Earth and Planetary Science Letters 226 (2004) 1–15 www.elsevier.com/locate/epsl Frontiers How life began on Earth: a status report Jeffrey L. Bada* Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0212, United States Received 7 January 2004; received in revised form 16 July 2004; accepted 22 July 2004 Editor: A.N. Halliday Abstract There are two fundamental requirements for life as we know it, liquid water and organic polymers, such as nucleic acids and proteins. Water provides the medium for chemical reactions and the polymers carry out the central biological functions of replication and catalysis. During the accretionary phase of the Earth, high surface temperatures would have made the presence of liquid water and an extensive organic carbon reservoir unlikely. As the Earth’s surface cooled, water and simple organic compounds, derived from a variety of sources, would have begun to accumulate. This set the stage for the process of chemical evolution to begin in which one of the central facets was the synthesis of biologically important polymers, some of which had a variety of simple catalytic functions. Increasingly complex macromolecules were produced and eventually molecules with the ability to catalyze their own imperfect replication appeared. Thus began the processes of multiplication, heredity and variation, and this marked the point of both the origin of life and evolution. Once simple self-replicating entities originated, they evolved first into the RNA World and eventually to the DNA/Protein World, which had all the attributes of modern biology. If the basic components water and organic polymers were, or are, present on other bodies in our solar system and beyond, it is reasonable to assume that a similar series of steps that gave rise of life on Earth could occur elsewhere. D 2004 Elsevier B.V. All rights reserved. Keywords: prebiotic soup; metabolist theory; origin of life; pre-RNA World 1. Introduction the Universe. It is generally assumed that there are two fundamental requirements for life as we know it: the One of the major scientific questions that confront presence of liquid water and organic polymers, such as humanity is whether life exists beyond Earth. If the nucleic acids and proteins. Water’s unique properties conditions and processes that resulted in the origin of (excellent solvent, exceptionally large liquid temper- life on Earth are common elsewhere, then it is ature range, etc.) make it an ideal medium for chemical reasonable to expect that life could be widespread in reactions to take place. Polymers are needed to carry out the central biological functions of replication and * Tel.: +1 858 534 4258; fax: +1 858 534 2674. catalysis. Without these vital components, as far as we E-mail address: [email protected]. know, life is impossible. 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.07.036 2 J.L. Bada / Earth and Planetary Science Letters 226 (2004) 1–15 The origin and early evolution of life on Earth can be associated with life as we know it reacted to generate divided into several stages [1–3]: the prebiotic epoch; the first living entities and how these in turn evolved the transition to primitive biotic chemistry (the pre- into organisms that left behind actual evidence of their RNAWorld); the evolution of the early biotic chemistry existence in the rock record. of the pre-RNA World into self-replicating RNA molecules (the RNA World); and the evolution of the RNA World into modern DNA/protein biochemistry 2. Origin of life theories (DNA/Protein World) which was a common ancestor of all subsequent life on Earth. The appearance of the Two views on how the transition from abiotic first molecular entities capable of multiplication, organic compounds to autonomous self-replicating heredity and variation, which probably occurred in molecules capable of evolving by natural selection the later part of the pre-RNA World, marked the point into ones of increasing efficiency and complexity took of the origin of both life and evolution. place are presently dominant [1]: It is only the two end members in this series, the prebiotic epoch and the DNA/Protein World, that we (a) The prebiotic soup theory: Organic compounds in know the most about [3]. We can readily investigate in the primordial oceans, derived from a variety of the laboratory, using a variety of plausible geochemical possible sources, underwent polymerization pro- conditions, the possible routes by which compounds of ducing increasingly complex macromolecules, biological interest could have been produced on Earth. some of which by chance were capable of Meteorites can be studied to determine what clues they catalyzing their own self-replication. These sim- contain about natural abiotic organic chemistry. The ple self-replication entities evolved into increas- genes and proteins of modern organisms can be ingly complex ones and eventually into organisms dissected in order to ascertain information about their with modern biochemistry. possible origins. The first time that direct evidence of (b) The metabolist theory: A primitive type of life’s existence would have been preserved in the form bmetabolic lifeQ characterized by a series of self- of physical fossils in ancient rocks occurred in the sustaining reactions based on monomeric organic DNA/Protein World when the compartmentalization of compounds made directly from simple constitu- the biochemical machinery by cell-like membrane ents (CO2, CO) arose in the vicinity of mineral- structures comparable to those used in modern biology rich hydrothermal systems. According to this likely became widespread. Fossilized structures that theory, at first, blifeQ did not have any requirement resemble single-cell organisms similar to modern day for informational molecules. As the system of cyanobacteria have been found in rocks formed 3.5 self-sustaining reactions evolved in complexity, billion years ago (Ga) [4], although whether these genetic molecules were somehow incorporated in structures are indeed ancient fossils or artifacts has order for metabolic-based life to develop into recently become controversial [5]. Rocks older than 3.5 biochemistry as we know it. Ga have been so extensively altered by metamorphic processes that any molecular or fossilized evidence of Besides these two dominant theories, there have also earlier life has apparently been largely obliterated (for been numerous suggestions that life began elsewhere example, see [6]). and was transported to Earth (for example, see [7]), To evaluate how life may have begun on Earth, we but this only shifts the problem of the origin of life to must access what the Earth was like during its early a different location. history and under what conditions the processes According to the modern version of the prebiotic thought to be involved in the origin of life took place. soup theory [1], organic compounds derived from Considerable progress has been made in our knowl- bhomegrownQ chemical synthetic reactions on Earth edge of the early Earth and in how the transition from and the infall of organic rich material from space abiotic to biotic chemistry may have occurred. Never- accumulated in the primordial oceans. These com- theless, there are still enormous gaps in our under- pounds then underwent further reactions in the primal standing of how the simple organic compounds broth, producing ones with increasing molecular J.L. Bada / Earth and Planetary Science Letters 226 (2004) 1–15 3 complexity. Some of these reactions took place at RNA molecules and even primitive cells [12]. How- interfaces of mineral deposits with primitive ocean ever, given the transient, short-lived nature of hydro- water, while others occurred when the primitive ocean thermal systems, the plausibility of this process under constituents were concentrated by various mecha- geochemical conditions seems questionable. nisms, such as evaporation in shallow water regions In principle, self-sustaining autotrophic reactions or the formation of eutectic brines produced during the conceivably could have arisen in any type of environ- freezing of parts of the oceans. ment as long as the reactant/product molecules sur- From the assortment of simple organic compounds vived long enough to continue to be part of the overall in the primitive oceans, geochemical processes next reaction chain. Of the various reaction schemes that resulted in the synthesis of polymeric molecules. As the have been proposed, however, none have been dem- variety of polymers (oligomers) that were assembled onstrated to be autocatalytic with one possible excep- from the simple monomers by polymerization pro- tion [13]. The exception is the formose reaction, where cesses became more varied, some by chance acquired the formation of a diverse variety of sugars from for- functions, such as the ability to catalyze other reactions. maldehyde in the presence of alkaline catalysts appa- With the rise of catalytic molecules, increasingly rently involves an autocatalytic cycle that can result in complex macromolecules were produced and eventu- the continuous autocatalytic synthesis of sugars as long ally by chance molecules with the ability to catalyze as there is an unlimited supply of formaldehyde [13]. their own imperfect replication appeared. Although Advocates of the metabolist theory generally favor these first replicators at first probably represented only hydrothermal environments (for example, see [9,14]). a tiny fraction of the large array of macromolecules, However, even if the metabolic-type reaction schemes with the ability to catalyze their own replication, they that have been proposed were feasible, they would not would have soon become increasingly more abundant. have been unique to hydrothermal temperatures. This would have marked the transition from purely Reactions that take place rapidly at elevated hydro- abiotic chemistry to primitive biochemistry. These first thermal temperatures would also occur at the more self-replicating molecular entities began the evolu- moderate temperatures characteristic of the overall tionary cascade that next led to the RNAWorld and then surface of the Earth, albeit at slower rates.