September 16, 1995
Editors
American Scientist
P.O. Box 13975
Research Triangle Park, NC 27709
To The Editors:
In "The Beginnings of Life on Earth" (September-October), Christian de Duve presents compelling "facts and ideas that are being considered to account for the early stages in the spontaneous emergence of life on earth"; especially, his hypotheses about the central energetic role that thioesters must have played in pre- and protobiosis. His last-page conclusion that these protometabolic "processes must have been highly deterministic. In other words, these processes were inevitable under the conditions that existed on the prebiotic earth" (emphasis, his) is also quite persuasive.
However, his subsequent opinion that "It also seems likely that life would arise anywhere similar conditions are found", and his final unqualified assertion (poetic license?) that "The universe is awash with life", require some scrutiny:
Science and history teach us that although processes in the universe proceed relentlessly, according to laws which are deterministic on a large scale, the details and course of the unraveling of such processes are often contingent on "accidents" that occur on the microscopic scale (e.g., the old saw that a flap of a butterfly's wing may determine the ultimate development of a distant massive weather system). Even if we allow that the universe may be awash with planets with flourishing "protometabolism", and even "protocells", it does not necessarily follow that contingent events that contributed to the next step in the origin of life have been mimicked on even one of these hypothetical worlds.
The arguments that de Duve presents for the general deterministic course of prebiosis must implicitly include the expectation of a racemic mix of left-handed and right-handed versions of the assymetric molecules in the prebiotic soup. If protometabolism is so certain to lead to life, we should expect to find evidence, on earth, of organisms which use molecules of both kinds of handedness in their proteins and nucleic acids. Where is the evidence?
As first discovered by Louis Pasteur, living organisms often use only one kind of each enantiomer. And the evidence is now overwhelming that, in all known organisms, only the levo amino acids and the dextro sugars are used in the building of proteins and nucleic acids. Likewise, all use the same genetic code. Together these facts seem to provide strong evidence that all organisms arose from a single common ancestor. Either, after multiple origins, life passed through a very tight evolutionary bottle-neck, or else it arose only once on earth. Therefore it might be unique to earth.
Can at least the "handedness paradox" be explained?
1. Could some pervasive assymetry in the properties of the inorganic substrate of the prebiotic world have acted to imprint its handedness on the molecules of the organic soup? No evidence for such pervasive assymetry exists.
2. Could some aspect of the assymetry of the conservation laws of physics; i.e., the non-conservation of parity, and the possible effects of so-called neutral currents, explain a large bias in the handedness of the soup's components? No compelling arguments or experiments yet support this position.
3. Is it possible that approximately equal numbers of proto-organisms arose which used opposite-handed building blocks, but through competition, only our ancestors survived? This argument requires a mechanism to "explain" the selective advantage of one kind of handedness over the other (such as 1. or 2.).
4. Is it possible, as in 3. that equal numbers arose, but through "random accident or accidents", only our ancestors survived? This would be plausible only if the numbers of each kind of progenitor were extremely small at the time of the accidents (i.e., the sine qua non for an evolutionary bottle-neck) .
5. Could the spontaneous aggregation of a survivable, mutable, self-replicatable RNA-like molecule, have been so improbable, that it happened only once, even in the earth's very hospitable prebiotic soups (the ultimate bottle-neck)?
If either 4. or 5. are true, the probability that "The universe is awash with life" might be vanishingly small, despite the enormous numbers of galaxies and starsý.
In such an enlightening tutorial on such an important subject, it was less than helpful for de Duve to skirt by the evidence that some "...single, freak, highly improbable event...", (i.e., an extremely narrow evolutionary bottleneck). still seems to be a required part of the story of the beginnings of life on earth.
ý Ornstein, L. 1982. Extraterrestrial Intelligence: The debate continues; A biologist looks at the numbers. Physics Today, March, pp. 27&endash;31.
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