The origin of life remains today one of the most challenging puzzles to science. The challenge is twofold: (1) qualify the essence of life and (2) explain its appearance on Earth. Although both aspects have been subject to much scientific investigation, no satisfactory explanation has been formulated so far.
Life is often defined as the distinctive property of particular physical systems: living organisms. One way of defining life is to say that a physical system is alive if and only if it can replicate with variation and therefore be submitted to natural evolution. From a temporal perspective, the origin of life can therefore be defined as the point in time when, for the first time on Earth, a particular physical system simultaneously displayed a set of given properties, namely replication and variation.
How life is defined has a strong impact on the question of its origin: Taking a more restrictive definition of life, one that would for instance require that such replicating systems with variation be able to complete at least one thermodynamic cycle, would move forward the point in time when the origin of life might be traced back to. In addition, defining life as a collective set of properties of living organisms also raises the question of the temporal order of appearance of each of these properties: For instance, which appeared first, replication or variation?
Roughly speaking, life most likely appeared on Earth some 3.8 billion years ago. This dating is framed, on the one hand, by the formation of the planet some 4.5 billion years ago, with still some heavy meteoritic bombardment until about 4 billion years ago; and, on the other, by the oldest cellular fossils dated 3.6 or potentially even 3.8 billion years ago. As such, the time frame available for life to appear is about 200 to 400 million years.
The first modern hypotheses of the origin of life were formulated in the 1920s by Alexander Oparin and John B. S. Haldane, separately: The first living systems would have appeared in a primitive ocean of organic molecules, all of them resulting from prebi- otic chemical processes, that is, chemical processes compatible with the physicochemical conditions believed to be those of the primitive Earth, before the appearance of life. The first scientific experiments supporting these hypotheses are those of Stanley Miller in 1953, who demonstrated the possibility of synthesizing certain organic molecules— amino acids—under prebiotic conditions. Since then, the prebiotic synthesis and chemical behavior of numerous other organic molecules in prebiotic conditions have been investigated, including those of proteins, lipids, and nucleic acids. Today, the scientific field of research on the origin of life draws upon a large number of disciplines: molecular biology, biochemistry, prebiotic chemistry, and theoretical biology but also planetology, geology, or even astronomy, in order to define the environmental conditions of the primitive Earth or to search for primitive life forms on alternative planets.
Far from having occurred at a particular moment or point in time, as a sudden event, the origin of life is likely to have been the result of a long and gradual process. The question remains whether this gradual process is truly continuous or might otherwise display some sorts of sudden steps akin to phase transitions, for instance. In the latter case, such steps could be used as particular landmarks for defining the origin of life, even if these are still speculative today. Due to this specific time frame of several hundred million years during which life is thought to have appeared on Earth, experimental research on the origin of life has to consider a broader scope of physicochemical possibilities than usual, for instance chemical reactions with longer characteristic times than in usual laboratory experiments, or environmental conditions that would be much different from current ones (temperature, pressure, pH, etc.). Over such long periods, the role of chance also might come to play an important role, enabling unlikely molecular encounters to happen or precellular components to assemble and disassemble in many different ways as in a “tinkering” process. Therefore, life as we know it on Earth would somehow also keep the trace of numerous specific events or “frozen accidents,” all of them highlighting the very historical nature of life.
Life as we know it on Earth might be one particular instance of a more generic phenomenon, the characterization of which is among the goals of artificial life research. What is more, not only could life appear under different forms, but it might also appear in different places in the universe. Exobiology research focuses in particular on the quest for extraterrestrial signs of life. Such a discovery would not only be an astounding discovery, but it would also shift the question of the origin of life and raise the question of its potential perennial existence over the known history of the universe, an old theory known as panspermia. In addition, life as we know it today might not be life as it has been in past, or, as a matter of fact, life as it will be in the future. Life as we know it today might not be life wherever nor forever.
Could there be, as well, multiple parallel pathways leading to the origin of life? If life is defined as a set of collective properties of particular physical systems, its origin might thereby be traced back to the chains of events that led to the appearance of each property separately and, as such, of those specific components enabling the physical systems to display such properties. Hence, for instance, the origin of life on Earth might be traced back to the origin of prebiotic proteins, prebiotic nucleic acids, or even prebiotic lipidic compounds, all of them being basic components of current living systems. In such a case, the origin of life could dissolve into as many origins as there are components necessary to build a living system.
Could life still be originating on Earth as we speak? This would make the origin of life a continuously recurring event, continuously generating new origins, so to speak. Such a hypothesis appears unlikely: It would require not only abiotic chemical processes, that is, chemical processes able to produce organic molecules without appealing to compounds of existing living organisms, but also physicochemical processes that could lead to the organization of these molecules into living systems. However, it appears extremely likely that most abiotically synthesized organic molecules today would end up being immediately assimilated by existing living organisms, without any possibility to further assemble themselves into new living systems, life thereby preventing any new origination of itself.
See also Creationism; Evolution, Chemical; DNA;
Evolution, Issues in; Evolution, Organic; Genesis, Book of; Gosse, Philip Henry; Materialism; Oparin, A. I.
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Brack, A. (Ed.). (1999). The molecular origins of life: Assembling pieces of the puzzle. Cambridge, UK: Cambridge University Press.
De Duve, C. (1995). Vital dust: The origin and evolution of life on earth. New York: Basic Books.
Eigen, M. (1992). Steps towards life: A perspective on evolution. Oxford, UK: Oxford University Press.
Fry, I. (2000). The emergence of life on earth: A historical and scientific overview. New Brunswick, NJ: Rutgers University Press.
Popa, R. (2004). Between chance and necessity: Searching for the definition and origin of life. Heidelberg, Germany: Springer-Verlag.