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      A laboratory-created 'chemical garden' made of a combination of black iron sulfide and orange iron hydroxide/oxide is shown in this photo. Chemical gardens are a nickname for chimney-like structures that form at bubbling vents on the seafloor. Credi

      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 1977, the first deep sea hydrothermal vent was discovered in the East Pacific Rise mid-oceanic ridge. Named ‘black smokers’, the vents emit geothermally heated water up to 400°C, with high levels of sulfides that precipitate on contact with the cold ocean to form the black smoke. This was followed in 2000 by the discovery of a new type of alkaline deep-sea hydrothermal vent found a little off axis from mid-ocean ridges. The first field, known as the Lost City, was discovered on the sea floor Atlantis Massif mountain in the mid-Atlantic.

      The vents are formed by a process known as serpentinization. Seabed rock, in particular olivine (magnesium iron silicate) reacts with water and produces large volumes of hydrogen. In the Lost City, when the warm alkaline fluids (45–90°C and pH 9–11) are mixed with seawater, they create white calcium carbonate chimneys 30–60m tall.

      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, 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 specializes in self-organizing 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.

      ‘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 favors 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 synthesizing 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

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