Pumice and the Origin of Life
Is pumice the solution to many of the problems with origin-of-life scenarios?
Science news sources are abuzz about a paper that came out in Astrobiology. This paper proposes pumice as a key player in making possible the origin of life on Earth. Pumice is a low-density volcanic rock, formed from solidified lava froth, whose uses include the preparation of stonewashed denim. (You may find bits of it in the pockets of your newly purchased "distressed" jeans.) The authors of the paper begin by addressing some of the major hurdles facing origin-of-life scenarios, and they subsequently outline the important features of pumice that make it a viable contender in aiding early Earth chemical reactions. Many people are interested in this because the authors present a well-researched paper that is up-front about some of the problems within origin-of-life research. They offer a creative hypothesis in the hope of overcoming these problems.
Origin-of-life scenarios try to explain either the origin of RNA (RNA-first world) or the origin of metabolic pathways (Metabolism-first world). Of the two, the RNA-first hypothesis is the stronger proposal and has gained popularity among scientists. Paul Nelson and Stephen Meyer discuss some recent findings in RNA-first research in their recent BIO-Complexity paper that can be found here. For ENV's summary of their paper, see here. Both RNA-first and the Metabolism-first advocates have been looking for ways to explain the closed-loop systems within living organisms. DNA gives us RNA, which in turn codes for proteins, but proteins are needed to make DNA. Metabolic pathways are closed loops with bi-products that are part of other closed-loop pathways (e.g., the tricarboxyllic acid cycle). Origin-of-life scenarios need to account for the way these closed-loops began, which means coming up with a novel reaction mechanism that must occur under naturalistic conditions. Natural conditions involve change coupled with chemical laws.
Ultimately, scientists who wish to give a naturalistic explanation for the origin of life must explain the origin of information (the problem is well outlined by Stephen Meyer in Signature in the Cell). DNA codes for the proteins that are the operators of cellular function. Without these two -- DNA and proteins -- cells could not exist. Naturalistic explanations presuppose a bottom-up approach to the origin of life, where things are built from their fundamental chemicals to complex life. According to this reductionist approach, DNA is the fundamental chemical of life and therefore a viable origin-of-life scenario must account for the reactions that will eventually lead to the formation of DNA.
With a refreshing scholarly candor, the authors of the Astrobiology paper lay out some of the fundamental problems with current origin-of life-scenarios, clearly stating that
Following half a century of reductionist thinking in biology, we now have good concepts for how the simplest cells work and even how later eukaryotic cells evolved, but there is little agreement as to how the first cells emerged from the prebiotic world.They point out that there are a variety of factors needed for life to have emerged, including metal catalysts, compartments for membranes to form, abundant free energy sources, energy storage in the form of ATP, and information-rich polymers like RNA and proteins. Certainly, on a first consideration, bringing together all of these factors seems implausible, yet the authors propose that pumice has qualities that can overcome many of the pertinent challenges.
Pumice forms from rapidly cooled magma. Because of this quick cooling, pumice does not crystallize, but forms a very porous rock. This porosity makes pumice very lightweight, so it can float on water. It also has crevices and a high surface area allowing for chemisorptions and compartmentalization of chemicals as well as the potential for lipid formation. Pumice has been used as a catalytic substrate in industrial reactions such as chemical cracking, and could very well absorb some of the metal oxides present in the early Earth to form organic catalysts. Furthermore, pumice is able to withstand extreme temperatures and conditions, so chemicals that are absorbed into the pores might be protected from extreme energy sources.
For each special property of pumice, the authors offer a possibility of how the stone could have proved useful in the origin of life. For example, regarding its ability to float on water, in the form of "pumice rafts," the authors offer this hypothetical scenario:
Early pumice rafts likely maintained their position at the air-water interface for years, perhaps alongside slicks of oily hydrocarbons. This is of interest because lipid micelles and other organic complexes are believed to have existed as films at the surface of the Archean ocean... and to have been generated by hydrothermal or extraterrestrial processes. Such lipid-lined vesicles have considerable advantages for the synthesis of RNA-like polymers. Here, we suggest that lipids formed by Fischer-Tropsch or Miller-Urey-type reactions around hydrothermal vent systems floated on the nearby ocean surface, where they formed oily slicks.Crucial to this scenario are several conditions that need to be just right. The pumice needs to be near hydrocarbons, but the paper offers no explanation of how these hydrocarbons formed, or how abundant they were on the early Earth. The formation of amino acids, the building blocks of proteins, from hydrothermal vents is problematic because of the various specific factors that must be in place for the reaction to work. (See here for an ENV article on the problems with a hot origin-of-life scenario.) Granted, this paper is conceptual but within the explanation being offered is a host of specifics that need to be adequately addressed but are not. For instance, how will this lipid-lined vesicle form in such a way as to aid in RNA polymer synthesis?
Regarding pumice's ability to withstand extreme conditions, the authors offer this possibility:
On early Earth, where alkaline basaltic and komatiitic rocks and alkaline white-smoker hydrothermal vents were widespread, beached pumice and scoria were likely to have come under the influence of alkaline waters...Negatively charged functional groups (carboxylic acids, phosphatic compounds) within some pumice vesicles could have then been brought together alongside positively charged groups (amines, heterocyclic bases) within adjacent compartments that had catalytic surfaces (zeolites, titanium oxides) as outlined above.In order for this scenario to work, not only does the pumice have to absorb the right chemicals, but then the positively charged and negatively charged chemicals have to "find" each other while there are also catalysts present.
With each possible use of pumice, there is a scenario that has multiple levels of complexity. The authors state that further laboratory experiments need to be done. We predict that the scientists will have to set up very exact conditions with specific reactants mixed in a particular way that are exposed to high-level energy sources in a very controlled environment that does not really mimic the presumably extreme conditions of the early Earth.
At first glance, pumice seems to be an interesting suggestion because it was likely in abundance and certainly has some useful features for certain chemical reactions. However, the reactions required are not simple reactions, and the steps involved, even using a substrate such as pumice, are still too numerous and specific to have happened by chance. It appears highly unlikely that pumice is capable of solving the problem of the origin of life.