Читать книгу IN THE BEGINNING - Welby Thomas Cox Jr. - Страница 11

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.