IN THE BEGINNING. Welby Thomas Cox, Jr.

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СКАЧАТЬ – by mixing lipids and RNA components adenosine monophosphate and uridine monophosphate. When dried, the lipids self-assemble into membrane-like structures, and if nucleotides are trapped between lipid layers, they will undergo esterification to produce RNA-like polymers. Over multiple wet–dry cycles the yield increases to 50%.6

      Deamer has confirmed the presence of these polymers inside the ‘protocells’ by direct RNA sequencing techniques. ‘We really do have single-stranded molecules that are in the size range of biological RNA,’ but Deamer cautions that it is not RNA as it is in a biological organism. He created a mixture of RNA, some with phosphate groups bonded as they are in nature, but some bonded ‘unnaturally’, which he concludes then ‘must have been subject to selection and evolution in these little protocells’.

      But the deep-sea hydrothermal vent camp is not ready to throw in the towel just yet. Barge says the vent environment could allow for concentration of reactants and condensation reactions. ‘You have gels all over the sea floor, you have minerals that absorb things and in the [chimney micropore] membrane itself there are gels, so you can have dehydrating reaction conditions even though the whole system is aqueous.’

      Lane also rebuffs the idea that potassium or sodium ion levels might fix future metabolic processes. ‘The assumption that natural selection is incapable over 4 billion years of coming up with an improvement I think is mad,’ explains Lane. ‘In my view, selection drives intracellular ion balance.’ He thinks life would have been quite capable of evolving in a sodium-rich environment and over time developing the ion removal pumps that create the current potassium-rich cells.

       SEEING THE LIGHT

      One other point of contention is the presence or absence of ultraviolet (UV) light. This could be a strong influence in a terrestrial origin scenario with no protective ozone layer on the early earth, but completely absent in the deep-sea theory. The relative UV stability of RNA nucleotides suggests selection occurred in UV light – on the earth’s surface not in the sea.

      This would also support the groundbreaking 2009 synthesis of RNA proposed7 by John Sutherland of the UK’s Medical Research Council Laboratory of Molecular Biology in Cambridge and his 2015 suggested synthesis of nucleic acid precursors starting with just hydrogen cyanide (HCN), hydrogen sulfide (H2S) and UV light.8 Illumination with UV light over 10 days enriched the yields of the biological nucleotides, adding weight to their selection being advantaged in UV light. Mulkidjanian has also suggested zinc sulfide precipitates could have acted as catalysts for carbon dioxide reduction using UV light – an early form of photosynthesis which he calls the ‘zinc world’ scenario

      But according to Lane, ‘There is a big problem with life evolving with UV light, which is to say no life today uses UV as an energy source – it tends to destroy molecules rather than promote biochemistry.’ He also argues that the synthetic chemistry proposed in such terrestrial scheme just doesn’t look like life as we know it. ‘It starts with cyanides or with zinc sulfide photosynthesis and you end up with a kind of Frankenstein chemistry,’ Lane says. ‘The chemistry might work but to join that up with life as we know it, I would say is borderline impossible.

       DISCIPLINARY DIVIDE

      Looking closer, the divide between those who support a terrestrial and those supporting an oceanic origin is split between disciplines. Synthetic chemists generally favor a continental origin and geologists and biologist mostly deep-sea hydrothermal vents. Chemists argue it’s impossible to do the chemistry in hydrothermal vents, while biologists argue that the terrestrial chemistry proposed just isn’t like anything seen in biochemistry and doesn’t narrow the gap between geochemistry and biochemistry.

      So, is there a way to unite the disciplines? ‘At the moment there is not much common ground between these ideas,’ Lane says. Deamer agrees. ‘At this point, all we can say is that everyone has the right to do a plausibility judgement on the basis of their ideas but then they also must do experimental and observational tests.’

       The smaller problems will be solvable – that’s what gets me out of bed in the morning. (WTC)

      What is needed is that killer piece of evidence or experiment that could join the dots together and explain how and where life began from a prebiotic world. ‘It would really be a big breakthrough if we can find a ribozyme among all of these trillions of random polymers that we are making,’ suggests Deamer. Ribozymes are RNA catalysts that are part of the cell’s protein-synthesis machinery but are candidates for the first self-replicating molecules.

      Further evidence to support the origins of life in deep sea hydrothermal vents centers on showing a plausible set of metabolic steps leading to complex molecules. At JPL, they are looking at how amino acid behave in their chemical gardens, according to Barge. ‘We are working on making an amino acid, and then seeing whether [amino acids] get stuck in the chimneys and whether you can concentrate them and maybe make some peptides.’

      ‘There are problems and difficulties,’ Lane acknowledges. ‘Can we really make carbon dioxide react with hydrogen to make more complex molecules like amino acids and nucleotides? I’m fairly confident we can do that, but I am aware we have not demonstrated that yet.’ Other difficult questions include whether lipid membranes can be stabilized in seawater with its high calcium and magnesium ion concentrations. But says Lane the big problem of the thermodynamic driving force is solved by hydrothermal vents. ‘Which gives me confidence that the smaller problems will be solvable in that context too, even if they look difficult now – that’s what gets me out of bed in the morning.’

      Of course, there is one other possibility – that life did not start on earth at all. Panspermia – the theory that life was seeded from space, seems eccentric, but not everybody counts it out. ‘An argument can be made that life actually began on Mars,’ according to Deamer, because it was first to cool down to a temperature which could support life.

      Whether this is the case or not, life elsewhere is certainly feasible. Jupiter’s moon Europa and Saturn’s moon Enceladus are candidates because they both have oceans beneath icy shells. In the next five years, Nasa is planning to send a space probe to both these moons to look for signs of life. Understanding our own origin story could help us work out where to look.

      Chapter 5

      New Study Outlines 'Water World' Theory of Life's Origin

      Life took root more than four billion years ago on our nascent Earth, a wetter and harsher place than now, bathed in sizzling ultraviolet rays. What started out as simple cells ultimately transformed into slime molds, frogs, elephants, humans and the rest of our planet's living kingdoms. How did it all begin?

      A new study from researchers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the Icy Worlds team at NASA's Astrobiology Institute, based at NASA's Ames Research Center in Moffett Field, Calif., describes how electrical energy naturally produced at the sea floor might have given rise to life. While the scientists had already proposed this hypothesis -- called "submarine alkaline hydrothermal emergence of life" -- the new report assembles decades of field, laboratory, and theoretical research into a grand, unified picture.

      According to the findings, which also can be thought of as the "water world" theory, life may have begun inside warm, gentle springs on the sea floor, at a time long ago when Earth's oceans churned across the entire planet. This idea of hydrothermal vents as possible places for life's origins was first proposed in 1980 by other researchers, who found them on the sea floor near Cabo San Lucas, Mexico. Called "black smokers," those vents bubble with scalding СКАЧАТЬ