Some event or series of events early in the history of life must have “broken the mirror,” as biochemists put it, throwing life into molecular asymmetry. Scientists have debated why life became homochiral, and whether it needed to happen or if it was purely a fluke. Were chiral preferences impressed on early life by biased samples of molecules arriving from space, or did they somehow evolve out of mixtures that started out as equal parts right- and left-handed?

“Scientists have been mystified by this observation,” said Soumitra Athavale, an assistant professor of organic chemistry at the University of California, Los Angeles. “They’ve come up with all sorts of proposals over the years, but it’s difficult to come up with proposals which are actually relevant geologically.” Moreover, while many theories could explain why one type of molecule might have become homochiral, none of them explained why whole networks of biomolecules did.

Recently, a group at Harvard University published a series of papers that present an intriguing solution for how life’s homochirality emerged. They suggest that magnetic surfaces on minerals in bodies of water on the primordial Earth, charged by the planet’s magnetic field, could have served as “chiral agents” that attracted some forms of molecules more than others, kicking off a process that amplified the chirality of biological molecules, from RNA precursors all the way to proteins and beyond. Their proposed mechanism would explain how a bias in the makeup of certain molecules could have cascaded outward to create a vast network of chiral chemistry supporting life.

It’s not the only plausible hypothesis, but “it’s one of the coolest because it ties geophysics to geochemistry, to prebiotic chemistry, [and] ultimately to biochemistry,” said Gerald Joyce, a biochemist and president of the Salk Institute who was not involved in the study. He is also impressed that the hypothesis is backed by “actual experiments” and that “they’re doing this under realistic conditions.”

The CISS Effect

The roots of the new theory about homochirality reach back almost a quarter century to when Ron Naaman, a professor of chemical physics at the Weizmann Institute of Science in Israel, and his team discovered a critical effect of chiral molecules. Their work focused on the fact that electrons have two key properties: They carry a negative charge, and they have “spin,” a quantum property analogous to intrinsic clockwise or counterclockwise rotation. When molecules interact with other molecules or surfaces, their electrons can redistribute themselves, polarizing the molecules by creating a negative charge at their destination and a positive charge at their starting point.

Naaman and his team discovered that chiral molecules filter electrons based on the direction of their spin. Electrons with one spin orientation will move more efficiently across a chiral molecule in one direction than the other. Electrons with the opposite spin move more freely the other way.

To understand why, imagine throwing a Frisbee that glances off the wall of a hallway. If the Frisbee hits the right-hand wall, it will bounce forward only if it’s rotating clockwise; otherwise, it will bounce backward. The opposite will happen if you hit the Frisbee off the left-hand wall. Similarly, chiral molecules “scatter the electrons according to their direction of rotation,” Naaman said. He and his team named this phenomenon the chiral-induced spin selectivity (CISS) effect.

Because of that scattering, electrons with a given spin end up aggregating at one pole of a chiral molecule (and the right-handed and left-handed versions of the molecule gather opposite spins at their respective poles). But that redistribution of spins affects how the chiral molecules interact with magnetic surfaces because electrons spinning in opposite directions attract one another, and those spinning in the same direction repel one another.

Consequently, when a chiral molecule approaches a magnetic surface, it will be drawn closer if the molecule and the surface have opposite spin biases. If their spins match, they will repel each other. (Because other chemical interactions are also going on, the molecule can’t simply flip to realign itself.) So a magnetic surface can act as a chiral agent, preferentially interacting with only one enantiomer of a compound.

In 2011, in collaboration with a team at the University of Münster in Germany, Naaman and his team measured the spin of electrons as they moved through double-stranded DNA, confirming that the CISS effect is both real and strong.

That’s when research into the effect and its possible applications “started to boom,” Naaman said. He and his team, for example, developed several ways to use the CISS effect to remove impurities from biomedicines, or to exclude the wrong enantiomers from drugs to prevent major side effects. They’ve also explored how the CISS effect might help to explain the mechanisms of anesthesia.

But they only began working seriously on the idea that the CISS effect plays a part in the rise of biological homochirality after they were invited to collaborate on a hypothesis by a team at Harvard led by the astronomer Dimitar Sasselov and his graduate student S. Furkan Ozturk.

A Physics Perspective

Ozturk, the young lead author on the recent papers, came across the homochirality problem in 2020 when he was a physics graduate student at Harvard. Unhappy with his research on quantum simulations using ultracold atoms, he flipped through a science magazine detailing 125 of the biggest mysteries in the world and learned about homochirality.

“It looked really like a physics question because it’s about symmetries,” he said. After reaching out to Sasselov, who is the director of Harvard’s Origins of Life Initiative and who was already interested in the question of homochirality, Ozturk switched over to become a student in his lab.