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A series of chemistry experiments hint at how an energy-storing molecule found in all living cells could have first arisen on Earth.

All living things use adenosine triphosphate (ATP) as a source of energy for biological processes, such as breaking down food. Because it is so universal, it probably formed in the earliest stages of the origins of life. It is also a precursor to RNA and DNA so had it formed that early, it would have also been a meaningful step towards life as we know it today.

at the University of Strasbourg in France and her colleagues set out to determine how ATP could have first formed when the earliest life was developing.

The team started from the fact that one known way to make ATP is to mix charged iron atoms, water and two molecules called adenosine diphosphate (ADP) and acetyl phosphate (AcP). To determine whether this mixture is somehow special, the researchers ran a series of experiments where they swapped iron for one of a dozen other metals like magnesium or cobalt, or used molecules that were similar to ADP but had a different number of nitrogen, oxygen or hydrogen atoms.

Only the mixture of ADP, AcP and iron produced ATP every time. Using computer simulations, the researchers found that the three molecules have an arrangement of atoms uniquely suited for producing ATP. They say this all adds up to suggest that it was this combination of ADP, AcP and iron that probably contributed to the formation of life.

In experiments, the team also varied the temperature and acidity of the mixture. They found that mildly acidic conditions and temperatures between 20 and 50掳C (68-122掳F) were best for ATP production from the three molecules. This means that it could have happened in multiple places where life may have started, like freshwater ponds or geothermal vents in prehistoric oceans, says at University College London, part of the team.

at the Autonomous University of Madrid in Spain says that cells today make ATP through a more complicated process. This is a step backwards in time, towards more simple systems, he says.

However, pinning down plausible chemical reactions that make the molecules we see today may only be part of investigating ATP鈥檚 origins, says at the Foundation for Applied Molecular Evolution in Florida. He says that if there isn鈥檛 also a reason for evolution to favour the process then it may not be the one used by early life on Earth.