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A long-standing mystery about what determined the 鈥渉andedness鈥� of molecules inside living cells may finally have a solution, thanks to a happy accident in the lab.

鈥淲e were really surprised when we got this selection that exactly matches up to what life uses,鈥� says at the University of Oxford.

Just as our left and right hands are mirror images of each other that can鈥檛 be superimposed, many molecules also have left or right-handed forms, a phenomenon known as chirality. Chemical reactions normally produce an equal mix of the two forms, yet in all the living organisms on Earth today, there are distinctive patterns of chirality.

The deoxyribose and ribose sugars in DNA and RNA are always the right-handed forms, for instance, while the amino acids that are strung together to make proteins are always the left-handed forms.

The question is, why? Over the decades, scientists have proposed numerous explanations, many involving exotic physics, but no single idea is widely accepted.

Richards didn鈥檛 set out to study this mystery. He and his team were just creating 鈥渂ags鈥� made of cell membranes to do experiments in when they聽accidently discovered that the make-up of these membranes could offer an explanation of life鈥檚 handedness.

It has long been known that small molecules such as sugars and amino acids can pass through cell membranes, but what the team discovered is that there are big differences in how easily these can move through different kinds of membranes. All cell membranes are made of fatty molecules called phospholipids, but those found in bacteria are a bit different to those in archaea, the other main kind of simple cell.

The researchers found that archaeal membranes and a hybrid membrane that consists of a mix of bacterial and archaeal phospholipids are much more permeable to the right-handed form of deoxyribose and ribose than pure bacterial membranes are. The hybrid membranes are also more permeable to the left-handed forms of some amino acids.

That means these membranes are acting as a kind of sieve that separates out specific chiral forms of molecules, says Richards. If such membranes existed early in the evolution of life, they might explain why living organisms came to use these particular forms.

鈥淲e were excited about this match-up between the permeability characteristics and what core central metabolism evolved to use,鈥� he says. 鈥淚t鈥檚 a really neat explanation of why central metabolism evolved to be the way it is.鈥�

鈥淭his paper reveals a possible prebiotic route to left-handed amino acids and right-handed sugars,鈥� says at the Tokyo University of Science in Japan.

One issue is that phospholipids in cell membranes are also chiral, so one explanation for the findings is that left-handed membranes, say, only allow through left-handed molecules. But the team saw selection for right-handed deoxyribose and ribose whatever the chirality of the membrane, says Richards, so that can鈥檛 be the full explanation.聽

鈥淚 don鈥檛 doubt their data 鈥� this is nice work,鈥� says at University College London. But he is unsure how much it tells us about the origins of chirality.

Lane favours the idea that early life made chemicals such as amino acids and sugars, rather than acquiring them from external sources. If that was the case, the membrane-filtering effect might just be a coincidence, he says. Some other researchers, however, do think early life acquired many chemicals from the environment.