Physics and biology between the sheets

My spam filter is set at the most Cerberean level, so I was surprised to receive an e-mail last week bearing the title “The secret of life may be between the sheets.”

The saucy subject line turned out to be the work of the National Science Foundation. The foundation’s eager press office was touting a new paper by one of its grantees, Helen Hansma of the University of California, Santa Barbara. Her paper, which will appear in the next issue of the Journal of Theoretical Biology, proposes that life began in tiny cavities that form between the sheets of mica, a shiny, flaky mineral. Mica is chemically benevolent toward the protomolecules of life, but it’s the confinement afforded by the cell-like cavities that could have boosted the chance that biomolecules, once formed, later evolved.

Hansma’s paper caught my eye because it represents a trend I’ve been following for a few years: the growing role of physics in explaining the origin of life on Earth. Our home planet formed 4.5 billion years ago; the first living things showed up about a billion years later under conditions we can no longer observe. Because life began just once, either those conditions were rare or the probability of forming life from them was low—or both.

Forming life’s most basic building blocks is not a problem. In a famous experiment conducted at the University of Chicago in 1952, Stanley Miller and Harold Urey enclosed an aqueous solution of ammonia, hydrogen, and methane in a flask and zapped it with lightning-like discharges. After a week, the mixture contained amino acids, the building blocks of proteins. Last year, Matthew Powner, Béatrice Gerland, and John Sutherland at the University of Manchester found that ribonucleotides, the building blocks of RNA and DNA, could form under what they termed “plausible prebiotic conditions.”

But there’s a long way from amino acids and nucleotides to life. For one thing, amino acids are all left-handed isomers and the sugars in nucleotides are all right-handed isomers. Something broke the left–right symmetry, although it appears that mere stirring could do the trick . And while it’s plausible that large organic molecules could form spontaneously from chiral building blocks, what’s needed is a way to make sure that the ones that can catalyze and replicate have the chance to win out over the ones that can’t. That’s where physics comes in.

Hansma’s proposal is one of several in which physical processes help chemistry become biochemistry. The first such proposal I encountered came in 2002 from Dieter Braun, then a postdoc in Albert Libchaber’s lab at Rockefeller University in New York City. Quite by accident, Braun found that convection in tight spaces can concentrate large molecules by factors of 1000 . Now at the University of Munich, Braun continues to work on and test his idea. In May, he and Christoph Mast showed that convective flow within a glass capillary could sustain the unzipping and replication of DNA .

When I tell nonphysicists at parties that I work for a physics magazine, they sometimes ask, “What’s new in physics?” Having gotten good mileage from my previous answer, topological insulators, I’ll now tell civilians that life on Earth could have sprung from physical activity between the sheets.