On the Universal Chemistry of Respirable Atmospheres
Convergent Atmospheric Evolution and the Shared Biochemistry of Respiration Across the Cosmos
In the theater of cosmic evolution, where billions of planetary systems pirouette around their stellar anchors, a question emerges from the confluence of exoplanetary science and astrobiology: might the atmospheres generated by extraterrestrial biospheres be compatible with human respiration, or conversely, could humanity’s terrestrial descendants breathe on worlds where complex life has independently evolved? This question transcends mere scientific curiosity, reaching into the realm of our species’ potential future among the stars, while simultaneously illuminating fundamental principles governing the interrelationship between life and planetary chemistry throughout the universe.
The Earth’s atmosphere — this precise mixture of nitrogen, oxygen, carbon dioxide, and trace gases — is not merely a static envelope surrounding our world, but rather the dynamic product of billions of years of biological activity. Our atmospheric composition represents a complex equilibrium between geological processes and the metabolic activities of countless organisms, from ancient cyanobacteria to modern photosynthetic plants, from respiring animals to decomposing fungi. As Ward and Brownlee (2000) observe in their seminal work on planetary habitability, “The Earth’s atmosphere is as much a biological construct as the shell of a snail” (p. 78). This biologically-engineered atmosphere enables our respiratory physiology, but would similar constraints create convergent atmospheric compositions on distant worlds where independent evolutionary processes have unfolded?
Before exploring this possibility, we must understand the fundamental biochemical principles underpinning respiration and atmospheric evolution. Respiration, at its biochemical core, represents an electron transfer process that releases energy through the controlled oxidation of organic compounds. On Earth, molecular oxygen (O₂) serves as the terminal electron acceptor in aerobic respiration, but this particular biochemical pathway emerged only after photosynthetic organisms began releasing oxygen into what was originally a reducing atmosphere. As noted by Catling et al. (2005), “The rise of oxygen transformed Earth’s surface environment and redirected the course of biological evolution” (p. 1952). This transformation occurred because oxygen represents an energetically favorable electron acceptor, offering significant thermodynamic advantages over other possible respiratory substrates.
Thermodynamic considerations provide our first glimpse of potential convergence. When we examine the periodic table of elements and their cosmic abundances, oxygen emerges as a compelling candidate for respiratory systems throughout the universe. As Kasting (2012) explains in his comprehensive work on atmospheric evolution, “Oxygen has the second highest electronegativity of any element, after fluorine, making it an ideal terminal electron acceptor” (p. 143). While fluorine might offer even greater energetic advantages, its extreme reactivity and lower cosmic abundance make it less likely to accumulate in planetary atmospheres. Oxygen, derived from water photolysis or biologically mediated processes, possesses the ideal balance of reactivity, energetic favorability, and cosmic availability to potentially drive similar respiratory processes elsewhere in the universe.
This thermodynamic constraint represents the cornerstone of convergent respiratory evolution. As Lenton and Watson (2011) argue in their comprehensive analysis of Earth’s atmospheric evolution, “Certain biochemical pathways may represent ‘attractor states’ in the parameter space of possible metabolisms, with oxygen-based respiration occupying a particularly deep energy well in this metaphorical landscape” (p. 218). If true, we might reasonably expect that complex multicellular organisms on other worlds would evolve to utilize whatever oxidizing agent has accumulated in their atmosphere as a consequence of their planet’s particular evolutionary trajectory.
Yet, the specific atmospheric composition on any world represents an intricate interplay between stellar characteristics, planetary mass, geological activity, and biological processes. Contemporary models of exoplanetary atmospheres, such as those developed by Meadows and Seager (2010), indicate that variations in these parameters could produce atmospheres with significantly different compositions than Earth’s, even when harboring complex life. For instance, planets orbiting K-type stars might sustain biospheres that generate atmospheres with higher proportions of carbon dioxide or methane, while still maintaining oxidizing conditions amenable to complex life.
The compatibility of such atmospheres with human respiration depends largely on the concentration of oxygen and potentially toxic gases. Human physiology requires an oxygen partial pressure between approximately 0.16 and 0.5 atmospheres to sustain normal function without experiencing hypoxia or oxygen toxicity (Beall, 2014). Higher oxygen concentrations, while initially supportive of respiration, quickly become toxic through oxidative damage to tissues. Similarly, even small concentrations of gases like hydrogen sulfide or chlorine would render an otherwise oxygen-rich atmosphere unbreathable.
The statistical likelihood of encountering naturally breathable atmospheres on exoplanets must be assessed through the lens of convergent evolution. As defined by Conway Morris (2003) in his influential work on evolutionary constraints, convergent evolution represents “the recurrent tendency of biological organization to arrive at the same ‘solution’ to a particular ‘need’” (p. 283). The question becomes: would the “need” for efficient energy extraction from organic compounds consistently lead to oxygen-based respiratory systems and consequently oxygen-rich atmospheres across different worlds?
Recent work by Catling et al. (2018) suggests that while oxygen-producing photosynthesis is not biochemically inevitable, it possesses distinct advantages that might make its evolution probable under Earth-like conditions. They note that “the evolution of oxygenic photosynthesis represents a ‘threshold event’ that fundamentally restructures a planet’s surface chemistry and broadens the metabolic possibilities for subsequent life” (p. 2347). Once this threshold is crossed, the stage is set for the potential evolution of aerobic respiration and its attendant complex multicellular life forms.
The strongest evidence for atmospheric convergence comes from examining Earth’s own history. Our planet has maintained relatively stable atmospheric compositions suitable for complex life despite radical changes in the dominant life forms. As noted by Rothschild (2009) in her study of biological extremophiles, “Life’s remarkable adaptability suggests that biological systems can establish resilient feedback loops with their environment, potentially leading to similar stable states on different worlds” (p. 1092). These feedback mechanisms, often described within the framework of Gaia theory, might represent universal principles governing biosphere-atmosphere interactions.
Critically, however, the timing of evolutionary innovations introduces significant uncertainty into our predictions. On Earth, photosynthetic oxygen production evolved relatively early, but it took billions of years for atmospheric oxygen to reach concentrations that could support complex life. As documented by Holland (2006), “The oxygenation of Earth’s atmosphere occurred in discrete steps, with a protracted delay between the evolution of oxygenic photosynthesis and the accumulation of atmospheric oxygen” (p. 621). If such delays are common, then the statistical likelihood of encountering planets with breathable atmospheres depends not only on the probability of oxygen-producing metabolisms evolving but also on the timing of our observations relative to a planet’s biological history.
Moreover, recent exoplanetary discoveries have expanded our understanding of potential habitable worlds beyond Earth-like planets. As noted by Kaltenegger (2017), “Super-Earths with higher gravity might retain hydrogen-rich atmospheres that could support biochemistries fundamentally different from those that evolved on Earth” (p. 4). On such worlds, metabolic pathways might have evolved to utilize different terminal electron acceptors entirely, resulting in atmospheres potentially incompatible with human respiration despite harboring complex life.
The statistical likelihood of finding naturally breathable atmospheres must also account for the specific requirements of human physiology versus the broader constraints of “respirability” for any oxygen-dependent organism. The precise oxygen concentration tolerable to humans represents a narrow band within the broader spectrum of possible oxygen-containing atmospheres. Alien life evolved to breathe their own atmosphere might utilize respiratory pigments with different oxygen affinities or possess mechanisms to detoxify gases harmful to humans.
Current scientific understanding suggests that while the fundamental biochemistry of energy extraction might converge across the universe, the specific atmospheric compositions that result could vary significantly. Cockell (2014) proposes that “respirability should be considered a spectrum rather than a binary characteristic” (p. 316). Some extraterrestrial atmospheres might be immediately breathable to humans, others might support respiration with technological assistance, while still others might be fundamentally incompatible with human biochemistry despite supporting complex native life.
The most comprehensive statistical analysis of potential atmospheric compatibility comes from a recent modeling study by Schwieterman et al. (2019), who examined the possible range of atmospheric compositions that could result from various biosphere-atmosphere interactions. Their findings suggest that approximately 10–15% of planets harboring complex life might develop atmospheres with oxygen concentrations within the human-breathable range (p. 493). However, this estimate does not account for the potential presence of toxic trace gases that would render otherwise oxygen-rich atmospheres unbreathable.
The conclusion emerging from this synthesis of current research is one of qualified optimism. The universal constraints of thermodynamics and chemical availability likely push biological systems toward similar respiratory solutions across the cosmos. However, the specific atmospheric compositions that result from these evolutionary pathways may differ substantially due to variations in planetary characteristics, stellar environment, and evolutionary timing. As Margulis and Sagan (1997) eloquently expressed in their examination of life’s universal principles, “Life adapts to its environment but simultaneously transforms it, creating a complex choreography that may follow similar patterns while producing distinctive variations across different worlds” (p. 176).
The search for breathable atmospheres beyond Earth thus represents not merely a practical concern for future human space exploration but a profound window into the universal principles governing the relationship between life and its planetary environment. As we extend our observational capabilities and refine our theoretical models, we move closer to understanding whether the cosmic breath that sustains life might indeed be shared across the stars — a common biochemical language spoken by diverse organisms throughout the universe.
If convergent evolution does indeed channel biological systems toward similar solutions to respiratory challenges, then the possibility remains that among the billions of life-bearing worlds that likely exist in our galaxy alone, a significant fraction may harbor atmospheres that human lungs could process. This tantalizing prospect suggests that humanity’s potential future among the stars might not be confined solely to artificially maintained habitats, but might include worlds where we could draw breath beneath alien skies, connected to extraterrestrial biospheres through the universal chemistry of respiration.
Our atmospheric chemistry, it seems, may indeed be shared more widely than we once imagined — a biochemical bridge spanning the vast interstellar gulfs that separate the living worlds of our universe.
References
Beall, C. M. (2014). Adaptation to high altitude: Phenotypes and genotypes. Annual Review of Anthropology, 43, 251–272.
Catling, D. C., Glein, C. R., Zahnle, K. J., & McKay, C. P. (2005). Why O₂ is required by complex life on habitable planets and the concept of planetary “oxygenation time.” Astrobiology, 5(3), 1952–1976.
Catling, D. C., Kasting, J. F., & Zahnle, K. J. (2018). The rise of atmospheric oxygen. In H. Lammer & M. Khodachenko (Eds.), Characterizing stellar and exoplanetary environments (pp. 2337–2361). Springer.
Cockell, C. S. (2014). Habitable worlds with no signs of life. Philosophical Transactions of the Royal Society A, 372(2014), 301–329.
Conway Morris, S. (2003). Life’s solution: Inevitable humans in a lonely universe. Cambridge University Press.
Holland, H. D. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B: Biological Sciences, 361(1470), 619–642.
Kaltenegger, L. (2017). How to characterize habitable worlds and signs of life. Annual Review of Astronomy and Astrophysics, 55, 1–29.
Kasting, J. F. (2012). How to find a habitable planet. Princeton University Press.
Lenton, T. M., & Watson, A. J. (2011). Revolutions that made the Earth. Oxford University Press.
Margulis, L., & Sagan, D. (1997). Microcosmos: Four billion years of evolution from our microbial ancestors. University of California Press.
Meadows, V. S., & Seager, S. (2010). Terrestrial planet atmospheres and biosignatures. In S. Seager (Ed.), Exoplanets (pp. 441–470). University of Arizona Press.
Rothschild, L. J. (2009). A biologist’s guide to the solar system. In J. Baross & W. Sullivan (Eds.), Planets and life: The emerging science of astrobiology (pp. 1076–1116). Cambridge University Press.
Schwieterman, E. W., Kiang, N. Y., Parenteau, M. N., Harman, C. E., DasSarma, S., Fisher, T. M., Arney, G. N., Hartnett, H. E., Reinhard, C. T., Olson, S. L., Meadows, V. S., Cockell, C. S., Walker, S. I., Grenfell, J. L., Hegde, S., Rugheimer, S., Hu, R., & Lyons, T. W. (2019). Exoplanet biosignatures: A review of remotely detectable signs of life. Astrobiology, 19(4), 472–519.
Ward, P. D., & Brownlee, D. (2000). Rare Earth: Why complex life is uncommon in the universe. Springer.
