All life as we know it uses the exact same energy-carrying molecule as a kind of 'universal cellular fuel'. Now, ancient chemistry may explain how that all-important molecule ended up being ATP (adenosine triphosphate) a new study reports.

ATP is an organic molecule, charged up by photosynthesis or by cellular respiration (the way organisms break down food) and used in every single cell. Every day, we recycle our own body weight in ATP.

In both the above systems, a phosphate molecule is added to ADP (adenosine diphosphate) through a reaction called phosphorylation – resulting in ATP.

Reactions that release that same phosphate (in another process called hydrolysis) provide chemical energy that our cells use for countless processes, from brain signaling to movement and reproduction.

How ATP ascended to metabolic dominance, in place of many possible equivalents, has been a long-standing mystery in biology and the focus of the research.

"Our results suggest… that the emergence of ATP as the universal energy currency of the cell was not the result of a 'frozen accident'," but arose from unique interactions of phosphorylation molecules, explains evolutionary biochemist Nick Lane from University College London (UCL).

The fact that ATP is used by all living things suggests it has been around since life's very beginning and even before, during the prebiotic conditions that preceded all us animate matter.

But researchers are puzzled as to how this could be the case when ATP has such a complicated structure that involves six different phosphorylation reactions and a whole lot of energy to create it from scratch.

"There is nothing particularly special about the 'high-energy' [phosphorus] bonds in ATP," says biochemist Silvana Pinna who was with UCL at the time, and colleagues in their paper.

But as ATP also helps build our cells' genetic information, it may have been roped in for energy use through this other pathway, they note.

Pinna and team suspect some other molecules must have been involved initially in the complicated phosphorylation process. So they took a close look at another phosphorylating molecule, AcP, that's still used by bacteria and archaea in their metabolism of chemicals, including phosphate and thioester – a chemical thought to have been abundant at the beginning of life.

In the presence of iron ions (Fe3+), AcP can phosphorylate ADP to ATP in water. Upon testing the ability of other ions and minerals to catalyze ATP formation in water, the researchers could not replicate this with other substitute metals or phosphorylating molecules.

"It was very surprising to discover the reaction is so selective – in the metal ion, phosphate donor, and substrate – with molecules that life still uses," says Pinna.

"The fact that this happens best in water under mild, life-compatible conditions is really quite significant for the origin of life."

This suggests that with AcP, these energy-storing reactions could take place in prebiotic conditions, before biological life was there to hoard and spur the now self-perpetuating cycle of ATP production.

Furthermore, the experiments suggest that the creation of prebiotic ATP was most likely to take place in freshwater, where photochemical reactions and volcanic eruptions, for instance, could provide the right mix of ingredients, the team explains.

While this doesn't completely preclude its occurrence in the sea, it does hint that the birth of life may have required a strong link to land, they note.

"Our results suggest that ATP became established as the universal energy currency in a prebiotic, monomeric world, on the basis of its unusual chemistry in water," Pinna and colleagues write.

What's more, pH gradients in hydrothermal systems could have created an uneven ratio of ATP to ADP, enabling ATP to drive work even in the prebiotic world of small molecules.

"Over time, with the emergence of suitable catalysts, ATP could eventually displace AcP as a ubiquitous phosphate donor, and promote the polymerization of amino acids and nucleotides to form RNA, DNA, and proteins," explains Lane.

This research was published in PLOS Biology.