Mirror-Image Life

You’re twisted. Sorry, but we all are. The molecules most central to life twist one way or the other. Your most famous molecule, DNA, a spiraling helix like the thread of a screw, is right-handed. The molecules encoded by your DNA, proteins, are left-handed. Even humble sugars like glucose have a twist to their shape.

Why does this handedness, called chirality after the Greek word for hand, feature in the molecules (clusters of atoms) from which all life on Earth is constructed? How and when was the left-or-right chiral twist of life’s components decided? No one knows—even though the chirality of life has been recognized for more than a century and a half.

But we do know that living organisms are exquisitely sensitive to this handedness. Feed bacteria with left-handed amino acids, and they’ll incorporate them into their proteins. Feed them right-handed amino acids, and they are likely to ignore them, even though these are the same molecules but inverted as though in a mirror.

There might be a dark biosphere—a whole ecosystem of mirror-image life forms.

Maybe we just need to accept this is how life is. It exists on one side of the looking glass. But some researchers think there could be equivalent lifeforms on the other side too—if only we could make them. They are already making mirror-image versions of proteins and nucleic acids like DNA. These molecules could be valuable drugs—all the more effective because their looking-glass structure should allow them to operate out of sight of the body’s usual defense mechanisms for breaking apart foreign molecules.

But biochemist Ting Zhu of Westlake University in Hangzhou, China, is determined to go much further than that. He’s made it his mission to create mirror-image versions of the key molecular ingredients of life. In principle, it might be possible to assemble those components into synthetic cell-like entities that can replicate and metabolize: a kind of primitive form of life, but inverted relative to every known organism, and therefore the first truly non-natural life form.

“I believe there are many alternative possibilities for life,” says Zhu. “But among all