Life, Origin of

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 ther mo dynamic cycle, would move forward the point in time when the origin of life might be traced back to. In [Page 784]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 prebiotic 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 acidsunder 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.

...