From the tiniest bacteria to the complex human body, all living beings require an energy-transporting molecule called ATP to survive. Often likened to a "rechargeable battery," ATP stores chemical energy in a form that can be used by organic matter.
"You need enzymes to make ATP, and you need ATP to make enzymes," said researcher Terence Kee of the University of Leeds in England. "The question is: Where did energy come from before either of these two things existed? We think that the answer may lie in simple molecules, such as pyrophosphate, which is chemically very similar to ATP, but has the potential to transfer energy without enzymes."
Obscure but important Prior theories for how life emerged from mere chemistry have considered that a similar but separate compound known as pyrophosphate was the predecessor to the more complex yet more efficient ATP.
Phosphate has 4 oxygen atoms bound to a central phosphorus atom, and is present in all living cells. When two phosphates combine and lose a water molecule, they form pyrophosphate.
Pyrophosphite, on the other hand, is rarely encountered, chemist Robert Shapiro at New York University told Livescience. "Even in my Google search for it, I got the query: 'Don't you mean pyrophosphate?'"
The presence of "one or two thorny little problems" with its rival molecule [pyrophosphate] had left some unanswered questions, Kee said in a telephone interview.
The two main problems were that pyrophosphate didn't seem to be available in significant amounts in the geological mineral record, and it doesn't react well without catalysts (which weren't around then), according to Kee.
On the other hand, Kee's team has found that pyrophosphite would be "relatively straightforward to prepare from minerals that are known to exist in iron meteorites." The routes to the production of this molecule are simpler than those proposed for pyrophosphate, Kee said.
Though similarly produced through dehydration, and similar in composition except that it has some oxygen atoms replaced by hydrogen, pyrophosphite is rare. Only three pyrophosphite minerals exist, compared with "many phosphate minerals," Kee said.
The chemical's obscurity on Earth is not a sign of its irrelevance. It's highly unstable in today's oxygen-rich environment (meaning it breaks down into other molecules rapidly) but is a superior catalyst (jump-starter) for certain chemical reactions, Kee said, citing as-yet-unpublished evidence.
Lateral thinking Kee called the altered theory "more a lateral thought process" than a "new concept."
"It is as little strange that pyrophosphite and its ability to act as a phosphorus-transfer agent have been known for some time but it has not been proposed previously as being of any pre-biotic significance," he said. "I suspect because noone had considered the need for it or that it may have been accessible pre-biotically."
Interestingly, machines that manufacture artificial DNA for experiments regularly use pyrophophite in their assembly process, Shapiro said.
The researchers detail their theory on pyrophosphate as life's first energy source in a recent issue of the journal Chemical Communications.