Meteorites could have been source of life's batteries

THE first life on Earth might have acquired its "batteries" from an alien source. Rocks that crashed to Earth could have supplied early organisms with essential molecules that allowed them to store energy, ensuring that they could go on to give rise to all terrestrial life we see today.

Every organism has built-in batteries in the form of molecules that store energy from food until needed. These storage molecules are all based on phosphorus. But early life needed meteorites to supply it in a useful form because terrestrial phosphorus would have been locked away in minerals.

Today the most common energy store is adenosine triphosphate (ATP), used by millions of complex organisms. But how did these battery molecules first evolve? It takes enzymes to make ATP and to release its energy, but the first organisms wouldn't have been sophisticated enough to make them. So researchers think a simpler storage molecule must have preceded ATP.

According to Terry Kee of the University of Leeds in the UK, the first energy store could have been a molecule called pyrophosphite that contains phosphorus, oxygen and hydrogen. It has many of the same chemical properties as ATP but is more reactive so no enzymes would be needed.

To see whether pyrophosphite could have formed when meteorites landed on early Earth, Kee's team studied a Siberian meteorite that contained a lot of phosphorus. They incubated fragments of the meteorite in acidic water collected from volcanic ponds in Iceland, thought to be chemically similar to the water on primordial Earth. After four days in the water, the meteorite samples had released large quantities of phosphite. When this was dried out, it transformed into pyrophosphite (Geochimica et Cosmochimica Acta, doi.org/kzc). "We have shown that it's very easy to form," Kee says.

His idea is bolstered by the discovery in 2009 that geothermal pools in California contain lots of phosphite. These pools resemble the primordial environment, suggesting that early Earth was also rich in the material.

However, the notion of pyrophosphite as the first energy store has divided those researching the origin of life. The biggest problem is that all modern organisms use phosphates to store energy, not phosphites, says William Martin of Heinrich Heine University in D?sseldorf, Germany. Animals and plants use ATP and most microbes opt for pyrophosphate. "My hunch is that it was always that way," Martin says.

For this reason, many researchers think that pyrophosphate is the most promising ancient energy store. But there are problems. It would have to form from phosphates, but they are very reactive so any that were present on Earth's surface wouldn't have hung around for long. Pyrophosphate also reacts with water rather than dissolving in it as pyrophosphite does. "The community has favoured pyrophosphate because it is the simplest thing," says Steven Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida. There is "no other reason" to believe it was the first energy store.

Kee thinks pyrophosphite could have been a precursor to pyrophosphate, used until life had the molecular equipment it needed to work with phosphates. His latest work backs this up: in as-yet-unpublished experiments, his team is finding that pyrophosphite can be easily converted into pyrophosphate. This suggests that life could indeed have switched quickly from one to the other.

This article appeared in print under the headline "Did meteorites supply life with a vital spark?"

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