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Quantifying the Origins of Life on a Planetary Scale Quantifying the origins of life on a planetary scale Caleb Scharfa,1 and Leroy Croninb,1 aColumbia Astrobiology Center, Columbia Astrophysics Laboratory, New York, NY 10027; and bSchool of Chemistry, University of Glasgow, Glasgow, G12 8QQ, United Kingdom Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved May 17, 2016 (received for review November 23, 2015) A simple, heuristic formula with parallels to the Drake Equation is currently impossible to construct a first principles quantitative introduced to help focus discussion on open questions for the origins estimate of an abiogenesis probability for the young Earth. It is of life in a planetary context. This approach indicates a number of also the case that we typically conflate the idea of “microabio- areas where quantitative progress can be made on parameter genesis”— namely, a highly localized assembly of molecular estimation for determining origins of life probabilities, based on structures that meets the minimum criterion for a living system— constraints from Bayesian approaches. We discuss a variety of “micro- with “macro-” or planet-wide abiogenesis. In other words the idea scale” factors and their role in determining “macroscale” abiogenesis of life arising on a planet is treated as a singular event rather than probabilities on suitable planets. We also propose that impact ejecta a possible accumulation of many microscale events, each of which exchange between planets with parallel chemistries and chemical evo- might be considered an origin event in its own right, and are ar- lution could in principle amplify the development of molecular com- guably more accessible through laboratory or modeling investi- plexity and abiogenesis probabilities. This amplification could be very gations. Nonetheless, abiogenesis did clearly occur at least once on significant, and both bias our conclusions about abiogenesis probabil- Earth (4), and we do possess general information about Earth’s ities based on the Earth and provide a major source of variance in the composition and a number of notional histories for the surface probability of life arising in planetary systems. We use our heuristic and near-surface environment during the planet’s first few hun- formula to suggest a number of observational routes for improving dred million years following the solar system’s initial collapse and constraints on origins of life probabilities. condensation from the presolar nebula. origin of life | planetary scale | chemical search space | exoplanetary science Results and Discussion In this short paper, a simple formula is proposed to specifically EARTH, ATMOSPHERIC, he question of whether or not life exists beyond the confines of encourage further consideration of the issues at play in linking AND PLANETARY SCIENCES Tthe Earth environment is intimately related to the question of OoL (or abiogenesis) to planetary environments. This approach ’ life s origins on Earth or anywhere else. For example, if just one parallels similar descriptive and pedagogical methods applied in — instance of life with an independent origin is detected whether in assessing the likelihood of extraterrestrial communications in the — the solar system or on a suitable exoplanet estimates of the Search for Extraterrestrial Intelligence (SETI). A simple appli- Bayesian probability for life across the universe will be significantly cation of this formula is made, and the results are discussed below. improved. Specifically, the rate of abiogenesis per young planet Furthermore, a number of modifications and extensions are con- could, in principle, be constrained to be at least one event per Gyr sidered that should help bring focus to the regions of parameter (1). Equally, if a detailed mechanism for the terrestrial origins of space where tractable progress can be made on building a genu- life (OoL) were identified and tested, a complementary estimate inely quantitative evaluation of OoL likelihoods (5). of the rate of planetary abiogenesis should be possible. Further- In 1961, Frank Drake introduced an equation to illustrate the more, the architecture of a given solar system might also lead to factors involved in estimating the potential number of communi- widely different probabilities (e.g., the proximity of Mars and cative civilizations in the Milky Way galaxy. This formula, now Earth and their potential exchange of material might result in a generally known as the Drake Equation, has served as a useful tool fundamentally larger search space in which OoL events could for focusing discussion on the extent to which we do, or do not, occur). Discovering complex organic molecules on Mars could have constraints on its various factors and for stimulating ideas help elucidate this possibility (2). At present, OoL science involves a wide range of approaches, Significance theories, and opinions. A considerable body of work now exists on hypothetical early terrestrial life and its precursor chemistry in- cluding the RNA–world hypothesis, prior polymerization chemis- In this paper, we describe an equation to estimate the frequency of planetary “origin of life”-type events that is similar in intent try, autocatalytic processes, and metabolic origins, as well as the to the Drake Equation but with some key advantages—specifi- presumed transition to fully cellular organisms. Significant work cally, our formulation makes an explicit connection between has also been undertaken on inorganic “template” chemistry, the “global” rates for life arising and granular information about a planetary evolution of suitable material precursors, and important — planet. Our approach indicates scenarios where a shared chem- substrates for the emergence of a biology including the study of ical search space with more complex building blocks could be the electrochemical gradients in hydrothermal vents systems and critical difference between cosmic environments where life is processes such as serpentinization as energy sources. In addition, ’ potentially more or less abundant but, more importantly, points the genomic exploration of the modern Earth s microbial life is to constraints on the search. The possibility of chemical search- providing ever-deeper insight into the nature and evolution of the space amplification could be a major variance factor in planetary ’ biosphere s nested metabolic processes (3), which may provide abiogenesis probabilities. clues to the original energy landscape of prebiotic chemical cycles. Theoretical and computational work on emergent and dynamical Author contributions: C.S. and L.C. performed research, analyzed data, and wrote systems, complexity, and “dry” (not wet laboratory-based) artificial the paper. life (A-Life) are also contributing to our picture of the funda- The authors declare no conflict of interest. mental principles of biological system operations. This article is a PNAS Direct Submission. However, there is no strong consensus that supports any single Freely available online through the PNAS open access option. — OoL hypothesis or timeline whether for the specific OoL events 1To whom correspondence may be addressed. Email: [email protected] or on the Earth or for any plausible alternatives. It is therefore [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1523233113 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 Table 1. Plausible values for the factors in the OoL frequency However, life today does not necessarily reflect all of the details equation for the Earth of early biochemistry, or earlier complex chemistry that was able Factor Value Basis for value to self-sustain routes to competing biologies that ultimately led to biology as we know it. Life today is more likely to reflect the ∼ 49 ’ Nb 10 No. of atoms in Earth s modern consequence of billions of years of evolution of a series of earlier crust + ocean + atmosphere chemical toolkits, all with decreasing complexity (at earlier stages) ∼ 11 hnoi 10 No. of atoms in bacterium and, therefore, increasing probability of emergence over a short (dry weight) period. In that context, it might be advisable to break these ∼ −14 fc 10 Fraction based on the ratio of components down further and consider the raw elemental modern Earth’s total estimated (atomic) abundances as the first factor in the OoL equation. An biomass to the total mass of additional advantage in doing so is that element abundance sim- + + crust ocean atmosphere plifies the connection of Nb to bulk planetary properties, thereby ∼ −36 λ = −3 Pa 10 Assuming 10 per Gyr linking this expression back to the scope of the Drake Equation − ∼10 30 Assuming λ = 103 per Gyr and our direct physical knowledge of the Earth and other planets. ∼ −33 λ < 10 Assuming 1 per Gyr (12% chance) In this case, Nb represents a maximal set of building blocks for life that can be estimated from the total mass and composition of the outer planetary layers (see below). In the Earth’s case, it is about how to make scientific progress on the problem of finding self-evident that not all of the atoms of Earth’s outer layers are life in the universe over a given period (6). There are however incorporated into living states, nor are all available for life at any serious limitations to actually using the Drake formula to produce given time. To account for element availability, the availability meaningful estimates of life’s frequency in the universe. A par- factor, fc, is described further below. Also, any collection of ticular issue is that although the formula seeks to
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