Emergence of biosignatures on Earth and implications for life detection

Abstract

Earth’s organic chemical evolution (OCE) from abiotic to prebiotic to biotic provides critical insights that may have implications for how we interpret observations made by life-detection planetary missions. To search for life on other worlds, astrobiologists seek biosignatures: features that indicate the presence of extant or extinct life as we know it. However, because the age of putative extraterrestrial biospheres is unknown, biosignatures present at or shortly after life’s emergence may have been diluted with prebiotic or abiotic signatures. Thus, the emergence of biosignatures and their formation context are critical considerations for mission interpretations, especially where there is no obvious pervasive biosphere. Here, we provide a structured approach for interpreting chemical states across the continuum of OCE on a planetary body. We distinguish between universal biosignatures of a world vs. specific biosignatures that are tied to environmental or temporal conditions of that world. We then consider how the evolution of a planetary body would impact the expression of these signatures over time. The detection of active prebiotic chemistry—that is, signatures that fall in the ambiguous transition between prebiotic and living systems—would be a hallmark discovery, providing insight into life’s origins and justifying investment in further exploration of such worlds. Leveraging Earth as an example, we suggest that while mission observations may be more challenging to interpret than previously thought, they may also open doors to a more productive exploration of our planetary neighbors.

Strategies for searching for life beyond Earth are unavoidably intertwined with the path of a planetary body’s organic chemical evolution (OCE), meaning how the organic molecule inventory of a planet (whether sourced from abiotic, prebiotic, and/or biotic processes) changes in response to conditions over geologic time (1, 2) (Box 1). Understanding how abiotic, prebiotic, and biotic chemistry transition in response to environmental and temporal conditions is critical to distinguish a biological world from an abiotic or prebiotic one. The astrobiology and origin-of-life fields have made tremendous advances in understanding organic chemical processes that may have led to life; however, applying these advances to life detection requires consideration of how organic chemistry is expressed within different planetary environments. Here we integrate this knowledge and apply it to several terrestrial planetary examples to demonstrate how Earth-like worlds might be observed at a moment within an arbitrary OCE timeline and discuss the implications for life-detection strategies.

Box 1.

Biotic, abiotic, and prebiotic.

Here, “biotic” refers to factors related to or derived from life, while “abiotic” refers to factors related to or derived from nonliving conditions or processes. “Prebiotic” is a subcategory of “abiotic” specifically related to chemistry contingent on the emergence of life. We do not define these terms as chronological stages but as part of a spectrum: Abiotic and biotic are the endmembers, and “prebiotic chemistry” occupies the middle ground, sharing commonalities with biochemistry without being the product of a living organism, and without being limited to the period before the OoL. Thus, abiotic, prebiotic, and biotic chemistry can occur simultaneously on a planetary body.

Take, for example, early Earth just after the rise of the last universal common ancestor (LUCA). In this scenario, Earth is both habitable and inhabited by life. If a spacecraft equipped with organic chemistry instruments designed for detecting terrestrial life as we know it landed on Earth at this time, would it detect life? If LUCA-derived life had not yet proliferated and diversified globally, the probability of sampling the precise environment where nascent life emerged and existed would be near zero. Further, life from separate origins or other branches of pre-LUCA life might have coexisted, and it is possible that some cell-like objects from this time might not even qualify as “living” by present-Earth standards. While Earth’s transition to a biosphere was likely geologically rapid, the duration of the “prebiotic Earth” remains unknown. If prebiotic chemistry (Box 1) (the specific subset of abiotic synthesis leading toward biology) was more widespread than the earliest cellular organisms, it could have been the primary source of organic production until life began to globally compete for these resources. As life expanded, it would have overprinted and recycled prebiotic chemistry, effectively destroying the record of early Earth’s OCE. By extension, worlds where life has not emerged or become globally widespread may still preserve organic features derived from prebiotic systems. Since the record of prebiotic chemistry on Earth has been largely destroyed, a critical question is: which specific chemical signatures might characterize prebiotic systems?

To answer this, we consider a continuum from prebiotic chemistry leading to the origin of life (OoL), LUCA, and life’s diversification. We hypothesize that a planetary body’s evolution would drive formation and distribution of both universal and specific biosignatures (Box 2). The implications of this thought experiment are profound for life detection: If a hypothetical mission were fortunate enough to encounter an inhabited planet with active prebiotic chemistry, concluding the presence or absence of life (as we know it) could remain theoretically intractable. Before life reaches a critical threshold of biological productivity, it could evade unambiguous detection for millions or potentially billions of years, as some signatures of very early life (e.g., from the interval between the OoL and LUCA) may be indistinguishable from signatures of prebiotic chemistry.

Box 2.

Universal and specific biosignatures.

Biosignatures are observable features that are interpreted to have an actual or perceived relationship to life. Different science communities refine the definition by adding specific qualifiers to the relationship and its probability. Here, we do not attempt to define “biosignature” since the definition has no bearing on how we interpret OCE. Instead, we consider two broad categories: 1) universal biosignatures that are expected to relate to all biology on a planetary body and 2) specific biosignatures that are limited in time and space to a particular environment. Since biosignatures may have a perceived relationship to life and prebiotic chemistry has commonalities with biochemistry, biosignatures are not necessarily derived from life.

The astrobiology community has proposed various definitions and frameworks of biosignatures (3–8), including considering the detection of prebiotically generated signatures (9, 10), but, this warrants additional consideration of the above ecological and evolutionary contexts. Here, we suggest that the definition of a “biosignature” could include prebiotic possibilities, depending on a world’s OCE stage. On Earth, even post-LUCA, the inventory of biosignatures has evolved over geological timescales. Because abiotic, prebiotic, or biotic mechanisms could contribute to generating biosignatures (11), life detection mission strategies that encompass this entire spectrum to establish interpretive confidence will be more productive. This is especially important for planetary targets of high astrobiological interest e.g., Mars, Titan, Enceladus, and Europa.

By focusing on the intersection between prebiotic chemistry and early life, thought experiments, like the early Earth case presented above, provide a theoretical basis for understanding transitions between abiotic, prebiotic, and biotic states (Fig. 1). Below, we explore the ecological and evolutionary implications of hypothetical biosignature discoveries, using examples of previously habitable planetary environments to illustrate potential effects of environment and timing on life detection investigations.

Fig. 1.

Evolution of biosignature properties in the prebiotic and biotic world. Assuming some biochemical processes and products emerged prior to the OoL, and could have continued increasing in molecular complexity and specificity after life emerged, then their diagnostic properties as “biosignatures” would change depending on the stage of life’s evolution. For example: Lipids may have emerged prebiotically as simple fatty acids, with post-OoL reaching some threshold of association with other biopolymers, then further increasing in specificity and functionality to what is observed for extant life. Prebiotic membranes (e.g., micelles, mineral pores) likely predate cells and membranes increased in functionality (e.g., bilayer exchange) over life history. Some degree of chiral enrichment could have emerged prebiotically and homochirality has persisted in all biochemistry across the tree of life. Simple peptides could have existed prebiotically and later evolved some functions and coassociation with other biopolymers (e.g., nucleotide oligomers) beginning prebiotically and continuing to today. A snapshot of feature properties at any point in time would therefore reveal in a set of biosignatures characteristic of that world’s stage of OCE; see Fig. 2.

Universal vs. Specific Biosignatures of Prebiotic and Biotic Chemistry

Biotic vs. Abiotic Chemistry.

Future life detection missions and concepts will most likely aim to measure organic properties (e.g., lipid-like hydrocarbon chains, small organic acids, cell-like structures, polymers, etc.) that might be interpreted as biosignatures based on predetermined criteria (3–5, 12–16). These criteria rely on known biological properties, such as relative abundances, structural complexity, homochirality, polymerization, isotopic fractionations, and metabolic activity. The discovery of multiple, independent biosignatures with high confidence of a biological origin could lead to the conclusion that extant or extinct life has been detected, especially if the environmental context is consistent with present or past habitability (4).

Paramount to a life detection interpretation is differentiating between organic chemistry generated abiotically vs. biotically (1, 17). Unlike biology that maintains products far from thermodynamic equilibrium, abiotic organic chemistry tends to progress toward thermodynamic equilibrium. Features of abiotic chemistry include an abundance of small and simple compounds, a broad spectrum of isomers, racemic mixtures, simple polymers made from a potentially much larger available set of monomers, and colocation of certain minerals with select organics. Examples of abiotic chemistry span the interstellar medium (remotely observed) and dust, the protoplanetary disk, to carbonaceous chondrites and comets (18). Larger bodies, such as asteroids (e.g., Ceres, Bennu, and Ryugu), experience aqueous and thermal transformations that produce a broader spectrum of organic compounds interpreted as both abiotic and/or prebiotic (19–22). Icy aggregations, including comets (e.g., 81P/Wild 2 and 67P/Churyumov-Gerasimenko) contain mineral and macromolecular organic components similar to carbonaceous chondrites (23, 24). Lab simulations and models provide the grounds for identifying abiotic synthesis (e.g., Fischer-Tropsch, Strecker synthesis, mineral-surface catalysis, radiolysis, and hydrothermal carbon reduction) and products in localized modern environments on Earth and more broadly beyond (25), including within Martian mafic rocks exogenously delivered to Earth as meteorites (26).

Biotic and abiotic organic chemistry represent two end members of planetary OCE. Today, the contrast between specific biosignatures and simple abiotic inventories is more distinctive; however, the chemistry that bridges them is much more complicated and carries the burden of many unknowns.

Prebiotic and Biotic Coexistence.

While abiotic chemistry occurs under various geological conditions, the set of conditions that could facilitate prebiotic chemistry moving toward life-like chemistry is still a matter of debate. If life does emerge, prebiotic chemistry and biology would likely coexist for a period of time on the planet. The duration of this prebiotic-biotic overlap would depend on the timing and conditions of the OoL and the biosphere’s ability to spread and diversify. Alternately, on worlds with limited habitability, it is possible that prebiotic and biotic systems might coexist indefinitely, or, life could fail to take hold and the planet could revert to a prebiotic state.

Following the OoL is LUCA, the population of cells that is the most recent common ancestor of all known extant life and its extinct related diversity. Comparative genomic analyses strongly support that LUCA already had properties that astrobiologists target, such as amino acid or lipid compound distributions, chirality, functional cell membranes, and peptide and protein functions, including translation (27, 28). However, on Earth there was a significant evolutionary distance between the OoL (the first biological system) and LUCA (29–31) (Fig. 2). During this time, it is possible that early life could have remained metabolically dependent on prebiotic synthesis (32), further blurring the distinction between the two. Over time, biology was shaped by selection which imparted greater distinction from the prebiotic/abiotic background. While life on Earth retained some specific “solutions” (e.g., our genetic code), it is unknown to what extent life on other planets would converge on the same solutions. Consequently, this transitional phase between prebiotic and biotic chemistry on other worlds might constitute a significant evolutionary phase where “universal” biosignatures may manifest from both biotic and advanced prebiotic chemistry.

Fig. 2.

Example of an evolutionary tree of life extending to life-like processes prior to the OoL. Illustration of how lipid (L), membrane (M), homochirality (H), and protein (P) related features, and features regarded as universal or agnostic (A) to life as we know it, may present in different (biological and prebiological) lineages. Placement of specific features is for illustrative purposes since the exact timing of emergence of each of these properties is not known; for example, whether abiotic chiral enrichments led to full homochirality prior to or after the emergence of life. Dotted lines trace the history of pre-OoL chemical systems not necessarily showing genetic inheritance. We are aware that the reality of the complexities of chemical evolution is omitted from this schematic, but it is meant primarily to show the different states of these systems with regard to biosignatures, rather than the full process of the emergence of these different states. For properties that emerged prior to the OoL (Fig. 1), extinct prebiological lineages (†) could exhibit a particular biosignature but are not actually living organisms (1). Some biosignature properties may be inherited from the prebiotic state and persist in all descendants post-OoL; however, others might appear only in certain lineages [e.g., lipid specificity (2)]—meaning that it is possible to detect life containing some, but not all, defined biosignature properties. Some universal biosignatures may even be lost in a subset of more specialized, derived lineages. Other biosignatures may emerge independently multiple times across lineages, resulting in a history similar to that of selected loss (3). Finally, it is possible that lineages diverging before LUCA evolved different combinations of universal and derived biosignatures, without leaving any descendants (4).

Universal vs. Specific Biosignature Evolution and Preservation.

If life evolved elsewhere in the Solar System via similar transitions to Earth’s history, we might expect nascent extraterrestrial biospheres to exhibit properties common to their own versions of pre-LUCA life and ancestral prebiotic chemistry. Such “universal biosignatures” (e.g., homochiral biochemistry) could be advantageous from a mission perspective since they could be used as criteria to search for life on other worlds. However, persistent coexistence of prebiotic and biotic chemistry could exhibit similar properties to one another. For example, if enantiomeric enrichments emerged before the OoL, that signature would then by definition also be associated with prebiotic chemistry. Similarly, a mission to early Earth might encounter: 1) prebiotic universal biosignatures because they emerged pre-OoL (e.g., if excesses in enantiomeric enrichment that exceed a purely abiotic threshold could emerge before the OoL) (Fig. 3); 2) post-OoL but pre-LUCA lineages containing only a subset of LUCA’s biosignatures, if some of those properties (e.g., a complete genetic code) emerged or were significantly refined post-OoL; or 3) extinct biological lineages that only contain a subset of LUCA’s biosignatures. Therefore, if advanced prebiotic chemistry is prevalent elsewhere, then it could make it difficult or impossible to resolve whether early living systems exist or have existed.

Fig. 3.

Evolution of universal and specific biosignature diversity over time across different planetary scenarios. Gray regions indicate peak biosignature diversity. (A) A prebiotic null hypothesis: a planet undergoes prebiotic chemical diversification that plateaus without leading to an OoL. (B) A planet with initial emergence of life and increase in biosignatures, but global extinction events make the planet uninhabitable (solid line); or alternative evolution if niche life survives only in specific habitats (dotted line). (C) A planet with initial diversification of life but few niche habitats to promote further diversification (e.g., a global ocean world monoculture), might host a constant degree of specific biosignatures. (D) An Earth-like planet with continued sustenance of a biosphere due to diverse habitats/niches and consistent nutrient cycling would exhibit overall increasing biosignatures over time, even with periodic extinction events.

Biosignature Preservation.

The preservation of the geological OCE record is also expected to be specific to each world’s environmental conditions. A planetary body’s history impacts the overall diversity and types (universal and specific) of biosignatures produced, and how organic compounds, including biosignatures, may be preserved. Regardless of origin, organic compounds would be subject to environmental alteration and recycling by life. Diagenesis occurs because organic compounds are no longer physically or chemically stable—a stability that living organisms can actively maintain. Ultimately, biological and chemical modifications to molecular structures erode many of the properties we identify as biosignatures: such as through loss of functionality, changes in stereochemistry, oxidation or reduction, hydrolysis, bond breakage, and recombination to form macromolecules, diminishing their diversity and quality and increasing the probability of false negatives. Therefore, measuring features closest to their time of formation will reduce the risks of complications introduced by diagenesis in interpreting their abiotic, prebiotic, or biotic affinity.

Lastly, the pool of universal and specific biosignatures is governed by the planetary body’s chemical inventory and conditions, reflected in its purely geochemical vs. post-OoL OCE. In this paradigm, an earlier phase of OCE might be more likely to host universal signatures from prebiotic processes, whereas a later phase (characterized by increased biological diversification) would produce more specific biosignatures. In early biospheres, advanced prebiotic systems and nascent life might be more similar across different worlds, exhibiting shared universal biosignatures. As life evolves and diversifies into different niches, environmentally specific biosignatures particular to that planet’s biosphere might become more prevalent. Universal biosignatures may still be present, but might constitute a lesser fraction of the total detectable biosignatures. Detecting universal biosignatures would therefore be an ambiguous signal for living organisms, but still inconsistent with completely nonprebiotic, purely abiotic explanations. Despite the lingering ambiguity, a discovery of universal biosignatures would transform our understanding of life’s emergence and its possible distribution in the Universe.

Proteins as an Example Evolving Biosignature.

OCE leading to biochemistry and life involves multiple types of compounds and their interactions in a self-consistent network. Peptides are a small part of such networks, and not sufficient on their own to support an OoL; however, their evolution illustrates biosignature emergence and the increased biochemical complexity necessary to transition from prebiotic to early biotic chemistry.

A major component of Earth’s biochemistry consists of peptides that fold into functional proteins. Protein synthesis is directed via translation of the genetic code, wherein messenger RNA codons are base-paired and decoded by tRNA binding of specific amino acids. Aside from a handful of exceptions (33), Earth’s schema of 20 amino acids and their designated codons is universal across all modern life, indicating that the code reached its mature form before LUCA. The modern genetic code depends upon many specific interactions for the synthesis of amino acids and their assembly into functional proteins, including enzymatic processes (amino acid synthesis, polypeptide synthesis, aminoacylation, and tRNA maturation) as well as semantics (codon–anticodon pairings, tRNA acceptor stem–amino acid pairings, tRNA–aminoacyl tRNA synthetase pairings) (34–37). These semantic relationships represent an abstraction that only has meaning within a living system, a break from the chemical determinism of abiotic and possibly even prebiotic systems. Given this end-member, how might advanced prebiotic peptide-rich systems differ in their respective chemical signatures, and might these be interpreted as possible biosignatures?

Prebiotic selection on amino acids.

Abiotic peptide synthesis has been reported in many environments relevant to early planetary history. Amino acids have been detected within meteorites and comets (38–42) possibly derived in part from interstellar materials (43, 44), and synthesized in experimental systems simulating hydrothermal environments (45) and primordial atmospheres (46). Simple peptides have been reported in meteorites (47, 48) and their formation pathways explored (49). These amino acid and peptide sources could serve as important raw materials for early prebiotic chemistry; however, they lack distinctive properties (e.g., structural complexity) linked to life, despite containing some identical compounds to Earth’s organisms. As selection begins to favor specific molecules within the precursor set of nonbiological products, the transition to prebiotic systems could be a shift away from a larger molecular inventory. We can explore various scenarios: for example, either prebiotic chemistry and the earliest life share a continuity with early peptide chemistry, imposing selection on “favorable” amino acids; or in another scenario, the modern amino acid alphabet is entirely a biological invention, with no direct connection to abiotic or prebiotic peptide availability. In either case, there is potential for a peptide biosignature independent of functional proteins, characterized by a restricted set of amino acids in predictable ratios: a collective property that may be difficult to explain in the absence of prebiotic or biotic processes.

Amino acid biosignatures of living systems.

In the biotic case, a universal biosignature of template-directed protein synthesis could be a discrete set of amino acids with sidechains sparsely and efficiently covering chemical space (i.e., the space defined by all possible compounds) (e.g., refs. 50 and 51), while prebiotic and abiotic systems may contain less balanced, much larger or much smaller sets. Based on combinatorics underlying chemical selection and functional protein and peptide space, the biotic vs. prebiotic/abiotic distinction is agnostic to the specific evolutionary history or biochemical functions of the system.

Functional proteins depend on a constituent set of amino acids with sidechains varying in length, volume, hydrophobicity, and charge. As such, any observed sampling of amino acid sidechain chemical space could be assessed for biogenicity, in terms of diversity and being chemically complete (52). This “completeness” appears to be a primordial trait, as Earth life’s core amino acid biochemistry essentially stopped at 20. Within this set of 20, some acids are very similar (e.g., isoleucine and valine, glutamate and aspartate), likely reflecting the selective advantage of fine-tuning; but, this advantage clearly has its limits, as the code has not continued to evolve to contain 63 different amino acids and a single stop codon (the limit under a trinucleotide codon schema). Therefore, a balance between fine-tuning and coverage of a broad chemical space may serve as a universal biosignature of biotic peptide systems (e.g., ref. 6).

Polypeptides and related oligomers.

The formation of peptide bonds represents an important step in prebiotic chemistry. Abiotic reactions that form peptides can occur in a limited fashion (e.g., refs. 49 and 53–55), but without a connection to genetically encoded information, they typically produce random short polypeptides of varying composition that readily hydrolyze. It is unlikely that such random oligopeptides would catalyze reactions or have specific biochemical roles. Nevertheless, they may provide a means for chemical selection that enables later, more complex prebiotic chemistry. For example, amino acids forming peptides that favor binding to surfaces such as minerals may be favored in these interactions, and preferentially resist hydrolysis increasing local peptide compositions that favor binding and secondary structure formation (56). In this way, even prebiotic peptide interactions could favor a restricted set of amino acids with collective properties.

On the abiotic end member of peptide-related OCE, it has been suggested that “depsipeptides,” oligomers of amino and hydroxy acids, may have been the precursors to peptides (57). Depsipeptides, characterized by a mix of ester and amide bonds, form more readily in planetary environments than pure peptides. In prebiotic experiments, a range of depsipeptides can be formed that increase in amino acid content over successive wet/dry cycles (57, 58). Repeated cycles of hydration and heating/dehydration promote hydrolysis of less stable ester bonds, shifting the population toward the amino acid-rich peptides that are more biochemically functional (58).

Prebiotic and biotic chemistry consist of multiple, complex chemical systems and their interactions. Peptide or depsipeptide synthesis is only one such system, illustrating how biosignatures can be generated and how their interpretation can be based on universal principles. Furthermore, the contrasts between prebiotic vs. biotic peptide systems shows that these distinctions ground our confidence in interpretations of abiotic, prebiotic, and biotic affinity of detected signatures.

Planetary Biosignature Evolution

Prebiotic and Biotic Diversity Over Planetary Timescales.

Using the prior discussions of prebiotic and biotic chemistry, we can consider how biosignatures would be reflected across different planetary bodies (Fig. 3). A purely prebiotic world serves as the null hypothesis in which the biosignature inventory plateaus once all environments conducive to prebiotic chemistry have explored the available chemical space (Fig. 3A). If prebiotic systems lead to an OoL, different processes favoring or disfavoring diversification of biosignatures might emerge. It is possible that life on Earth may have even arisen multiple times and in multiple locations, showing a spatially sparse but diverse biochemistry when the entire planet is considered, and leading to early confrontations and extinctions that would decrease the diversity of specific biosignatures. This decrease could occur even as the total amount of biomass and detectable biosignatures increased, and universal biosignatures could persist as well. A similar pattern may hold across longer planetary timescales.

We hypothesize that biosignature diversity should follow similar universal trends, shaped by gradual biosignature diversification and punctuated by decreases during major planetary changes. For example, during the Hadean and Archean, development of the atmosphere, oceans, and crust enabled the transition from prebiotic chemistry, to OoL, to LUCA. As early Earth environments continued to adjust, microbial communities adapted to changes in Earth’s habitability, which likely led to diversification of both specific and universal biosignatures.

Implications for Biosignature Histories across Planetary Bodies.

A purely prebiotic system (Fig. 3A) and an Earth-like planet (Fig. 3D) may be considered endmember states in molecular “biosignature” diversity. For example, consider a system in which an OoL occurred but later the planet evolved away from habitability (Fig. 3B). Biosignature diversity may have increased after life emerged, then decreased dramatically (or ceased entirely). It is possible that Venus and/or Mars fall into this category, if their early history was similar to early Earth, perhaps with many disparate habitable environments promoting rapid diversification of specific biosignatures and a slowly decreasing diversity in universal biosignatures. If a widespread biosphere emerged on such a world, biological diversity may have increased, followed by a marked decline as the planet’s environments became less habitable. On longer timescales, these changes could have led to mass extinction—perhaps, for example, ultimately resulting in the extinction of all life on Venus, and life possibly only persisting in refugia with a low but stable biosignature diversity on Mars.

In contrast to these scenarios is a planetary body that maintained long-term habitable conditions and in which life emerged, yet did not result in an overall increase in biosignature diversity. For example, if life emerged on an ocean moon, such as Saturn’s moon Enceladus, life might diversify (Fig. 3C) to fill available ecological niches, including perhaps hydrothermal systems at the seafloor, a chondrite core, and the ice–ocean interface (59–63). If water-rock alteration producing geochemical energy were limited to a short timescale (64), diversification may have plateaued fairly early in the moon’s history. In contrast, if Enceladus’s early seafloor was fairly heterogeneous in terms of its hydrothermal activity and therefore mineralogy, biodiversity may have increased in response to varied chemical systems, temperatures, and even seafloor topography. However, fewer specific biosignatures would be expected if seafloor activity became more homogeneous over time, implying fewer available niches driving diversification.

Summary and Implications

Here we explore the concept of biosignature emergence on Earth and apply it to hypothetical discoveries on other planetary bodies, considering OCE along with environmental habitability and broader planetary conditions. The production of organic chemical signatures by both prebiotic and biotic chemical processes that we might expect on another planetary body would increase across each pre- to post-OoL transition. Consequently, detection of universal biosignatures would not necessarily indicate a biogenic source and would be ambiguous, since these could be potentially consistent with any point in the evolutionary range from prebiotic chemistry up to and including modern life as we know it on Earth. However, a discovery of specific biosignatures would have the greatest probability of validating the presence of life.

With respect to environmental conditions and dynamics, the diversity of organic molecular biosignatures could vary greatly based on changes in relative habitability over time. We hypothesize here that peak biosignature diversity on a world should coincide with the greatest habitability potential. Thus, in a mission scenario, high organic-molecular biosignature diversity, especially including specific biosignatures, may be indicative of an ecology. In summary, our thought experiment shows a narrow window for “life detection” success, requiring specific biosignatures and diversity timed with habitability.^*

Our conclusions compel us to recast goals of “searching for life” into a goal of “understanding the state of OCE,” which is broadly inclusive of the abiotic, prebiotic, and biotic possibilities (2) for all planetary bodies. To support such a goal, space mission measurement capabilities would target both universal and specific biosignatures, including a broader organic molecular inventory to enable exploration of features unique to other worlds. The astrobiological value of such a mission would extend beyond one with a search-for-life goal, if it could differentiate universal from specific biosignatures. Discovery of any molecular features from Fig. 1 (e.g., lipids, membranes, or proteinaceous components) would be an astounding discovery regardless of abiotic, prebiotic, or biotic affinity, since access to natural prebiotic systems is not possible on Earth. A strategy focused on OCE would still focus on interpreting a broad suite of organic molecular biosignatures and habitability, but would also require foundational systems-level modeling of planetary chemistry, to explore the possible synthesis and diagenetic pathways for universal or possible specific biosignatures, with considerations for environmental stability and evolution.

A mission finding of biosignatures would be the next giant leap in discovery. A mission built on this revised goal would include measurements to identify universal biosignatures, since attempting to predict specific biosignatures at a particular moment in time on a planetary body is a likely futile exercise. However, broader organic molecular measurement capabilities would allow for exploration of unknown specific biosignatures. True positive discoveries could point to a range of possibilities along the emergence timeline in Fig. 1, where only a diverse set of observations would enhance the probability of establishing a scientific consensus that extraterrestrial life is present beyond Earth—the holy grail of astrobiology.

Data, Materials, and Software Availability

There are no data underlying this work.