The environment of abiogenesis. There two alternative environments where abiogenesis likely took place: (1) Alkaline thermal vents in the mid ocean ridges. (2) Thermal terrestrial chemical vents and clay volcanic thermal vents. This is a very good reference that describes the science behind both, and the controversy. Both possible environments provide the necessary energy sources for abiogenesis to take place.
From:
Hydrothermal vents and the origins of life
"The question ‘How did life begin?’ is closely linked to the question ‘Where did life begin?’ Most experts agree over ‘when’: 3.8–4 billion years ago. But there is still no consensus as to the environment that could have fostered this event. Since their discovery, deep sea hydrothermal vents have been suggested as the birthplace of life, particularly alkaline vents, like those found at ‘the Lost City’ field in the mid-Atlantic. But not everyone is convinced that life started in the sea – many say the chemistry just won’t work and are looking for a land-based birthplace. With several hypotheses in play, the race is on to replicate the conditions that allowed life to emerge.
. . .
In 1993, before alkaline vents were actually discovered, geochemist Michael Russell from Nasa’s Jet Propulsion Laboratory (JPL) in California, US, suggested a mechanism by which life could have started at such vents.1 His ideas, updated in 2003,2 suggest life came from harnessing the energy gradients that exist when alkaline vent water mixes with more acidic seawater (the early oceans were thought to contain more carbon dioxide than now).
This mirrors the way that cells harness energy. Cells maintain a proton gradient by pumping protons across a membrane to create a charge differential from inside to outside. Known as the proton-motive force, this can be equated to a difference of about 3 pH units. It’s effectively a mechanism to store potential energy and this can then be harnessed when protons are allowed to pass through the membrane to phosphorylate adenosine diphosphate (ADP), making ATP.
Russell’s theory suggests that pores in the hydrothermal vent chimneys provided templates for cells, with the same 3 pH unit difference across the thin mineral walls of the interconnected vent micropores that separate the vent and sea water. This energy, along with catalytic iron nickel sulfide minerals, allowed the reduction of carbon dioxide and production of organic molecules, then self-replicating molecules, and eventually true cells with their own membranes.
Chemical gardens
Chemist Laura Barge, also a research scientist at JPL, is testing this theory using chemical gardens – an experiment you might have carried out at school. Looking at chemical gardens ‘you think its life, but it’s definitely not’, says Barge, who specialises in self-organising chemical systems. The classical chemical garden is formed by adding metal salts to a reactive sodium silicate solution. The metal and silicate anions precipitate to form a gelatinous colloidal semi-permeable membrane enclosing the metal salt. This sets up a concentration gradient which provides the impetus for the growth of hollow plant-like columns.
Chemical gardens in the lab mimic the conditions of hydrothermal vents and are a useful model for studying how life could have started
‘We started simulating what you might get with a vent fluid and the ocean and we can grow tiny chimneys – they are essentially like chemical gardens,’ explains Barge. To mimic the early ocean she has injected alkaline solutions into iron-rich acidic solutions, making iron hydroxide and iron sulfide chimneys. From these experiments her team have illustrated that they can generate electricity: just under a volt from four gardens, but enough to power an LED,3 showing that the sort of proton gradients that provide energy in deep sea vents can be replicated.
Nick Lane, a biochemist at University College London in the UK, has also been trying to recreate prebiotic geo-electrochemical systems with his origins of life reactor. He favours Russell’s theory, although is not happy with the ‘metabolism first’ label it is often given, in opposition to the ‘information first’ theory which supposes that synthesising replicating RNA molecules was the first step to life. ‘They are portrayed as being opposing but I think that’s silly,’ says Lane. ‘As I see it, we are trying to work out how you get to a world where you have selection and can give rise to something like nucleotides.’
Lane has been persuaded by how closely the geochemistry and biochemistry align. For example, minerals such as greigite (Fe3S4) are found inside vents and they show some relationships to the iron–sulfur clusters found in microbial enzymes. They could have acted as primitive enzymes for the reduction of carbon dioxide with hydrogen and the formation of organic molecules. ‘There are differences as well, the barriers [between micropores in vent chimneys] are thicker [than cell membranes] and so on, but the analogy is very precise and so the question becomes “Is it feasible for these natural proton gradients to break down the barrier to the reaction between hydrogen and carbon dioxide?”’
Lane’s simple bench-top, open-flow origins of life reactor4 is simulating hydrothermal vent conditions. On one side of a semiconducting iron–nickel–sulfur catalytic barrier, an alkaline fluid is pumped through to simulate vent fluids and on the other side, an acidic solution that simulates sea water. As well as flow rates, the temperatures can be varied on both sides. Across the membrane, ‘The first step is trying to get carbon dioxide to react with hydrogen to make organics, and we seem to be successful in producing formaldehyde in that way,’ says Lane.
So far yields have been very low but Lane considers they have ‘proof of principle’. They are working 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 synthesise 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.
Next is the description of possible terrestrial origins:
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 vapour coming out of the earth,’ he explains. It is only pools created from vapour 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 realise 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 polymerisation."
A read of the full article is worth while for those interested in this subject.
This much only can be said from POV of empicism. Science, I believe, is vaster.
