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Free from the archives: The origin of life II: How did it begin?

Posted on 9. August, 2014.

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The problem of how a mixture of chemicals can spontaneously transform themselves into even a simple living organism remains one of the great outstanding challenges to science. Various primordial soup theories have been proposed in which chemical self-organization brings about the required level of complexity.

Major conceptual obstacles remain, however, such as the emergence of the genetic code, and the “chicken-and-egg” problem concerning which came first: nucleic acids or proteins. Currently fashionable is the so-called RNA world theory, which casts RNA in the role of both chicken and egg. Other theories assume that protein chemistry and even clay crystal life came before nucleic acids. To be fully successful, a theory of biogenesis has to explain not merely the emergence of molecular replication and chemical complexity, but the crucial information content and information processing capabilities of the living cell. 

In paper I, I considered recent developments in the problem of when and where life began. I argued that a favourable setting was the deep subsurface on either Earth or Mars, some 4 billion years ago, with Mars being somewhat favoured. However, I said almost nothing about what actually took place to bring life into being. In this paper I shall discuss the problem of how life originated, which remains one of the great outstanding challenges to science.

The central difficulty is easy to grasp. How can undirected physical forces produce a state of such immense complexity and specificity as a living organism? The simplest autonomous organisms contain millions of large specialised molecules. The living cell consists of an elaborate web of inter-dependent chemical substances, many of which are not found anywhere except in living systems. If we want to understand how life came to exist from nonliving substances, we cannot appeal to the prior existence of molecules that are only created by life.

In the nineteenth century it was widely supposed that life was some sort of “magic matter,” often called protoplasm, and that the formation of life resembled baking a cake – a question of mixing the right ingredients in the right proportions and the right order. Today we know that the cell is made of perfectly ordinary substances, the essential elements being carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur, although the term “organic chemistry” remains as a hang-over from the time when it was believed that life obeyed different physical laws from inanimate matter. 

The revolution in the biological sciences, particularly in molecular biology and genetics, has revealed not only that the cell is far more complex than hitherto supposed, but that the secret of the cell lies not so much with its ingredients as with its extraordinary information storing and processing abilities. In effect, the cell is less magic matter, more supercomputer. In paper I, I mentioned the genetic code as one example of the computational prowess of the cell. Life performs its amazing feats not because of a special form of chemistry, but because organisms can harness chemical processes and subordinate them to an agenda encoded in DNA. So any successful theory of biogenesis has to account not only for the stuff of life – the myriad customised molecules that are vital to its operation – but also for its informational aspects. 

The living cell is so complex that it clearly didn’t spring into being all in one go, as a result of a single amazing chemical reaction. There must have been a long sequence of physical processes leading up to the first microbe. There seem to be three distinct aspects of life that must be explained on the way: reproduction, metabolism and cellular structure. Organisms must be able to replicate, i.e. pass on genetic information. But without metabolism, they wouldn’t be able to do anything (including replicate). And because Darwinian evolution depends on a competition between individuals, it is hard to see how life could evolve without cells. But researchers cannot even agree on the sequence of these major features. Some argue that cellular structures formed first, others that the road to life started with self-replicating molecules, and yet others that complex energy-harnessing chemical cycles preceded both.

Read the complete article, free of charge, in Science Progress, Volume 84, Number 1, February 2001, pp. 17-29.

Author: Paul Davies

DOI:10.3184/003685001783239096

Image courtesy of Mopic