The earliest known microfossils, preserved in Archean rock, date back roughly 3.5 billion years. But even these ancient microbes had ancestors—possibly represented in the geologic record by chemical “fossils” interpreted to reflect biological activity some 600 million years earlier [e.g., Bell et al., 2015]. Exactly when and how the planet’s first organisms emerged onto the scene remain among the most elusive scientific questions. However, one key reality is well established: The earliest steps in life’s emergence were interwoven with the evolving chemical and physical conditions of Earth’s earliest environments.
Scientists from a variety of fields have long speculated about how life might have developed from compounds that were not biological in origin (i.e., prebiotic molecules). However, prebiotic chemistries tested to date, including the specific kinds of molecules and their surrounding environments, have not been demonstrated to function under realistic planetary conditions. These environments would have helped define the abiotic chemical pathways that ultimately gave rise to life. Because of this gap, along with the many unknowns about the geological, geophysical, and geochemical details of the early ocean, atmosphere, and continents, we lack coherent linkages between experimental work synthesizing biomolecules from prebiotic beginnings and work illuminating plausible early Earth conditions.
Traditionally, research into the origins of life too often has been performed in silos defined by a researcher’s specific expertise. Now that Earth’s earliest environments are coming into sharper focus, thanks to concerted efforts to understand Earth’s early rock record and the insights gained from numerical simulations, a more interdisciplinary approach in origins-of-life research is becoming increasingly possible.
The interconnected nature of these environments, the chemical evolution occurring at water-rock interfaces, and the interplays between life’s chemical building blocks and the local geologic conditions that hosted those molecules are all key to this research. Increasingly sophisticated models for the evolution of global and local environments on early Earth must also remain a research target for those asking essential questions about life’s beginnings. Viewing life’s origins within the necessary environmental context must thus include contributions from prebiotic chemists, biogeochemists, astrobiologists, atmospheric scientists, geologists, geophysicists, astronomers, and planetary scientists.
Connecting and coordinating these groups is a considerable undertaking, requiring that current scientists reach beyond their comfort zones and that the next generation cut their teeth in collaborative, interdisciplinary settings. Here we highlight examples of what we know about early Earth environments and the beginning of life, as well as what we do not, and we introduce a consortium of researchers helping to bridge traditional disciplinary boundaries and spur research to illuminate these unknowns.
Earth Under an Active Young Sun
Solar evolution models constrain the total power output, or luminosity, of the early Sun, just after the birth of our solar system, to roughly 70% of its current level. Even so, investigations of young solar analogues found among other stars indicate that the Sun’s ultraviolet (UV) output may have been elevated compared with modern levels. Not only do these analogues exhibit elevated high-energy emissions, but also they have more frequent coronal mass ejection events [Airapetian et al., 2020].
If our young Sun exhibited such activity, carbon dioxide (CO2), methane (CH4), water, and nitrogen-bearing species like molecular nitrogen (N2) in the early atmosphere would have photodissociated (broken apart because of interaction with light) at higher rates than we see today. Fluxes of UV light at Earth’s surface also would have been greater, likely influencing the synthesis, degradation, and transformation of prebiotic compounds. Further, robust photodissociation of these molecules would have contributed key components that may have helped jump-start primitive chemosynthetic metabolisms, including those that used carbon monoxide and nitrogen oxide (NOx) compounds [e.g., Kasting, 2014]. Accurate models of early solar spectra, developed by computational methods or through observational studies of early Sun analogues, are a key input required for photochemical models of early Earth atmospheres.
Earth’s Early Atmosphere and Ocean
Volatile emissions from the solid Earth contributed massively to the composition of the planet’s early atmosphere. The influence of other factors, like frequent and often large impactors (e.g., meteorites and planetesimals), is still debated. Chemical interactions caused by impactors striking Earth may have produced reduced gases like CH4 and molecular hydrogen [Zahnle et al., 2020] (Figure 1), whereas early volcanism likely produced relatively oxidized gases like CO2 and N2 (Figure 2).
The distributions of these gases over time and space in the atmosphere would have dictated the availability of chemical reactants essential to prebiotic chemistry. They also would have modulated surface temperature and habitability under the presumably subdued solar input and at times of high UV fluxes during massive solar ejections. The presence and strength of the geomagnetic field—also the subject of intense study and debate—would have further regulated the retention or loss to space of atmospheric gases.
Geochemical data from the oldest zircons suggest that early recycling of crustal materials that had been previously altered in the presence of liquid water occurred during the Hadean eon (i.e., from the birth of the planet to about 4 billion years ago). This observation provides evidence for hydrologic cycling; relatively cool surface temperatures; early crustal differentiation to form silica-rich rocks; and even incipient plate boundary interactions, possibly including subduction, during this period [Harrison, 2020]. Arguments for the presence of very early oceans in the Hadean—a time formerly thought to have been too hot for liquid water to exist on Earth’s surface—have shifted the conversation about Earth’s earliest environments toward a consensus that they may have been favorable for life’s emergence.
Despite progress in our understanding, the composition of Hadean oceans and their evolution through the first several hundred million years of Earth’s history—and even whether liquid water was continuously and pervasively present—remain largely unknown. Oxidized compounds that we see in the modern ocean, like sulfates, would have likely been absent in Hadean oceans, which instead were probably acidic and rich in reduced iron as a result of the CO2-rich, oxygen-poor atmosphere [Halevy and Bachan, 2017] (Figure 3). Bulk Hadean ocean chemistry, water-rock interactions, and hydrothermal processing would have determined the properties of all near-surface and deep marine fluids while helping to modulate climate and set conditions in which prebiotic reactions could occur.
Plates and Planetesimals
Another important question is when crust emerged above the ocean surface. Some tectonic models indicate that conditions favorable to the formation of nascent, emerged continents might have occurred in the Hadean. Hot spot volcanism and large impacts also may have generated topographically high places that rose above the waves (see the opening image).
In addition to influencing the emergence of land, tectonic processes would have affected ocean and atmosphere composition, Earth’s climate, and diverse hydrothermal landscapes that may have shaped prebiotic chemistry (Figure 3). For example, weathering of primitive crust would have drawn down levels of atmospheric CO2, dampening the global greenhouse effect and altering ocean chemistry. And the rate of weathering would have accelerated, as more subaerial land was exposed.
Exposed (subaerial) landmasses also could have been crucial to the evolution of prebiotic chemistry [e.g., Benner et al., 2020]. Land exposed to the atmosphere experiences wet-dry cycles, and evaporation concentrates chemical compounds (Figure 2). These processes could have driven the assembly of cellular building blocks, such as lipid-like compounds that form enclosed vesicles, with the potential to encapsulate the precursors of genetic information and metabolic networks. Similar processes also might have supported transitions of these precursors into self-sustaining functional systems [Damer and Deamer, 2020]. However, although subaerial land is central in some views of the prebiotic world, the likelihood of such landmasses during the Hadean remains an open question.
As early Earth’s skies, ocean, and crust were evolving, the planet was being bombarded by large meteorites and planetesimals (Figure 1). Early collisions likely destroyed near-surface environments [Sleep et al., 1989], but also they may have delivered key prebiotic compounds to the planet, such as amino acids, sugars, purines (nitrogen-containing organic compounds that form the building blocks of modern DNA and RNA), and reactive phosphorus [e.g., Furukawa et al., 2019]. These collisions are also credited with creating a transient, highly reducing atmosphere, in contrast to the highly oxidizing conditions we have today [Benner et al., 2020; Zahnle et al., 2020]. Most models of prebiotic chemistry suggest reducing conditions as the most probable path to generating essential prebiotic compounds.
Beyond the delivery and production of essential organic molecules through impacts, such events may have helped fuel life’s beginnings in other ways, specifically by stimulating hydrothermal activity (Figure 1). The full range of consequences from early impacts, both conducive to and challenging for the beginnings of life, remain an important topic of research.
Filling In the Blanks Together
Studies and hypotheses related to the origins of life are fraught with uncertainties and debate, which is not surprising considering that scientists are looking back more than 4 billion years to piece together highly complex processes. The uncertainties, consequences, and interrelationships linked to these processes must be explored further.
None of the processes or hypotheses discussed here can be illuminated convincingly in isolation. To date, the diverse research communities separately investigating life’s origins and Earth’s early environments, who typically have collaborated and interacted across disciplines only superficially, have not achieved landmark progress toward resolving very difficult questions. The new Prebiotic Chemistry and Early Earth Environments (PCE3) Research Coordination Network (RCN) within NASA’s Astrobiology Program was designed to bridge this gap. This consortium of researchers aims to transform origins-of-life research by enhancing communication across the disciplinary divide between early Earth geoscientists and prebiotic chemists.
The principal goal of PCE3 is to cultivate a new research culture in which potential prebiotic pathways to life are tested within realistic planetary conditions and the dynamics and constraints of early Earth environments are fully integrated into origins hypotheses. We envision an iterative process wherein the needs and uncertainties of one group help motivate the research trajectories of the other in an ongoing back-and-forth.
For example, in efforts to chemically synthesize informational polymers, the products of such experiments often define next steps for research with little or no consideration for whether the conditions that yielded a successful reaction (e.g., salinity, pH, oxidation state, dissolved aqueous species) were likely to have existed in early Earth environments. Considering how much has been learned about Hadean conditions, geoscientific knowledge can and should inform next steps and goals in this work. In this new collaborative scheme, possible prebiotic chemical scenarios can be filtered by minimizing assumptions and by environmental plausibility.
New Discussions and Directions
Members of the PCE3 consortium are part of a growing community that includes many early-career scientists, poised to better integrate environmental knowledge of early Earth with models of life’s prebiotic chemistry. An important step toward this community building and integration was a PCE3-sponsored workshop in fall 2020 that hosted scientists from diverse disciplines. Themes covered in the workshop included Earth’s planetary formation; interactions between the crust and the volatile reservoir; the nature, sources, and inventories of life’s building blocks; the geologic settings where they occurred on early Earth; the reaction pathways that could result in increasingly complex prebiotic molecules; and ways of tracing our ancestors’ origins through investigations of present-day biochemistry.
Workshop participants compiled the most important open questions from each theme by identifying critical unknowns in studies of early Earth environments and their relationships to prebiotic chemistry. Topics of particular interest included the chemical and physical necessities shared among prebiotic chemical models and experiments and the prebiotic chemistry scenarios that likely lie outside planetary reality. For example, workshop participants highlighted as “critical unknowns” several aspects of the dynamic nature of local environments. These aspects include wet-dry cycles, temperature-pressure gradients, freeze-thaw cycles, atmospheric production rates of key molecules, redox fluctuations, and volcanic outgassing. Participants also identified mineral surface chemistries and the identity and concentration of metal ions in solution as important unknowns; these variables likely affected the types, rates, and the range of complexities of prebiotic reactions in local environments.
We are confident that the interdisciplinary approach stressed at the workshop will be an effective model going forward. We anticipate planning more workshops and working groups and look forward to novel opportunities for multidisciplinary funding, communication and collaboration across communities, and continuous evaluation of the assumptions in long-standing models.
Despite dramatic strides in studies of Earth’s history and its coevolving biosphere, life’s beginnings remain unknown. The uncertainties that have emerged in origins-of-life research can serve as nuclei for building a community engaged in interdisciplinary Earth science research. The PCE3 RCN is one important step toward a cohesive expression of shared needs, critical unknowns, and the unifying threads among different views of life’s beginnings.
It is essential that experiments in prebiotic chemistry account for what we already know about early Earth at global and local scales and that we translate progress made in our understanding of the Hadean into refined boundary conditions for these experiments. At the same time, scientists must question assumptions and continue to reexamine unknowns about early Earth—such as those discussed in this article—and integrate these uncertainties into their experiments and models.
How life began is an integral question central to the human experience, and it is intrinsically linked to the even bigger question of whether we are alone in the universe. The importance of focusing on early Earth is amplified as we expand our exploration of planetary habitability and life on other planets and moons, as well as planetary systems beyond our solar system. Identifying the specific conditions and chemical pathways that fostered the emergence of life is certain to factor prominently in that search.
We are indebted to Janet Iwasa for helping to draft the illustrations. We thank Simone Marchi, Miki Nakajima, Eddie Schwieterman, and Nick Tosca for their insight and scientific expertise, especially as we worked on the overview image. Funding for this project was provided by Earth’s First Origins NASA grant 80NSSC19M0069.
Airapetian, V. S., et al. (2020), Impact of space weather on climate and habitability of terrestrial-type exoplanets, Int. J. Astrobiol., 19, 136–194, https://doi.org/10.1017/S1473550419000132.
Bell, E. A., et al. (2015), Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon, Proc. Natl. Acad. Sci. U. S. A., 112, 14,518–14,521, https://doi.org/10.1073/pnas.1517557112.
Benner, S. A., et al. (2020), When did life likely emerge on Earth in an RNA-first process?, ChemSystemsChem, 2, e1900035, https://doi.org/10.1002/syst.202000010.
Damer, B., and D. Deamer (2020), The hot spring hypothesis for an origin of life, Astrobiology, 20, 429–452, https://doi.org/10.1089/ast.2019.2045.
Furukawa, Y., et al. (2019), Extraterrestrial ribose and other sugars in primitive meteorites, Proc. Natl. Acad. Sci. U. S. A., 116, 24,440–24,445, https://doi.org/10.1073/pnas.1907169116.
Halevy, I., and A. Bachan (2017), The geologic history of seawater pH, Science, 355, 1,069–1,071, https://doi.org/10.1126/science.aal4151.
Harrison, T. M. (2020), Hadean Earth, 291 pp., Springer, Cham, Switzerland.
Kasting, J. F. (2014), Atmospheric composition of Hadean–early Archean Earth: The importance of CO, Spec. Pap. Geol. Soc. Am., 504, 19–28, https://doi.org/10.1130/2014.2504(04).
Sleep, N. H., et al. (1989), Annihilation of ecosystems by large asteroid impacts on the early Earth, Nature, 342, 139–142, https://doi.org/10.1038/342139a0.
Zahnle, K. J., et al. (2020), Creation and evolution of impact-generated reduced atmospheres of early Earth, Planet. Sci. J., 1, 1–21, https://doi.org/10.3847/PSJ/ab7e2c.
Dustin Trail (email@example.com), University of Rochester, Rochester, N.Y.; Jamie Elsila, NASA Goddard Space Flight Center, Greenbelt, Md.; Ulrich F. Müller, University of California, San Diego; Timothy Lyons, University of California, Riverside; and Karyn L. Rogers, Rensselaer Polytechnic Institute, Troy, N.Y.