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In a collection of glass jars at NASA’s Jet Propulsion Lab, Michael Russell’s team is trying to recreate the moment before life on Earth came into being.

The jars are filled with seawater and stirred with a blend of chemicals designed to match the seas of Earth as they were 4 billion years ago. A syringe at the base of each jar pumps in another concoction – crafted to approximate the alkaline fluid emitted by hydrothermal vents on the sea floor. Where the two fluids come together, minerals precipitate out to form a delicate tower reminiscent of the crystal gardens made with toy chemistry kits.

Russell has spent the past three decades working on a theory that life began on Earth, and perhaps elsewhere in the solar system, in places like this. Now, with these miniature crystal gardens, he is putting that idea to the test.

Ocean or pool?

A competing theory holds that life started in shallow freshwater pools over a hydrothermal field, like those in modern-day Iceland. The pools swelled when rain fell, but when they evaporated, the organic molecules in them became more concentrated, forming films like cell membranes, or joining into longer chains like strands of RNA.

In this account, lifelike structures can easily come together – and lab experiments have backed that up. For Russell, however, easy assembly isn’t enough. The key is to find chemical imbalances that could power a metabolism. And for that, he says, the best place to look is at a particular kind of hydrothermal vent, gently bubbling away on the ancient sea floor

Russell’s theory is attractive because it takes account of how life exploits energy sources. The process involves pumping positively charged hydrogen ions, called protons, across a membrane.

Pumping brings more protons to one side, causing an imbalance. But when they cross back the other way, an enzyme called ATP synthase steals energy from the flow and stores it in molecules of ATP.

Proton power

Living cells thrive by exploiting similar imbalances in nature – of protons, and also electrons, which are exchanged in chemical reactions between oxidants and reductants. “We would say evolution is merely a search engine for similar disequilibria,” says Russell.

So how could life harvest these chemical imbalances before soft, permeable membranes existed to keep the ingredients organised?

According to Russell, the answer starts with water percolating down into the rocky mantle, where it reacts with minerals to produce an upwelling of alkaline, hydrogen-rich effluent – a process called serpentinisation. The fluids react with relatively acidic seawater to produce mineral structures full of tiny pores, where hydrogen and carbon dioxide come together to form simple molecules.

And here’s where the energy comes in. The difference in pH – basically a measure of proton concentration – between the seawater and effluent also gives rise to a proton gradient across the walls of these pores, creating electrochemical potential that molecules inside the chambers could tap.

When Russell first came up with the idea, we did not know of any such vents. In 2000, however, we discovered the Lost City at the bottom of the Atlantic – a system of alkaline vents with mineral towers supporting microbes and other forms of life.

Chemical gardens

Russell has since embellished his theory, adding that a mineral called green rust, which can form in vent chimneys, can exploit the flow of protons at a vent in much the same way as ATP synthase. He argues that the built-in proton gradient would be enough for a primitive organism to feed on.

Later, it could have brought with it other enzyme-like minerals from the chimney walls, allowing it to establish a proton gradient on the go. “Life had to figure out how to make its own vent, eventually,” says Laurie Barge, one of Russell’s collaborators.

More recently, Russell, Barge, and their teams have begun to test their ideas experimentally by growing their own vents in jars. Their chemical gardens at the Jet Propulsion Lab in Pasadena, California, have already confirmed that hydrothermal vents can provide that all-important proton gradient, measured as electrical potential between the inside and outside of a chimney.

“If you grow it around an electrode, you can see the voltage appear as the chimney starts growing,” Barge says. Indeed, hooking several vents together produces enough juice to power a light bulb.

Now they are trying to figure out how organic molecules can assemble and evolve in the pores of chimneys. “It’s kind of like a mineral sponge, which will keep anything in there and concentrate it,” Barge says. In 2015, Russell and his collaborators showed that a vent surrounded by an approximation of Earth’s early ocean could link RNA molecules into a two-part strand, and Barge says they will soon release more results.

Other worldly implications

A recent analysis suggests that the last common ancestor of all life on Earth had the genes to feed on a proton gradient but not the genes required to make its own, lending support to Russell’s theory.

Even so, its supporters still have to provide firm evidence that larger molecules and membranes can be synthesised at alkaline hydrothermal vents, says David Deamer of the University of California, Santa Cruz, who is a leading proponent of the idea that life evolved in evaporating pools instead. “Until they’re able to show that, they really don’t have evidence to substantiate the claim that life can begin under these conditions,” says Deamer.

If Russell is right, then our view of life would be transformed: rather than some lucky accident, life starts to looks like an unavoidable consequence of the thermodynamic and chemical conditions in the oceans of early Earth. And the implications of these experiments go beyond the origins of life on our own planet.

We now know that distant moons like Europa, which orbits Jupiter, and Enceladus, which circles Saturn, house vast subsurface oceans that some astrobiologists suspect host hydrothermal vents akin to Lost City. NASA will send a probe to Europa to find out in the next decade, and plans are afoot for a similar mission to Enceladus – where there are hints that the sea floor could be bubbling with hydrogen-rich fluids.  

If so, the odds of finding life out there would be slashed. At the very least, Russell is convinced that wherever these alkaline vents exist, mineral chimneys will form, and the molecular precursors of life could organise and enjoy ready access to the energy they need to grow and replicate. “It’s extremely likely – almost 100 per cent, we would say – that the same kind of processes happen on any wet rocky world,” he says.

Quelle: NewScientist