The Origin of Life: Not so Simple (Part I)
In an article titled "A Simpler Origin for Life"--a title which hides the implication of the article, Robert Shapiro, writing in Scientific American, highlights many problems with chemical origin of life scenarios. Shapiro quotes Richard Dawkins on his worship of the first self-replicating molecule and says "[a]t some point a particularly remarkable molecule was formed by accident. We will call it the Replicator." (emphasis in original) That's "Replicator" with a capital "R". But, as Shapiro explains, the conventional explanation is not nearly so simple:
Unfortunately, complications soon set in. DNA replication cannot proceed without the assistance of a number of proteins--members of a family of large molecules that are chemically very different from DNA. Proteins, like DNA, are constructed by linking subunits, amino acids in this case, together to form a long chain. Cells employ twenty of these building blocks in the proteins that they make, affording a variety of products capable of performing many different tasks--proteins are the handymen of the living cell. Their most famous subclass, the enzymes, act as expeditors, speeding up chemical processes that would otherwise take place too slowly to be of use to life. The above account brings to mind the old riddle: Which came first, the chicken or the egg? DNA holds the recipe for protein construction. Yet that information cannot be retrieved or copied without the assistance of proteins. Which large molecule, then, appeared first in getting life started--proteins (the chicken) or DNA (the egg)?Shapiro also takes aim at the hypothesis that Miller-Urey type chemistry may have led to life's building blocks in meteorites:
(Robert Shapiro, "A Simpler Origin for Life," Scientific American, February 12, 2007)
By extrapolation of these results, some writers have presumed that all of life's building could be formed with ease in Miller-type experiments and were present in meteorites and other extraterrestrial bodies. This is not the case. A careful examination of the results of the analysis of several meteorites led the scientists who conducted the work to a different conclusion: inanimate nature has a bias toward the formation of molecules made of fewer rather than greater numbers of carbon atoms, and thus shows no partiality in favor of creating the building blocks of our kind of life. (When larger carbon-containing molecules are produced, they tend to be insoluble, hydrogen-poor substances that organic chemists call tars.) I have observed a similar pattern in the results of many spark discharge experiments.Shapiro then recounts that in the 1980's some suggested that "Life began with the appearance of the first RNA molecule." But Shapiro explains that RNA cannot be the first life, because "the clues I have cited only support the weaker conclusion that RNA preceded DNA and proteins; they provide no information about the origin of life, which may have involved stages prior to the RNA world in which other living entities ruled supreme." He goes on to critique the RNA world hypothesis, and lamenting that "despite the difficulties that I will discuss in the next section, perhaps two-thirds of scientists publishing in the origin-of life field (as judged by a count of papers published in 2006 in the journal Origins of Life and Evolution of the Biosphere) still support the idea that life began with the spontaneous formation of RNA or a related self-copying molecule."
(Robert Shapiro, "A Simpler Origin for Life," Scientific American, February 12, 2007, emphasis added)
More of Shapiro's discussion will be mentioned in the next post...