IN THE BEGINNING. Welby Thomas Cox, Jr.
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on replicating their results and proving that the formaldehyde seen is not coming from another source such as degradation of tubing. From the same conditions, Lane says they have also been able to synthesize low yields of sugars, including 0.06% ribose, from formaldehyde, although not at the formaldehyde concentration produced by the reactor alone.
DIGGING DEEPER
Investigating hydrothermal vents, geochemist Frieder Klein from Woods Hole Oceanographic Institution in the US has discovered a variation on the deep-sea origin story. He has found evidence of life in rock below the sea floor which might have provided the right environment for life to start.
Klein and colleagues were looking at samples from cores drilled from the Iberian continental margin off the coast of Spain and Portugal in 1993. The samples came from rock 760m below the current sea floor, which would have been 65m below the early unsedimented ocean floor. He saw some unusual looking veins in the samples, composed of minerals also found at the Lost City hydrothermal system. ‘That was intriguing to me because this mineral assemblage is only formed when you mix hydrothermal fluids with seawater,’ says Klein. This suggests similar chemistry could be going on below the sea floor.
Within these veins, dated to 120 million years ago, Klein’s team found inclusion of fossilized microbes. He suggests the desiccating properties of the mineral brucite (Mg (OH)2) might explain the preservation of organic molecules from the microbes. These included amino acids, proteins and lipids which were identified by confocal Raman spectroscopy. Klein says he was initially skeptical, but analysis of extracted samples confirmed unique lipid biomarkers for sulfate-reducing bacteria and archaea, which are also found in the Lost City hydrothermal vents system.5 SEM imaging showed carbon inclusions which he says ‘seemed to look like micro-colonies of micro-organisms’
While obviously these samples are much younger, ‘The presence of these microbes is telling us that life is possible in sea floor environments in hydrothermal systems, that were probably present and active throughout most of the early earth,’ Klein observes. ‘The sub-sea floor represents another more protected environment.’
LANDLOCKED
But not everyone agrees that life began in deep sea hydrothermal systems. Armen Mulkidjanian at the University of Osnabruck in Germany says there are several big problems with the idea, one being the relative sodium and potassium ion concentrations found in seawater compared to cells.
Mulkidjanian invokes what he calls the chemistry conservation principle – once established in any environment, organisms will retain and evolve mechanisms to protect their fundamental biochemical architecture. He says therefore it makes no sense for cells that contain 10 times more potassium than sodium to have their origins in seawater, which has 40 times more sodium than potassium. His assumption is that protocells must have evolved in an environment with more potassium than sodium, only developing ion pumps to remove unwanted sodium when their environment changed.
Mulkidjanian thinks life could have sprung from geothermal systems, such as the Siberian Kamchatka geothermal fields in the Russian Far East. ‘We started to look for where we could find conditions with more potassium than sodium and the only things that we found were geothermal systems, particularly where you have vapor coming out of the earth,’ he explains. It is only pools created from vapor vents that have more potassium than sodium; those formed from geothermal liquid vents still have more sodium than potassium. A handful of such system exist today, in Italy, the US and Japan, but Mulkidjanian suggests that on the hotter early earth you would expect many more.
David Deamer of the University of California Santa Cruz in the US has been studying macromolecules and lipid membranes for over 50 years. He comes to the field from a slightly different angle, which some have called ‘membrane first’. But, he says, ‘I’m pretty sure that the best way to understand the origin of life is to realize that it is a system of molecules all of which work together, just as they do in today’s life.’ The location ‘comes down to a plausibility judgement on my part’, he muses.
One of the biggest arguments against a deep-sea origin is the fact that so many macromolecules are found in biology. DNA, RNA, proteins, and lipids are all polymers and form via condensation reactions. ’You need a fluctuating environment which is sometimes wet and sometimes dry – a wet period so that the components mix and interact and then a dry period so that water is removed and these components can form a polymer,’ says Mulkidjanian. ‘There is no way for this kind of a thing to happen in [a deep sea] hydrothermal vent because you cannot have wet–dry cycles there,’ adds Deamer. Wet and dry cycling occurs every day on continental hydrothermal fields. This allows for concentration of reactants as well as polymerization.
The assumption that natural selection is incapable over 4 billion years of coming up with an improvement I think is mad (WTC)
Deamer has been trying to create his own protocells in the lab – 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