Cracking how life arose on Earth may help clarify where else it might exist
July 30, 2013 — Does life exist elsewhere or is our planet unique, making us truly alone in the universe? Much of the work carried out by NASA, together with other research institutions, is aimed at trying to come to grips with this question.
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A novel and potentially testable theory of how life arose on earth, first advanced more than 25 years ago by Michael Russell, a research scientist in Planetary Chemistry and Astrobiology at the NASA Jet Propulsion Laboratory, was further developed in a recent paper published in Philosophical Transactions of the Royal Society B (PTRSL-B) -- the world's first science journal -- by Russell, Wolfgang Nitschke, a team leader at the National Center for Scientific Research in Marseille, France; and Elbert Branscomb, an affiliate faculty member at the Institute for Genomic Biology (IGB) at the University of Illinois at Urbana-Champaign.
This just-released paper, together with a companion paper by Russell and Nitschke in the same journal issue significantly advance the hypothesis and bring it substantially closer to experimental testing -- an effort already underway by Russell and his collaborators who are developing experimental model systems that can recreate, and test, the essential principals of the theory.
Notably, the Russell theory provides potential explanations for several puzzling aspects of how life works, including, for example, how it taps into and exploits sources of energy.
This strange energy conversion process works by using externally supplied energy to constantly pump a lot more protons onto one side of a biological membrane than the other and then having them flow back "downhill" immediately, but only through a turbine-like molecular engine which creates a chemical fuel called ATP; a fuel that cells "burn" in order to power vital cell processes. This process would be an exact parallel to the hydroelectric generation of electrical energy if some other source of energy was first required to pump water up hill behind a dam to then flow down to drive turbines. The question is why is this strange and seemingly inefficient process used?
Russell's theory suggests an answer: when life began it wasn't necessary to pump protons, the proton gradient was already there for the taking. Russell's theory sees life coming into being as a natural physical consequence of a geochemical process called serpentinization that produced, for free, the system's major components: cell-like compartments surrounded by membranes, the right proton concentration differences between the inside and outside of these mineral membranes, and primitive, "mineral-based" forms of the "turbine" motors needed to make a molecule like ATP.
The process of serpentinization occurs when water gravitates down cracks in hot, newly formed ocean crust, where it reacts chemically with the minerals in the rocks in a reaction that produces an extremely alkaline (pH ~13) effluent, rich in hydrogen and methane and the metal molybdenum, so important as a catalyst in life. This effluent is then driven back to the surface where, in the ocean of early earth, it contacted cooler, acidic (~ pH 5.5), and CO2-rich water to create precipitates that form chimney-like submarine towers. These precipitates are highly structured with a myriad of cell-like compartments surrounded by mineral membranes -- akin to our own. And across these membranes, separating acidic ocean and alkaline effluent, stood essentially the same proton gradient -- in strength and direction -- that the cells of all living things on earth are constantly recreating and then immediately using today.
"So, if the Russell theory is correct, it is suddenly obvious why we pump protons and use this silly method," Branscomb said. "We got stuck on this 'free lunch' energy system when life was born, developed a lot of fancy machinery to use it, and have never severed that umbilicus since."
Branscomb, a member of the IGB's Biocomplexity research theme led by Swanlund Professor of Physics Nigel Goldenfeld, was funded in part by a recently awarded, five-year grant totaling $8 million from the NASA Astrobiology Institute. The grant funds the University of Illinois's Institute for Universal Biology, a member of the NASA Astrobiology Institute, which includes many members of the Biocomplexity theme who are studying the origin and evolution of life. Find out more about Illinois's Institute for Universal Biology and the IGB's Biocomplexity theme.
"We have a sample of only one planet known to harbor life," Goldenfeld said. "Thus it is critical that we be creative in extracting the most information from Earthly life as possible, if we are to ever understand the existence, likelihood, and nature of life elsewhere in the Universe. Russell, Nischke, and Branscomb's work lays an intriguing foundation for that endeavor, by cleverly bringing together concepts from thermodynamics, geochemistry and biology to advance a major new hypothesis for life's origins."
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