Astrobiology

Questions about our earliest beginnings have filled thinking minds for millennia across cultures, faiths, and wide-ranging frontiers of research. Asking where we come from is as fundamental as astrobiology’s driving query of “are we alone?” These days, we often link these questions as we explore life beyond our planet and solar system. Scientific steps toward answers have been big and frequent, but the pathways remain highly varied. Achieving something even close to a consensus has been elusive. The one thing we all can agree on, however, is that our understanding of Earth’s earliest stages, and that of our solar system, has advanced by leaps and bounds over recent decades. No longer must we explore life’s beginnings with little knowledge of how and when planetary habitability first developed and, more specifically, what the world was like roughly 4.4–4.2 billion years ago—a reasonable time estimate for the initial steps in the progression toward life. The simple distillation of this view is that models for life’s earliest chapters, including experimental simulations of prebiotic chemistry, can and should be designed around an increasingly sophisticated understanding of Earth’s initial boundary conditions, including the timing and controls on the emergence of oceans, the atmosphere, and tectonics—along with their coupled evolutions.

This article reviews the early history of our solar system from an astrobiological perspective and presents evidence from meteorites and astronomical observations. The purpose is to trace the formation of key molecules that participated in the building blocks of life. The Sun and its planetary system started from a section of a molecular cloud that collapsed into a protoplanetary disk. In the center of the protoplanetary disk, the protosun heated the surrounding material. The dust and gas inherited from the cloud remained pristine farther away from the protostar, while new compounds were created in the gas and on the icy mantles of the dust. The dust accreted into pebbles, pebbles formed planetesimals, and planetesimals collided and accreted pebbles to create planets. Meanwhile, the protosun became the Sun when its core reached the pressure and temperature required to transform hydrogen into helium. During this process, the Sun emitted high-energy radiation and particles that impacted the chemistry in the disk and the early evolution of the terrestrial planets.

The presence of major volatile elements—carbon, hydrogen, nitrogen, and sulfur—on Earth is critical for establishing life. The origin of these life-essential volatile elements (LEVEs) on Earth has been studied for many years. Here, we present a brief compilation of the prevailing ideas regarding volatile delivery to Earth and evaluate their origins, strengths, and weaknesses. Motivated by the fact that one model of LEVE delivery is via a giant impactor to Earth, we subsequently present a geochemical model aimed at understanding the possible volatile inventory and fractionation between the core, the silicate magma ocean (MO), and the atmosphere of a Mars-mass embryo. We looked at various end-member accretion scenarios of the embryo and their influence on the embryo’s LEVE budget and the LEVE ratios. We varied various chemical (initial concentration of volatiles in the undifferentiated bodies and the oxygen fugacity [fO₂] of geochemical fractionation) and physical parameters (silicate-mass fraction of the accreting bodies, MO depth) to observe their effects on the absolute and relative LEVE budgets of the embryo. Our results show that an oxidizing condition (logf O₂ ≥ IW−1 [Iron-Wüstite]) is critical in establishing the relative LEVE budget of the embryo’s MO, closer to that of present-day bulk silicate Earth. Furthermore, the accretion of larger bodies to form the Mars-mass embryo results in the closest match of the LEVE ratios to that of the present-day bulk silicate Earth (BSE). However, the absolute LEVE budget of the MO of Mars-mass embryo is depleted by at least 1–2 orders of magnitude compared with the BSE under all model calculation scenarios. In contrast, the CI-chondrite-normalized LEVE budget of the embryos’s core, in many of the scenarios, especially from the reduced (e.g., IW−2) bodies, overlaps or exceeds the present-day BSE estimate. We argue that for a Mars-mass, differentiated embryo, the cores provide a better prospect for LEVE delivery to proto-Earth, through core breakups and subsequent mixing in the MO or solid mantle. Future studies need to better assess whether the fractional retention of core materials in the silicate reservoir can match the present-day BSE LEVE budgets and how such a process compares with the LEVE delivery via less-processed primitive asteroids.

The topography and chemical composition of Earth’s early crust likely shaped the conditions under which self-replicating biomolecules emerged. The stability of these molecules depended on dynamic interactions across Earth’s interior and surface, from the core to the atmosphere. Tracing the origin of a biosphere on Earth requires understanding its transformation from an initially uninhabitable planet into a temperate world with a stable crust, active rock recycling, volatile cycling, and surface oceans. These features are closely linked to plate tectonics, a process unique to Earth in our solar system. Before the onset of modern plate tectonics, Earth evolved from a global magma ocean (∼4.5 Ga) into a differentiated planet with a primordial crust, mantle, and core. The co-evolution of the lithosphere, hydrosphere, and atmosphere played a fundamental role in establishing surface conditions suitable for life. Here, we review current perspectives on the evolution of tectonic regimes from Earth’s formation (∼4.567 Ga) to the emergence of mobile-lid tectonics and the implications for crustal environments that may have supported the origin of life.

The mode of tectonics that governed early Earth is controversial. This makes it challenging to infer surface environments relevant to the origin of life. The majority of the literature published in the past two decades was inclined to favor the appearance of plate tectonics sometime around the mid-Archean (∼3 Ga), with the operation of stagnant lid convection (or its variants) dominant in the earlier part of Earth’s history. However, the available and increasing geological record from early Earth is actually equivocal, and there is no theoretical basis to prefer stagnant lid convection over plate tectonics. In fact, such a delayed onset of plate tectonics would inhibit the emergence of life in the Archean, let alone in the Hadean. On the contrary, rapid plate tectonics in the early Hadean, enabled by the fractional crystallization of a magma ocean, could quickly transform inclement young Earth into a habitable planet, with formation of multiple surface environments potentially conducive to abiogenesis.

To address the chemistry responsible for life’s emergence, we examine the simple chemicals accessible on early Earth and the processes that could have transformed them. We focus on carbon reduction as a process central to the direct precursors of life. Two main approaches for the emergence of life’s building blocks and their associated functions are discussed. One approach investigates chemical routes for forming life’s building blocks via processes that worked differently from those of extant life, referred to as “unrestricted” prebiotic chemistries. The other approach establishes a direct mechanistic connection between prebiotic building blocks and extant biochemistry, referred to as “lifelike” prebiotic chemistries. Significant gaps remain in our understanding of early Earth’s conditions, which makes it difficult to constrain the possible locations and mechanisms of life’s emergence and the sources of precursors. Although numerous differences remain between the main origin-of-life hypotheses, consideration of the geochemistry of carbon reduction is a common denominator of these historically divided hypotheses.

The scientific study of the origins of life is a deep pursuit that exists at the intersection of multiple disciplines. Prebiotic chemistry focuses on understanding how biopolymer building blocks such as amino acids, nucleotides, and sugars emerged and how their chemical and structural space evolves toward life (chemical evolution). Simulation experiments have been essential for exploring plausible pathways for the origin of building blocks under early Earth conditions and planetary environments. Key examples include the seminal Miller-Urey experiment and the polymerization of hydrogen cyanide. Research highlights the role of environmental cycles and geochemistry in shaping the prebiotic chemical space. These processes facilitated the condensation and stabilization of biopolymer precursors, particularly in terrestrial or small-pond scenarios. Noncanonical building blocks, including triazines and alternative amino acids, may have contributed to proto-biopolymer formation. This expands our understanding of chemical evolution. Despite significant progress, challenges remain, particularly in understanding nucleoside formation and the transition to modern biopolymers. This review provides a general overview of the prebiotic formation of biopolymer building blocks and examines both classic and seminal experiments and recent experimental approaches. Insights provided by extraterrestrial samples, such as carbonaceous meteorites and asteroids, also contribute to offering a comprehensive perspective on abiogenesis.

Chemical and geological processes on prebiotic Earth are believed to have resulted in the emergence of life through the increasing organization and functionality of organic molecules. This primer provides an overview of some key abiotic chemical and physical processes that could have contributed to life’s building blocks (amino acids, nucleotides, fatty acids, and monosaccharides) becoming more ordered during the early stages in the origin of life. The processes considered include polymerization, intramolecular folding, multimolecular assembly, and chemical evolution through various selective mechanisms. Our goal is to provide an accessible, high-level synopsis of these key general concepts for a diverse audience.

The tree of life is rooted at the “origin of life.” One model holds that core biochemistry, which includes the genetic code, the ribosome, biopolymer backbones, and amino acid and nucleotide monomer alphabets, was inherited vertically from a single origin of life. In this model, core biochemistry is a frozen accident that reflects prebiotic chemistry. In an alternative model explored here, life arose across diverse planetary environments and generated diverse biochemistries that competed and cooperated. These biochemistries converged through selection driven by the “network effect.” The network effect conferred greater fitness on participants in increasingly dominant biochemistries: the more extensive the adoption of a biochemistry, the greater the benefits for systems using it. In this model, the evolution of core biochemistry was driven, in part, by compatibility, integration, and coordination. The last universal common ancestor (LUCA) in this model represents a diffuse tipping process—where biochemical convergence reached critical mass. LUCA is a process of convergence rather than a specific organism or collection of organisms. At the tipping point, the biosphere committed to the transition from competing biochemical platforms to a universal standard. After the tipping point, biological innovation exploded, with fixed core biochemistry. This model makes testable predictions: core biochemistry should show evidence of evolutionary optimization rather than frozen accidents; core biochemistry should show molecular entanglement that reflects incremental coevolution; and biosynthetic pathways should differ from prebiotic chemistry. These predictions appear to be supported by observations.

Both the geologic and genomic records provide information on the earliest history of terrestrial life. Despite the challenges inherent to the fragmentary and highly metamorphosed earliest rock record, microfossils as old as ∼3.5 billion years (Ga) and potential morphological and isotopic traces in earlier rocks suggest the origin and diversification of life within the planet’s first billion years. This is consistent with a growing body of evidence for liquid water in Earth’s surface or near-surface environment by around 4.3 Ga, which provides at least one requirement of habitability. Genomic evidence further informs our understanding of early life, including some aspects of its environment, physiology, and metabolism. In addition, ancestral proteome reconstruction and molecular clocks provide clues to the nature and timing of the last universal common ancestor (LUCA), although the latter methodology remains challenging and highly uncertain. Improved molecular clock methods may be able to better constrain the timing of LUCA and the divergence of the major domains of extant life, while further interrogation of the earliest geologic and geochemical records may be able to reveal even earlier evidence for an inhabited planet.

This review of martian organic geochemistry aims to contextualize recent findings of organic molecules in martian meteorites and from Mars missions within the broader study of origins of life on Earth. Analyzing martian organic inventories helps us understand the abiotic processes in planetary environments that are common wherever rocks interact with liquid brines and that likely contributed to the emergence of life on Earth. Mars is only the second planetary body studied for organic molecules; while carbonaceous meteorites, comet missions, and sample-return analyses of comets and asteroids have shown the diversity of organics across the solar system, studying Mars reveals what these molecules are on another planet. Although a definitive sign of extraterrestrial life has not yet been found, the findings provide insights into abiotic synthesis mechanisms that would have occurred on early Earth. At worst, these observations represent the oldest planetary record of organic and prebiotic chemical synthesis pathways that could have led to life, as inferred from the alteration of Earth’s oldest rocks. They may also point to potential habitats for past martian life. Currently, samples collected by the Perseverance rover represent a unique opportunity to verify which of the two questions, “Are we alone?” or “How did we get here?” will be true for Mars. Without doubt, these questions would be best addressed through the use of higher resolution analyses by more advanced and sensitive instrumentation after sample return to Earth. Even if no definitive signs of life are found in returned samples, they would give us the opportunity to study the missing link to life on Earth, that of the primordial abiotic organic chemical processes that could have led to life. Therefore, there are no wrong answers to exploring Mars for signs of life; its secrets will illuminate our understanding of ourselves and our place in the universe, whatever the answer.