage-specific “hacks” atop an established translational architecture, akin to regional dialects of a shared language (Acevedo-Rocha and Budisa, 2016). The deep structure - the core translational machinery, the synthetase system, the ribosomal framework - remains universally conserved, tracing to a single evolutionary origin. Horizontal gene transfer reinforced this unity: interoperability was existential, selecting for a common “lingua franca” across the early Biosphere (Luu et al., 2025).

Universality, therefore, is not metaphysical but system architectural. Complex, code-dependent systems converge on single standards because compatibility is a precondition for complexity (Whitacre, 2010). Recognizing this shifts the debate away from claims of “non-universality” (Di Giulio, 2025) and toward the true philosophical stakes (Vincent, 2015): the unity of life is informational, grounded in a common operating logic. Once this is understood, the implications of rewriting that logic - ethically, existentially, and ideologically - become profound.

Finally, the most precise recent definition of xenobiology is provided by Schmidt, Kubyshkin, and Budisa: “Xenobiology is the science of estranged life forms. More specifically, it is an emergent technoscience that combines advances in genetic engineering with the design of biological systems based on unusual biochemistries delivered by chemical compounds of mostly anthropogenic origin. Xenobiology enables us to create and study strange new life forms - not as science fiction imagines them, but as objects of enlightened science, design, and engineering.” (Budisa et al., 2020) This definition captures the conceptual horizon of the field and marks the foundation for all discussions that follow.

3 What is life? The search for a minimal unit and a definition

The question “What is life?” has long transcended the boundaries of biology. From antiquity to modern science, the problem has demanded both philosophical clarity and empirical rigor. Erwin Schrödinger’s seminal 1944 essay (Schrodinger, 2012) provided a turning point: he argued that life’s stability must rest on an aperiodic crystal able to store and faithfully transmit hereditary information. This was a striking conceptual leap that effectively anticipated the genome concept (Stencel and Crespi, 2013) grounded in the double-helical structure of DNA (Watson and Crick, 1953).

Despite its extraordinary diversity, all known life rests on a universal biochemical platform. Organisms share a common core chemistry (Smith and Morowitz, 2004) built from roughly 100–125 conserved metabolites (Srinivasan and Morowitz, 2009) - a metabolic network so ancient that it predates the oldest surviving rocks on Earth (Morowitz et al., 2000). They rely on the same triad of macromolecules - nucleic acids, proteins, and lipids - and deploy a conserved organization of information processing and metabolism (Kubyshkin and Budisa, 2019a). This deep unity suggests an unavoidable question: what is the minimal functional unit of biological organization? (Ellinger and Ellinger, 2013).

Other sciences have clear answers: the atom in physics, the molecule in chemistry, the unit cell in crystallography. In biology, however, the minimal unit is not a single structure but a self-sustaining system. It is relational rather than material; defined not only by what it is made of, but by what it does.

This realization is not new. Leibniz famously described organisms as “divine automata,” (Klockenbusch et al., 1992) machines whose smallest parts are themselves machines - an early and remarkably accurate intuition of biological nestedness. The 20th century translated these metaphysical insights into formal models. John von Neumann envisioned a self-reproducing automaton (Von Neumann and Burks, 1966) in an abstract, “solid-state” geometry: a logical entity able to build a copy of itself from encoded instructions. Tibor Gánti provided the first chemically plausible analogue: the Chemoton, a “fluid-state automaton” representing the minimal organization needed for life (Gánti, 2003b).

Gánti’s Chemoton (Gánti, 2003a) consists of three inseparably coupled subsystems: An autocatalytic metabolic cycle (energetics and material flux). An information carrier (a hereditary program). A membrane (a boundary that creates individuality). From this architecture, Gánti derived a rigorous set of criteria for life (Griesemer and Szathmáry, 2009). The absolute criteria - individuality, metabolism, and inherent information storage - are indispensable. The facultative criteria - growth, reproduction, evolution, and mortality - are emergent properties arising when many such units exist in a population (Scheme 1).

Seen through this lens, life is a process governed by two fundamental principles: (i) The deterministic laws of physics and chemistry, which set the possibilities. (ii) The genetic program, which selects among those possibilities and directs biological form and function. It is a notion widely accepted among thinkers on the origin of life - not only within empirical science but also in philosophy and even in certain strands of scientific theology, which represent distinct yet complementary forms of knowledge (Soriano and Herce, 2024).

It is precisely at this interface that the genetic code sits. The code is the algorithmic bridge between blind physico-chemical dynamics and the teleonomic organization of living systems (Vane-Wright and Corning, 2023): a symbolic mapping that turns chemical sequences into functional proteins. In doing so, it anchors life’s autonomy, allowing matter to become information-bearing and information to become causally efficacious (Kay, 2000). Without this mapping, Gánti’s Chemoton would collapse into prebiotic chemistry; with it, chemistry becomes biology.

4 A compressed historical arc - from material unity to informational grammar

Reopening what Pasteur called the “closed door” of life’s origin (Pasteurian Wall) (Budisa and Levin, 2025) is thus not a mystical endeavor. It marks a historical transition from studying life-as-given to engineering new biological a priori. This shift is simultaneously experimental, conceptual, and societal, forcing a confrontation with the foundations of biology and broadening the meaning of “life” beyond its inherited, historical grammar (Schmidt et al., 2017). Starting with Thales of Miletus, the claim that “everything is made of water” (Glasgow, 2009) may look naïve today, but it captures a surprisingly modern intuition: life emerges from the physico-chemical conditions of the planet itself (Spitzer et al., 2015). In Thales’s time, “water” stood for the underlying, unifying substrate - what we would now call the material continuum out of which more complex forms arise. In that sense, Thales was the first to argue that life is not imposed from outside but generated from the world’s own matter and processes (Schulze-Makuch and Irwin, 2004).

Contemporary origin-of-life research gives this early intuition real scientific footing. As Smith and Morowitz argue (Smith and Morowitz, 2016), life did not begin with a single spectacular lightning bolt à la Miller–Urey (Miller and Urey, 1959), but through a planetary-scale transition in which segments of geochemistry gradually reorganized into geo-biochemistry. Metabolism-like networks, redox gradients, mineral surfaces, and catalytic cycles coalesced first; genetics arrived later. Life was not an event but a phase transition of the Earth’s surface. This culminates in Vernadsky’s insight (Vernadsky, 1998): the Biosphere is not simply a layer of organisms sitting on top of geology but a geological force - a structured chemical regime that stabilizes and reshapes the planet. Life becomes the Earth’s fourth geosphere, a thermodynamic engine integrated into planetary chemistry (Vernadsky, 2012). This materialist narrative finds a parallel in a more formalist tradition, beginning with Pythagoras (Rowett, 2013), which would see the code not as emergent chemistry but as immutable number (Scheme 1).

Placed in this long arc, the genetic code appears not as an arbitrary convention but as the deep informational crystallization (Gaiseanu, 2020b) of this planet-wide transition. The code is the grammar through which geochemistry became biology, the point where self-sustaining chemistry acquired a universal rule set. It is the moment where Thales’s primordial material unity gave rise to Vernadsky’s planetary biochemical order. And xenobiology? It represents the first deliberate attempt in human history to reopen that transition, to create new informational grammars and new forms of biochemistry with the potential to become alternative geo-biochemistry (Budisa, 2014). Where early philosophy asked from what does life arise? Xenobiology asks what other forms of life could arise if the grammar itself were changed? (Kubyshkin and Budisa, 2017). This shifts the question from metaphysics to experiment - still operating in the spirit of Thales, but with better tools than “everything is water.”

Pythagoras’s claim that “number is the ruler of forms and ideas” (Burgin, 2022) resonates uncannily with modern molecular biology (Scheme 1). Life’s deepest architecture is numerical: four nucleotides, twenty canonical amino acids, and a strictly finite set of sixty-four codons (Kubyshkin and Budisa, 2019a). The genetic code is not just a chemical mapping - it is a discrete combinatorial system, a kind of arithmetic through which matter becomes organized function (Rakočević, 2018; Zolyan, 2022). In this sense, biology realizes a Pythagorean intuition: form and function arise from numerical constraint (Partridge and Sibly, 1991). Xenobiology extends this insight by altering the numerical parameters themselves - changing the alphabet, expanding the codon table, and thereby exploring entirely new spaces of biochemical “form.”

The maxim attributed to Pythagoras (Riedweg, 2008) in later tradition - ‘Reason is immortal, all else mortal’ - maps surprisingly well onto biology’s deepest hierarchy. Species, lineages, and entire kingdoms rise and vanish over evolutionary time (Cavalier-Smith, 1998), yet the genetic code - the rule of translation - remains unchanged, the enduring logic beneath life’s shifting forms. In this sense, the code functions as the “immortal reason” of terrestrial biology: a stable, universal grammar persisting while everything built upon it is transient. Xenobiology is the first deliberate attempt to rupture of this pattern, not altering the mortal forms but intervening in the supposedly immortal logic itself - rewriting the very reason that has governed life since its origin (Luu et al., 2025).

Plato distinguished between the shifting shadows of the sensible world and the stable, underlying Forms that give them structure (Naas, 2018). Biology mirrors this hierarchy. The phenotypes we observe - species, traits, behaviors - are the shadows: transient, contingent, endlessly variable (Scheme 1). The genetic code functions more like a Platonic Form (Whittaker, 1968): a stable, universal rule system that shapes all possible biological appearances without itself changing. Life’s observable diversity is the play of shadows; the code is the underlying ideal grammar that makes those shadows coherent. Xenobiology is philosophically radical precisely because it reaches into the level of the Form itself, altering the generative structure rather than the fleeting appearances - rewriting the “ideal” rather than rearranging its shadows.

5 Ontological foundations - essence, existence, and the code

Lucretius, in his Epicurean atomism (Johnson and Wilson, 2007), understood life as arising from countless random combinations of atoms, with only the successful configurations persisting. Replace atoms with amino acids and nucleotides as well as universal metabolites (Smith and Morowitz, 2004) and his view becomes a proto-evolutionary insight: life’s forms are contingent outcomes of combinatorial chemistry, while the genetic code imposes the deep constraints that channel which combinations can ever exist. Xenobiology intervenes precisely at this level, rewriting those constraints and exploring molecular possibilities nature never attempted - an experiment Lucretius would have welcomed as a new chapter in De rerum natura (Lucretius, 1744).

Avicenna (Ibn Sīnā) first articulated the foundational metaphysical distinction between essence (mahiyya - what a thing is) and existence (wujūd - that it is), arguing that an essence does not entail its own existence (El-Bizri, 2001) This framework, later adopted and Christianized by Aquinas, was transformed: where Avicenna treated existence as a philosophical and structural principle, Aquinas reinterpreted it as an act of being (actus essendi) bestowed by God (Corrigan, 1984; Kenny, 1991). Aquinas treats existence as an act given by God, whereas Avicenna frames it more philosophically and structurally. Strikingly, this ancient conceptual structure maps with remarkable clarity onto modern biology. The genetic code corresponds to essence - a universal, abstract, and necessary informational rule system that defines the possible forms of proteins, metabolisms, and organismal architectures (Scheme 1). In contrast, individual organisms and species represent existence - the contingent, historically evolved instantiations of that deeper essential blueprint.

Life’s diversity, in this view, arises from countless realizations of a single biochemical essence - an essence encoded in the universal genetic code. Xenobiology is revolutionary precisely because it intervenes not in existence but in essence, altering the rule system rather than its manifestations. It creates new biochemical conditions for what life can be, not merely new variations of what already exist Therefore, the genetic code corresponds to essence: a universal, abstract rule system defining what kinds of proteins, metabolisms, and organismal architectures are even possible (Kubyshkin and Budisa, 2019b). Individual organisms and species are existence - contingent instantiations of this deeper informational essence shaped by environment and evolution. In this sense, the code is the necessary form underlying life’s many contingent realizations. Xenobiology becomes revolutionary because it intervenes not at the level of existence but at the level of essence itself - creating new informational conditions for what life can be, not merely new variations of what already exists.

From an ontological perspective, the genetic code acts as a formal cause, in the Aristotelian and Thomistic sense (Evans, 1959). It is an informational structure that predetermines which proteins, metabolic architectures, and organismal forms can ever be realized. Without the code, genomes would remain chemically uninterpretable. With it, sequences become functional living matter. Thus, the code specifies the essence of terrestrial life, while cellular instantiation provides its existence.

The genetic code can be philosophically situated between Aquinas and Kant. In a Thomistic sense (Garrigou-Lagrange, 2021), the code functions as a formal cause: an informational structure that defines the range of biological potentialities but requires cellular instantiation to become actual - mirroring Aquinas’s distinction between essence and existence (Kenny, 1991). At the same time, in a naturalized Kantian sense (following Konrad Lorenz), the code operates as an a priori constraint: an evolution-shaped framework that conditions all possible forms and functions an organism can realize (Lorenz, 1982). Thus, the genetic code embodies both an ontological role - specifying what life can be - and an epistemic-structural role - constraining how life can appear and operate. It stands at the intersection of classical metaphysics and evolutionary epistemology: a formal blueprint with the status of a biologically grounded, non-transcendental a priori.

Sartre’s slogan “existence precedes essence” (Linsenbard, 2010) collapses the distinction Avicenna and Aquinas understood far more deeply. In living systems, essence very clearly precedes existence: the inherited biochemical constraints - the genetic code above all - define what an organism can be long before it comes into the world (Kubyshkin and Budisa, 2019a). Sartre’s existential voluntarism (Sartre, 1966) simply does not survive contact with molecular biology. Avicenna and Aquinas grasped a truth Sartre missed entirely: there are necessary structures without which no being - biological or otherwise - could exist at all. The genetic code exemplifies this perfectly. It is the essence that conditions existence, not an optional script we invent afterward.

The remarkable stability and near-universality of the Standard Genetic Code, defended against common misconceptions in the previous chapter, presents a profound biological fact: all known life shares a common rule system for translating information into function. This invites a philosophical question. What is the ontological status of this rule system? Is it merely a historical accident, or does it play a more fundamental, constraining role in biology?

A useful lens for this inquiry comes from classical metaphysics, specifically the distinction between essence (what a thing is) and existence (that it is), as explored by Avicenna and Aquinas. This framework finds a striking analogue in biology. (i) The Genetic Code as Essence: It is the universal set of rules that defines what forms of protein-based life are possible. It is the necessary, formal cause -the “what-it-is-ness” - of life as we know it. (ii) The Organism as Existence: Any individual creature is a contingent instantiation of those rules, shaped by historical accidents, environmental pressures, and evolutionary chance - the “that-it-is.”

From this perspective, the radical nature of synthetic xenobiology comes into sharp focus. Most genetic engineering - editing genes within an organism - operates at the level of existence, creating new variations within the given rules. Xenobiology, by contrast, aims to alter the rule system itself. It intervenes not in the contingent products of biology, but in its essential framework, thereby redefining the space of what life can be.

This is not a crude biological determinism. The genetic code does not dictate specific phenotypes any more than the rules of chess dictate specific games. Rather, it determines the space of possible phenotypes. Altering the code changes the boundaries and contents of that space, launching biology into a new realm of possibility.

6 The paradox of origin - contingency, closure, and the frozen accident

Kant’s insight that experience depends on non-negotiable a priori structures (Ginsborg, 2006) find a direct biological analogue in the genetic code. As Konrad Lorenz argued, evolution installs species-wide constraints that function as “phylogenetically acquired a priori” (Lorenz, 1982). The genetic code is precisely such a constraint: inherited rather than learned, fixed within an organism’s lifetime, and delimiting the space of possible phenotypes and Umwelt. Importantly, describing the genetic code as a biological a priori does not imply a deterministic mapping to phenotype; rather, it constrains the space of realizable biological forms, within which phenotypic diversity emerges through regulatory, developmental, and ecological layers built upon this shared grammar. Organisms do not choose how codons are interpreted - they receive the rulebook in advance.

Bacon’s dictum knowledge is power (Vickers, 1992) foreshadows the shift from describing life to engineering its conditions of possibility. The genetic code transforms biological knowledge into operational power: understanding life’s grammar grants the capacity to rewrite it. Hobbes and Descartes, in their mechanistic visions of organisms as automata (Sorell, 2014), anticipated the informational skeleton revealed by molecular biology. This mechanistic framework - especially Descartes’ dualism of res extensa (extended substance) and res cogitans (thinking substance) - led to the speculation that sufficiently advanced machines might become indistinguishable from humans (Descartes, 2008). Today we know that life is indeed composed of coded instructions and molecular machines (Mavroidis et al., 2004) - but that even this machine operates under a fixed logical architecture. Xenobiology pushes this insight further: the rules governing mechanistic life are not metaphysical necessities but editable design principles. Bacon, Hobbes, and Descartes mark the transition from explanation to control; the code provides the machinery, xenobiology the power.

Locke’s tabula rasa (Locke, 1847) offers a sharp contrast. Organisms do not enter the world as blank slates but with inherited molecular “handwriting,” encoded in the genome and interpreted through the universal genetic code. Experience shapes behaviour, but the biochemical a priori structures the very capacity for experience. Xenobiology sharpens this point: by altering the genetic code, we do not merely write new content on the slate - we change the slate’s material, the grammar through which any writing becomes meaningful.

Leibniz (1989) described living beings as divine automata: machines whose components are themselves machines, descending through nested layers of organization. Modern molecular biology confirms this view with precision: cells are hierarchies of coded molecular machines, all arising from the universal rule system of translation (Scheme 1). The genetic code is the primitive rule of construction—the generative logic from which all subordinate biological “machines” emerge. Monod captured this intuition in his famous dictum that “what is true for Escherichia coli is true for the elephant” (Ball, 2023). Xenobiology is revolutionary precisely because it intervenes at this foundational level: it rewrites the rule that builds the machines that build the machines.

Hume dismantled the idea that we observe necessary connections in nature; we encounter only constant conjunctions (Hume, 2014). Under this lens, the genetic code appears as a frozen historical habit rather than a rational law. A mapping such as UUU → Phe is not metaphysically necessary, but a contingent regularity propagated by inheritance. Hume would therefore see the code’s universality as custom, not reason - and xenobiology proves him right by showing that codon assignments can indeed be changed.

Kant’s response to Hume illuminates the code anew. For Kant, experience requires a priori structures that render the world intelligible (Ginsborg, 2006). The genetic code functions as such a structure: not a cognitive a priori but an operational one, a precondition for any genome to be meaningful. Hume reveals the code’s contingency; Kant reveals its necessity within life as we know it (Molina, 2010). Xenobiology unites both insights by demonstrating that the biological a priori is historically contingent yet experimentally rewritable - a Kantian operation performed on a Humean substrate.

Darwin completes the picture (Gilson, 2009). What is a priori for the organism is a posteriori for evolution (Díez and Lorenzano, 2015). The genetic code is Crick’s “Frozen Accident”: a contingent, emergent structure that became the deep constraint on all subsequent diversification - the closure arising from contingency. Xenobiology intervenes at precisely this level, modifying not the products of evolution but the biochemical conditions - the frozen accident itself - under which evolution operates (Scheme 1).

7 Contemporary philosophy of biology: the genetic code between mechanism and meaning

Why this is not simply Kant again? In Kant’s epistemology, the a priori is transcendent and immutable (Kant, 1999): it structures experience but cannot itself be changed by experience. The biological a priori discussed here differs fundamentally. The genetic code is historically stabilized by evolution and materially realized through molecular mechanisms, but it is not fundamentally immutable.

Contemporary xenobiology shows that the genetic code can be experimentally altered, reassigned, or expanded, directly probing the limits of this constraint. In this sense, xenobiology transforms the a priori from a primarily philosophical category into an experimentally accessible object: not a timeless condition of knowledge, but a historically contigent, technically modifiable condition of biological possibilities. This shift - from transcendental necessity to operational mutability - characterizes the conceptual novelty of the present framework.

Hume showed that observed regularities need not be necessary (Hume, 2014); the code’s early assignments were contingent rather than necessary: not arbitrary in the literal sense yet not enforced by immutable physical laws. Kant showed that experience depends on structured conditions (Kant, 1999); the code functions as an operational a priori for cellular “experience” (Lorenz, 1982).

Darwin revealed how contingent structures become fixed constraints through selection. Modern philosophy of biology adds complementary views: (i) Mechanistic philosophy (Schaffner, 1969): translation as a multi-level mechanism; (ii) Process philosophy (Gilson, 2009): life as dynamic self-organization (Maturana and Varela, 2012); (iii) Computational/informational views (Farnsworth et al., 2013): codon assignments as semantic rules (Stegmann, 2016). The a priori analogy is therefore heuristic, not literal - but scientifically powerful.

Nietzsche’s claim that “man is something to be surpassed” (Nietzsche, 1910) was existential, not biochemical, but it gains literal force once the contingency of the genetic code is understood. The entire architecture of human experience - from perception to emotion - rests upon the substrate defined by the twenty-amino acid alphabet. Nothing in nature requires this particular set; it is merely the contingent outcome that evolution stabilized. Once this code becomes editable, human biology becomes one biochemical possibility among others - not a metaphysical endpoint, but a historical configuration open to revision.

Haraway’s cyborg is posthuman (Haraway, 1994) at the level of the interface: a hybrid of organism, machine, and information systems. Xenobiology is posthuman at a far deeper ontological stratum. It intervenes not in the body but in the grammar that makes bodies possible at all. If the cyborg forges new couplings, xenobiology engineers new biochemical worlds. It shifts the locus of posthuman transformation from prosthetics and computation to the very molecular conditions of embodiment: to the invention of alternative chemical spaces for life itself.

The relevance of Foucault, Derrida (Calcagno, 2009), and other thinkers of similar orientation is historical rather than explanatory. They help illuminate how the very notion of a ‘genetic code’ emerged within the 20th-century epistemic framework shaped by cybernetics and information theory. But the operation of the code is not discursively constructed - it is enforced by chemistry. A codon reassignment that kills a cell is not a linguistic event or a power relation; it is biochemical necessity asserting itself. When applied too broadly, constructivism (Suchting, 1992) becomes corrosive in this domain: it obscures objective constraints and fosters the illusion that empirical truths can be negotiated. Xenobiology, by contrast, forces hypotheses to collide directly with the recalcitrance of matter.

Merleau-Ponty emphasized that organisms inhabit the world through pre-reflective, embodied structures (Reuter, 1999). The genetic code constitutes the deepest layer of this embodiment - it sets the biochemical parameters that define what kinds of lived worlds (Umwelten) (Chebanov 2001) any organism can have. Rewriting the code thus reshapes the preconditions of embodiment rather than its secondary manifestations.

For Carnap, frameworks determine what counts as a meaningful statement (Quine, 1960). Analogously, the genetic code is the biochemical framework that makes sequences interpretable. Xenobiology becomes philosophically significant because it modifies this framework itself, producing new rules for what can count as a “sentence” in biological reality. Popper would interpret the genetic code as a universal hypothesis about the logic of translation - corroborated but never proven. Xenobiology intend to perform exactly the “severe tests” he advocated: codon reassignment and code expansion probe the boundary between what is chemically necessary and what is historically contingent. By attempting to rewrite the code, biology moves from passive explanation into active falsification (Popper, 1963).

Finally, Midgley’s pluralism (Midgley, 1992), Comte’s empirical discipline (Comte, 1975), and Husserl’s concern with constitutive structures (Sandmeyer, 2009) converge on a single point: the genetic code is a foundational layer - empirical, constraining, constitutive. Xenobiology does not undermine this insight; it extends it. Even foundational layers can be modified, provided we respect the objective constraints that make living systems coherent.

Processual and systems biologists argue that life is best understood as dynamic organization rather than as discrete informational structures (Gaiseanu, 2020a). This view complements rather than contradicts the present thesis. While the genetic code is evolutionarily dynamic, its rate of change is negligible on ecological and organismal timescales (Barbieri, 2019b); thus, it still functions as an operational a priori within living systems. The processual view highlights that the code is not metaphysically fixed but materially embedded, evolving slowly while constraining faster layers of biological organization.

8 Bio-philosophy and theoretical biology - towards a naturalized transcendentalism

This inquiry stands within a specific and fertile tradition of thought: naturalized transcendentalism (Ramstead, 2015). This study does not seek to revive orthodox Kantianism but to reinterpret its core insight - that experience is structured by pre-existing, non-negotiable conditions - in biological terms. We transpose the a priori from the realm of pure reason to the domain of embodied, evolutionary information. The genetic code, in this framework, is not a cognitive structure but an operational a priori.

This move is fundamentally heuristic. It is a powerful analogy, a category-bending metaphor that is conceptually fertile precisely because it is not a literal mapping. The genetic code is not a priori in the strict, Kantian sense of a timeless, necessary condition for any possible rational being. It evolved; it is contingent, a “frozen accident” of evolutionary history that could have been otherwise, as Hume would rightly insist. Yet, for any individual organism, it is given as necessary. It is a phylogenetically acquired framework that functions, within the lifespan of a being, as the absolute condition of possibility for its biological “experience” - its structure, its functions, and its Umwelt. It is the formal cause that must be in place before any material cause can take effect.

This position is not an isolated speculation but finds robust parallels in a lineage of 20th-century theoretical biology and bio-philosophy that sought to biologize Kant’s transcendental framework: The project was pioneered by Konrad Lorenz, who, in works like Behind the Mirror (Lorenz, 1977), argued for an “evolutionary a priori,” positing that the innate categories of understanding are biological adaptations shaped by phylogeny. I extend this logic from the categories of the mind to the grammar of the cell. This biological reframing of purpose is central to Ernst Mayr’s concept of teleonomy (Mayr, 1974). In his Toward a New Philosophy of Biology (Mayr, 1988), Mayr describes organisms as acting as if guided by purpose, a directedness made possible by a programmed, mechanistic substrate. The genetic code is the foundational layer of this programming, the a priori informational ground for all teleonomic functions.

The work of Francisco Varela in Principles of Biological Autonomy (Varela, 2025) further radicalizes this view. Varela interprets living systems as autopoietic, or self-constituting, unities characterized by operational closure (Maturana and Varela, 2012). This closure - the circular, self-referential network of processes that defines the organism - functions as a kind of transcendental condition for biological cognition and identity, a direct parallel to the code’s role as a precondition for biological function.

A similar line of thought appears in Hans Jonas’s The Phenomenon of Life (Jonas, 2001). For Jonas, metabolism is the fundamental a priori of existence: a continual self-renewal that constitutes life before any reflective cognition or representation. This is not far from the reasoning of Smith and Morowitz on the universality and inevitability of intermediary metabolism (Smith and Morowitz, 2004), Wong’s co-evolution theory of the genetic code (Wong, 1975) and the Budisa–Kubyshkin Alanine World model (Kubyshkin and Budisa, 2019a). Jonas explicitly links this metabolic teleology to Kant’s notion of “natural purposes,” grounding purposiveness not in subjective reason but in the organism’s own material organization.

Within this conceptual framework, the genetic code becomes a deeply conserved, materially instantiated informational architecture, maintained by biochemical mechanisms rather than existing independently of them. This architecture enables and constrains metabolic “freedom” and forces the entire Biosphere to operate within the biochemical “bubble” of the Alanine World (Scheme 2). Life’s autonomy, in other words, is always already shaped by a prior molecular logic (Kubyshkin and Budisa, 2019b).

This line of thinking culminates in what philosophers like Michel Bitbol (Bitbol et al., 2009) and Jean Gayon (Gayon, 2009) have termed transcendental biology, an inquiry into the laws and structures that serve as the conditions of possibility for evolution and self-organization themselves. My argument places the genetic code as the primary candidate for such a transcendental structure.

Code Biology (Barbieri, 2012a) offers an important complement to this view. Barbieri systematically argues that life is defined by the emergence of organic codes (Barbieri, 2012b), with the genetic code being the first and most fundamental (Barbieri, 2019a). He explicitly frames it as a “semiotic rule system” and even as “the first organic a priori,” a formal, rule-governed realm that logically precedes the phenotype it specifies (Barbieri, 2003). The genetic code is the deepest of these organic codes: a symbolic mediation layer that stands as a genuine third party between genes and proteins, enabling biological organization by separating meaning (codon assignments) from mechanism (aminoacylation and translation). In this semiotic framework, communication processes are characterized by three components: the encoding of a message by the sender, the content of the message, and the process of decoding by the receiver.

Finally, the foundational work of Howard Pattee on the “epistemic cut” provides a crucial formalization (Pattee, 2001). Pattee’s cut distinguishes the dynamic, physical world of molecules from the symbolic, informational world of the code. This cut was not spontaneously generated; it required a material bridge. As Morowitz and Smith’s research (Smith and Morowitz, 2016) suggests, a universal metabolic core first generated simple oligomers - precursors to proteins and nucleic acids - from basic C, H, O chemistry. These oligomers formed stable chemical associations, creating a primitive “memory” - a record of successful chemistry that was first formalized in a suitable organic code and later fixed into the heritable genetic code. This mediating function - allowing a sequence of nucleotides to control chemical dynamics by building on this chemical memory - is a quasi-Kantian move that establishes the code as the necessary condition for the transition from mere matter to meaningful biological organization.

Therefore, this study is not a departure from theoretical biology but a synthesis of one of its most profound currents. By viewing the genetic code through the consolidated lens of naturalized transcendentalism, I integrate the insights of Lorenz’s evolutionary epistemology, Varela’s operational closure, Jonas’s metabolic purpose, and Barbieri’s semiotic rule systems. Together, these perspectives illuminate the code as a contingent but functionally a priori framework - an inherited structure that precedes and constrains all biological organization. On this basis, xenobiological intervention becomes more than a technical manipulation. It becomes a philosophical act: the experimental creation of a new biological a priori, the deliberate engineering of new conditions for what life can be - indeed, engineered conditions for possible future forms of life.

9 The “paradox” of the code’s origin: cause and effect in one system

The classical paradox - translation requires proteins, proteins require translation - reveals the co-emergence of information and function (Nida, 1991). Modern work on proto-ribosomes (Davidovich et al., 2010), tRNA minihelices (Buechter et al., 1993), and RNA–peptide coevolution (Frenkel-Pinter et al., 2020) suggests paths toward resolving this circularity. Understanding the genetic code as a kind of naturalized a priori must confront two biological realities. First, although the code is nearly universal, the discovery of a small number of variant genetic codes shows that it can undergo lineage-specific modifications. These rare rewritings do not undermine the a priori analogy; they refine it as discussed earlier. For any organism, whatever code it inherits - canonical or variant - functions as the non-negotiable operational ground of its biology. The extreme rarity and high cost of such changes highlight not plasticity, but the depth of the code’s integration. The code’s semiotic arbitrariness - arbitrariness in the Shannonian sense of a conventional mapping - solidified over evolutionary time into functional necessity, producing the genetic code’s well-known “frozen” character (Barbieri, 2007). Xenobiology directly challenges this integrated architecture through orthogonal translation systems, chemical estrangement, and genetic recoding, pushing against nearly four billion years of stabilization and conservation.

The second complexity is the classical paradox of the code’s origin, explored by Manfred Eigen (Eigen and Schuster, 1978), Stuart Kauffman (Kauffman, 1992), and others: the genetic code is both the cause of precise protein synthesis and the effect of a system that already required proteins to evolve. Translation demands enzymes, but enzymes require translation. This circularity reveals the code not as a transcendent platonic form but as an evolutionarily emergent solution that, once established, became a fixed constraint. In this sense, it is a phylogenetic a priori: an inherited structure that closes the loop between genetic information and material function. Jacques Monod saw this closure as the essence of biological necessity (Monod, 1974).

Any rigorous treatment of the code’s arbitrariness must begin with Claude Shannon’s information theory, which measures signal probability while deliberately excluding semantics (Shannon, 1948). This provides a useful counterpoint: a codon is simply a triplet of nucleotides. Its “meaning” arises not from chemistry but from the evolved interpretive machinery of the cell (Danchin, 2008). Biological information is inherently functional and semantic; the codon–amino acid linkage is not chemically inevitable but historically contingent. As Monod argued in Chance and Necessity (Monod, 1974), evolution “chooses” these mappings, which then become frozen by selection into systems of necessity.

Erwin Schrödinger’s notion of an aperiodic crystal (Schrodinger, 2012) provides the conceptual precursor to this logic. He envisioned a stable, non-repetitive molecular structure capable of storing a “code-script” for life - a prediction that found its physical embodiment in DNA. Early codon–amino acid assignments were likely arbitrary in precisely this semiotic sense (Barbieri, 2007). As translation machinery evolved and stabilized, these once-contingent assignments became increasingly conserved (Crick, 1968). This logic itself has deep evolutionary roots: as Szathmáry (Szathmáry, 1993) argues, early ribozymal systems in the RNA world established primitive coding relations through amino-acid cofactors and proto-tRNA handles, creating the first contingent genotype–phenotype couplings. When later biochemical evolution expanded the amino-acid set along alanine-derived scaffolds, these early assignments became fixed into the canonical code, transforming a contingent molecular beginning into a universal constraint. Once the system solidified, the expansion to the canonical twenty amino acids was no longer a matter of free choice but a historically constrained process shaped by metabolic availability, chemical and biophysical plausibility, requirements for folding hierarchies, increasing demands for translational fidelity, and overall biosynthetic burden - constraints collectively encapsulated in the Alanine World model (Kubyshkin and Budisa, 2019a) (Scheme 2).

This trajectory - from initial arbitrariness to deep evolutionary conservation - explains the code’s near-universality and its “frozen” character (Crick, 1968): once the system crystallized, any reassignment became overwhelmingly deleterious. It also clarifies why contemporary attempts to engineer alternative codes must rely on orthogonal translation systems, suppressor tRNAs, and extensive genome recoding. We are not simply editing a table of correspondences; we are pushing against four billion years of evolutionary integration, a planetary logic that resists modification because it underpins the very possibility of life’s coherence.

10 The limits and possibilities of knowing life and ethical glossing

10.1 Conceptual rescaling

Treating the genetic code as an operational a priori forces a change of scale. In a cosmic register, it suggests that life is not a sacred list of molecules but a particular kind of organized chemistry whose “grammar” could, in principle, be rewritten and re-instantiated - making astrobiology and xenobiology two sides of the same question. In an anthropological register, it destabilizes human exceptionalism: what we call “human nature” is built atop a historically frozen translation regime, and once that regime becomes editable, humanity becomes one biochemical configuration among others. Ontologically, the code functions like a rule-boundary between mere chemistry and teleonomic organization; altering it is not editing a text but revising the language itself. Ethically, this is precisely why xenobiology demands more than technical competence: it requires restraint, safeguards, and governance adequate to interventions in the biosphere’s deep interoperability layer. Religious rhetoric will keep showing up, but it should be handled as cultural semiotics; the operational issue is responsibility for a rewritable foundation.

10.2 Biosphere and humility

Understanding and rewriting the genetic code ultimately forces a humbling recognition: life is a planetary process (Smith and Morowitz, 2016). As Vernadsky (1998) argued, the Biosphere is a thermodynamic envelope whose core biochemical logic - metabolic universality, conserved macromolecules, and the shared genetic code, enforced by horizontal gene transfer, stereochemistry and error minimization has stabilized over billions of years (Woese, 1965). To re-engineer this logic is not routine molecular tinkering; it is a direct intervention into the informational substrate of the Biosphere (Marliere, 2009). This raises a legitimate doubt about whether our species, geologically young and ethically inconsistent, is prepared for such a responsibility (Schmidt et al., 2017). Even this doubt, however, is valuable: it anchors genetic code engineering (Budisa, 2006) in epistemic humility and reminds us that we are modifying the very historical conditions that enabled our own existence (Luu et al., 2025).

10.3 Kantian framing

Viewed through Kant’s four questions (Weiler, 1980), synthetic and xenobiology become fields that merge philosophy with engineering. What can I know? becomes an inquiry into the biological a priori - the recognition that genomes are meaningful only within the fixed grammar of the genetic code. What should I do? becomes a biological ethics, forcing reflection on how far we may reassign or expand this grammar, as exemplified by the adaptive laboratory evolution of organisms like E. coli. What may I hope? concerns the futures that emerge when evolution and human design converge. What is the human? becomes a question inseparable from xenobiology itself, because exploring possible life inevitably reframes our understanding of ourselves. The question “What should I do?” demands an ethical framework for code engineering, incorporating pre-project impact assessment, stakeholder inclusion, and safeguards against dual-use and inequity.

10.4 Governance and biocontainment

Such interventions require robust safeguards. Semantic and tropic genetic firewalls serve biosafety functions (Karbalaei-Heidari and Budisa, 2024), as demonstrated in recoded strains dependent on ncAAs for survival (Wannier et al., 2018), but they also create an engineered isolation: organisms incapable of natural gene flow and evolution (Herdewijn and Marlière, 2009). Designing lineages that exist only within controlled nutritional or informational constraints raises ethical questions about the status and stewardship of synthetic life (Acevedo-Rocha and Budisa, 2011). Moreover, the tools of code expansion - recoded organisms, orthogonal translation systems, XNA-based heredity - carry dual-use potential (Luu et al., 2025). Organisms with noncanonical signatures could evade standard diagnostic assays, as seen in the challenge of detecting pyrolysine or reassigned stop codons (Belinky et al., 2021) without structural or omics analysis, making proactive governance essential. This includes new detection standards for unfamiliar polymers, responsible disclosure practices, and international oversight comparable to frameworks for gain-of-function research (Selgelid, 2016).

10.5 From description to prescription

Genetic code engineering (Budisa, 2004) thus transforms the Central Dogma (Crick, 1970) from a descriptive framework into a prescriptive one. Crick’s “frozen accident” hypothesis (Crick, 1968) emphasized the fragility of codon reassignment; my and other laboratories (Koch and Budisa, 2024) successes have shown that this accident can in fact be strategically thawed, through codon capture or ambiguous intermediate mechanisms (Luu et al., 2025). This shift destabilizes essentialist notions of life and reframes biology as a pluralistic design space rather than a fixed natural kind. Directed laboratory evolution of the genetic code (Hoesl et al., 2015) operates teleologically - imposing goals and constraints - thereby becoming a new evolutionary force (Tolle et al., 2023). Whether this represents a realization of latent biological potential or the imposition of human logic onto evolution remains an open, and ethically charged, question (Schmidt, 2010).

10.6 Medical horizon (aging, oxidative mismatch)

The current era of biotechnology is defined by the democratization of genome editing, transitioning from a rare, bespoke intervention toward a routine and accessible clinical modality (Ernst et al., 2020). This progress in equitable access to genetic correction represents a monumental achievement in medicine. Yet, its scope is inherently limited. It operates within a system whose core components reflect ancient evolutionary constraints, not modern human needs. The canonical amino acids (Borrel et al., 2014) - the shared “alphabet” of our biological commons - were selected in a pre-oxygenated Biosphere (Ligrone, 2019). In our oxygen-rich world, they are intrinsically vulnerable to oxidative damage, a fundamental vulnerability that manifests as the protein misfolding and aggregation central to age-related diseases like Alzheimer’s and neurodegenerative diseases (Collin, 2019).

The next frontier, therefore, must be xenobiological. This paradigm moves beyond editing the inherited genetic text to rationally redesigning its foundational chemical lexicon, directly engineering proteome stability and resilience. It addresses a core mismatch: our biology is in significant part optimized for a world that no longer exists. Upgrading these universal building blocks for the modern metabolic environment (Völler and Budisa, 2017) is not a correction of an error, but a necessary adaptation - an endeavor aimed directly at eradicating the molecular causes of degenerative aging and expanding the human health span. This constitutes a profound ethical challenge and opportunity for global medicine.

The same rise in atmospheric oxygen that enabled large-scale technospheres (Balbi and Frank, 2024) - and ultimately modern medicine - also imposed a new oxidative regime on molecular biology, exposing the evolutionary antiquity of the canonical amino acids and other small-molecule metabolites. Xenobiology therefore addresses not a marginal defect, but a planetary-scale mismatch between the chemical conditions required for technological civilization and the molecular grammar inherited from pre-oxygenated life.

While this power to “write” biological grammar introduces valid concerns about a proprietary divide, the precedent of genome editing offers a roadmap for responsible translation (Church and Regis, 2014). The challenge of equitable access is not a reason for restraint, but a call for deliberate, global governance from the outset. The goal must be to ensure that this transformative technology - unlike the arbitrary lottery of evolution - is developed and distributed as a universal human good. We are expanding our capacity to cleanse genomes of lethal alleles using now-classical genome editing tools (Doudna, 2020). When combined with the potential to fundamentally improve human biology by recruiting novel, synthetic building blocks (Luu et al., 2025), this commits us to a technologically optimistic future. In this future, enhancing human resilience and longevity becomes the next chapter in our shared progress.

10.7 Ethical method

This general responsibility requires a concrete methodological expression. To navigate these complexities, I propose “ethical glossing” as an integral part of the research cycle: starting with pre-project mapping of potential long-term implications of codon reassignment or recoding, informed by evolutionary models such as Wong’s co-evolution theory and the Alanine World model (Scheme 2); continuing with stakeholder-inclusive design that brings ethicists, social scientists, and public interlocutors into early stages of development; emphasizing reversibility and proportional containment aligned with prospective ecological impact; incorporating narrative responsibility through transparent, non-hyped communication; and extending to post-completion stewardship, recognizing that responsibility persists beyond publication or strain storage.

Ethical glossing governs the intentional design and release of xenobiological systems, but it cannot eliminate risks arising from hostile appropriation or extrinsic misuse (e.g., analyses of hybrid and adversarial biosecurity threats (Kambouris, 2024)); these belong to a distinct domain of societal security and international governance that lies beyond the laboratory yet must be acknowledged by it.

Expanding the genetic code is an act of immense creative power, offering insights into life’s logic and enabling innovations in materials, therapeutics, and sustainable biotechnologies. But creative power demands ethical and epistemic discipline. The goal is not mastery over nature but informed co-creation- exploring possible biochemistries while preserving the integrity of the Biosphere and the dignity of life’s future forms. The “third phase” of genetic code evolution (Wong, 2024), guided by human intention, must therefore proceed with a balance of ambition and restraint, ensuring that our engineering of new biological a priori’s is matched by wisdom worthy of the responsibility they entail.

For example, human-derived cells engineered with a 21st amino acid could enable targeted drug delivery, metabolically safeguarded bio-implants, or enhanced biosensing, as previewed by biomedical applications of ncAAs (Chemla et al., 2024) in, e.g., prion-protein transformation (Wolschner et al., 2009), biocontainment, and bioorthogonal chemistry (Jann et al., 2024). Such interventions force a philosophical question: if we alter the biochemical alphabet underlying human embodiment, do we also revise what counts as “human essence”? (Wilson, 1978).

11 Conclusion

The genetic code is the deepest informational structure of life - an operational a priori that conditions what organisms can be. Xenobiology’s capacity to rewrite this structure transforms biology from a science of actual life into an exploration of possible life (Budisa, 2014) including alternatives grounded in distinct biochemical starting points, as illustrated by the Alanine World Model (Scheme 2). This model emphasizes that Earth’s alanine-centered biochemistry represents one historically realized solution among several plausible primordial amino-acid worlds, selected under specific planetary boundary conditions - such as the prevalence of reductive geochemistry and thioester-driven metabolism (Goldford et al., 2019) - rather than dictated by chemical necessity. The exploration of possible biochemistries is constrained not only by historical contingency but by chemical landscape topology - some regions of molecular space are more probable entry points for evolution, whether natural or engineered (Kirschning, 2021).

The logic of the model, however, extends far beyond amino acid selection. It suggests that alternative biologies could be founded on entirely distinct physico-chemical criteria - for instance, a sulfur-rich, acidic biochemistry analogous to Venusian conditions (Schulze-Makuch, 2021). In such frameworks, metabolism might bypass the citric acid cycle altogether, energy could be harnessed from pH, ionic, or photonic gradients rather than redox potentials, and information might flow through novel signaling and regulatory architectures (Lane, 2022). The Alanine World is therefore not a deterministic destination, but one contingent framework among many, illuminating the expansive landscape of biochemistries attainable through different foundational starting points.

This perspective aligns with the broader conjecture of a “shadow biosphere” (Davies et al., 2009) - the search for naturally evolved alternative life on Earth - while extending it into the domain of constructive possibility: where we not only imagine but deliberately engineer “hidden worlds” based on alternative physico-chemical starting points. This movement - from the Biological a priori to Xenobiological Possibility - traces a conceptual trajectory from the inherited, evolution-shaped a priori of terrestrial life (the universal genetic code, conserved metabolism, canonical macromolecules, and cellular embodiment) toward the deliberate creation of novel biological a priori. Xenobiology aims to establish new semiotic thresholds - engineered epistemic cuts - where arbitrary molecular assignments acquire biological function within a self-sustaining system. Process-oriented accounts (Tenti, 2022) that dissolve biological codes into morphogenetic interactions correctly emphasize life’s material creativity, but they cannot explain the historical fixation and inheritance of specific semiotic mappings that function as operational constraints across all terrestrial life.

The conceptual journey of this paper mirrors a progression in biological practice, one that can be mapped onto classical philosophical frameworks. The descriptive and mechanistic biology of Hume and Darwin gives way to the code biology of Kant (seeking constitutive conditions) and Leibniz (nested machines), culminating in the interventionist logic of xenobiology. Technically, this mirrors a shift from editing genomes and metabolic pathways, through engineering synthetic auxotrophies and orthogonal translation systems, and editing proteomes, to the ultimate expansion of the chemical alphabets of informational and catalytic macromolecules (foldamers).

Together, this progression represents biology’s transition from merely modifying observable phenomena toward deliberately altering life’s underlying essence. Xenobiology thereby marks the first deliberate attempt to rewrite the foundational rules of living systems, generating alternative biochemical grammars that constitute new, engineered biological a priori. With this power comes responsibility: to act with epistemic humility, technical precision, and ethical foresight. As humanity enters the “third phase” of code evolution (Wong, 2024), the challenge is not mastery of nature but wise co-creation - engineering new biological a priori’s with a sense of planetary stewardship and philosophical depth.

The future trajectory of xenobiological research is clearly delineated, moving beyond foundational science toward tangible global applications (Handal-Marquez et al., 2020; Herdewijn and Marlière, 2009; Marliere, 2009). This program includes: (i) the sustainable, bio-based production of chemicals, materials, and energy; (ii) the engineering of novel microbial metabolisms and orthogonal genetic coding systems; (iii) the design of synthetic ecosystems and food webs; and (iv) the expansion of core elemental cycles beyond CHNOPS to include fluorine, silicon, and boron. Its ultimate aims are the establishment of a safe, bifurcated evolutionary tree -separating natural from synthetic life - and the achievement of semantic biocontainment to prevent genetic pollution.

To realize this vision, the next decade of research must address three foundational questions at the intersection of molecular design, philosophy, and planetary stewardship: (i) Feasibility: What constitutes a minimal, viable orthogonal code coupled to a sustainable energy-gradient mechanism capable of supporting autonomous synthetic life? (ii) Integration: How would ecosystems and evolutionary dynamics function under these alternative grammatical architectures? (iii) Governance: What global, transdisciplinary frameworks - ethical, legal, and scientific - are required to guide the safe, just, and equitable development of this powerful technology?

By rendering the genetic code experimentally modifiable, xenobiology transforms a long-standing philosophical concept - the biological a priori - into an object of empirical inquiry, marking a qualitative shift in how life can be understood, explored, and designed. Life is thereby reframed not as a fixed outcome of Earth’s history, but as a chemically grounded space of possibilities, of which terrestrial biology is only one realized instance.

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Footnotes

Edited by: Patrick O’Donoghue, Western University, Canada

Reviewed by: Diogo Mayrinck, Rio de Janeiro State University, Brazil

Hassen Chaabani, University of Monastir, Tunisia