Theoretical origin of life; stop the hype! (Introduction)

by David Turell , Tuesday, June 19, 2018, 20:03 (2349 days ago) @ David Turell

This thoughtful article asks for reasonable thought and expectations:

https://www.the-scientist.com/?articles.view/articleNo/54863/title/Opinion--Constrain-S...

"There are almost daily reports (and rather excited press releases) of how some scientific observations argue for the plausibility of life on other planets, in other solar systems, or in other galaxies, when in fact, what has been found is one or another organic molecule, or hints of organic material, in meteorites and comets. Rarely is the fact that we have yet to find life anywhere but here on Earth mentioned explicitly.

***

“'Defining life is notoriously difficult; its very diversity resists the confines of any compact definition,” the real problem is that the diversity of life, as we know it, is superficial and something of an illusion—we know of only one type of life, one original organism, and all of the subsequent organisms derived from it by various evolutionary processes. Moreover, we cannot examine this “last universal common ancestor” or LUCA, although there is no scientific doubt that A) it existed, B) it used DNA to store information, C) information was expressed in the form of RNAs, many of which, in turn, encoded polypeptides/proteins, D) it was bounded by a lipid membrane, and F) it can be characterized as a nonequilibrium reaction system, one that has been running continuously for billions of years and whose descendants are present in every living cell since.

LUCA was pretty complex, with the machinery to maintain its nonequilibrium state, a specific nucleotide-to-polypeptide coding scheme, and the ability to carry out DNA replication, transcription factor–regulated RNA synthesis, and ribosome-mediated, RNA-directed polypeptide synthesis. We might go a little further, and speculate that LUCA arose in a special environmental niche, and given its membrane-nature, likely an iso-osmotic one.

"what came before and the exact steps leading to LUCA are unknowable. Moreover, the billions of years that have elapsed since LUCA’s origin and the active nature of evolutionary processes that result in new genes “popping out” of the noise and becoming essential in organisms from fruit flies to humans, combined with the reality of structural or functional convergences, the growing recognition of small and alternative open reading frames that encode functionally different proteins, and the ubiquity of various forms of horizontal gene transfer, means that historic details and their evolutionary drivers are often obscure.

"All pre-LUCA models are based on plausible chemistry, an idea pioneered by the Miller/Urey experiment in 1959 that found that passing electrical current through what the scientists presumed was a likely early terrestrial atmosphere led to the generation of a complex array of organic molecules, including precursors of molecules found in modern organisms. More-recent studies have begun to refine these hypothetical scenarios, defining environments for the synthesis of raw materials, and then more-complex proteins, RNAs, and lipids. These molecules came together in some presumably stable or stably oscillating environment to generate pre-LUCA systems, whether based on “RNA world” type replicators, such as ribozymes—RNA molecules with both enzymatic activity and the ability to replicate themselves—or metabolic (nonequilibrium chemical reaction) systems, leading to membrane-bounded systems. All this not withstanding, until non-LUCA-derived forms of life are discovered, it does not serve the integrity of science as a source of dependable knowledge to speculate on what came before LUCA much beyond that."

Comment: Which is why I show the silly hyper-hyped reporting about minor findings in a highly controlled lab experiments. The start of life still looks like a miracle.