Saturday, December 04, 2010

Many Metabolisms, Many Origins?

An unspoken but widely held belief among both evolutionary biologists (and some "intelligent design" supporters) is the idea that life (or, to be more specific, living organisms and/or metabolic processes) originated once a very long time ago. Along with my fellow biology majors, I was taught this by William T. Keeton in introductory biology at Cornell, where we also were told that if life (or biomolecules) somehow spontaneously started again today, it would immediately be scarfed up by already living organisms.

This idea ultimately derives from the last paragraph of Darwin's Origin of Species, in which he proposed that
"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." [Origin of Species, 1st edition, 1859]
Darwin asserted this partly to contrast his theory of evolution from that of Lamarck's, which included the idea that life was continuously arising spontaneously, generating new phylogenetic lines of organisms throughout deep evolutionary time. The discovery of the (almost) "universal" genetic code in the 1950s by Crick, Nirenberg, Holley, Khorana, et al provided strong evidence for the "one origin" hypothesis.

However, the fact that there is currently no evidence for an alternative "many origins" hypothesis doesn't necessarily support the conclusion that this hypothesis has been falsified. On the contrary, as the recent discovery by Felisa Wolfe-Simon of a "shadow arsenic metabolism" indicates, this lack of evidence is the result of lack of investigation, rather than actual lack of such origins. It is, in other words, quite possible that life (or at least biochemical processes similar to metabolic processes and molecules similar to "standard" biomolecules, and even cell-like structures incorporating both) is "originating" spontaneously all the time, but that we haven't noticed it because we haven't been looking. After all, nobody suspected the existence of an entire domain of living organisms (i.e. the Archaea) until Carl Woese starting looking two decades ago.

As J. B. S. Haldane — who formulated an early hypothesis for the origin of life — once quipped,
"[T]he Universe is not only queerer than we suppose, but queerer than we can suppose." [Haldane, J. B. S. (1927) Possible Worlds and Other Papers, page 227]

As always, comments, criticisms, and suggestions are warmly welcomed!


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At 12/04/2010 10:27:00 AM, Anonymous Billare said...

You can't possibly have missed this Nature paper, A formal test of the theory of universal common ancestry? From the abstract: "Among a wide range of biological models involving the independent ancestry of major taxonomic groups, the model selection tests are found to overwhelmingly support UCA irrespective of the presence of horizontal gene transfer and symbiotic fusion events." Off the top of my head, as I don't have access at the moment, I think the author the probability that there was not a LUCA to be 10^{-43} -- essentially zero.

At 12/04/2010 01:30:00 PM, Anonymous Billare said...

Why was my comment deleted?

At 12/04/2010 06:21:00 PM, Blogger Allen MacNeill said...

It wasn't; it got hung up in the spam filter. Sorry about that!

As for the Nature paper, it doesn't affect my blog post at all, as it applies only to the LUCA of organisms that use the "universal" phosphate-containing DNA code. It remains to be seen if there are other phylogenetic lineages with metabolisms so different from those currently recognized that we haven't studied them yet. The discovery of organisms that use arsenic in place of phosphorus in their DNA suggests that such "alternative phylogenies" may exist. Wouldn't it be fun to find one?

At 12/19/2010 02:21:00 AM, Blogger Tom English said...

1. I had read elsewhere that arsenate had replaced phosphate in the DNA backbone, and that there was a huge question as to how the structure, which should dissolve rapidly, was stabilized. In the NYT article, I see that Gerry Joyce questions whether the DNA structure has actually changed. He describes the bacterium as "clinging to every last phosphate molecule..."

2. I've given some thought to computational study of the emergence of autocatalytic sets. I'll never try it, because the problem of detecting them seems intractable. Detection can't be any easier in the real world.


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