Mica sheets: Bed for early life?
Lightning struck Earth’s gaseous atmosphere some 4 billion years ago, sparking chemical reactions that led to the formation of organic molecules in the young planet’s shallow oceans. In this primordial soup, these molecules began bumping into one another and bonding to form compounds that became the first living things. At least that’s what the scientific community thought 50 years ago. But biochemists quickly found flaws in this theory: How could a handful of tiny organic molecules floating in a vast sea find each other and how could the compounds that formed stay hitched? They’ve been searching for answers ever since.
A new hypothesis posits that the molecular trysts happened not in the open ocean, but in the sheltered gaps found between sheets of mica on the seafloor. There, life’s precursors would have been protected from the elements, says Helen Hansma, a biophysicist at the University of California at Santa Barbara who is currently serving as a rotator at the National Science Foundation.
The idea that life sprang from between mica layers came to Hansma, a newcomer to the study of life’s origins, last spring when she examined mica she picked up while hiking around an abandoned mine under a microscope she keeps in her apartment. “I saw some green algae and some brown crud at the edges of the mica sheets,” she says. The observation got her thinking.
Hansma didn’t know much about origins of life research, but she suspected that the properties that make mica a good place for algae to grow might be the very same properties that the first living organisms needed to form. The environment found between sheets of mica — rich in potassium and negatively charged — is similar to that found inside a cell. And the contraction and expansion of the mica with fluctuating temperatures and ocean currents would “stretch the molecules out and squish them together, giving them energy and maybe making and breaking bonds,” she says. Hansma began conducting literature searches and reading books on the topic. Gradually her hypothesis took shape.
Hansma, who plans to retire in February, presented her ideas in early December at the annual meeting of the American Society for Cell Biology, but she has yet to conduct any experiments in the lab.
Hansma is not the first to propose that minerals such as mica might have inadvertently played matchmaker for organic molecules. Lynda Williams and colleagues at Arizona State University in Tempe published a paper in Geology two years ago in which they presented evidence showing that non-expandable clays — such as illite, a type of clay composed of mica minerals — don’t show organic synthesis.
“Minerals are very attractive targets as templates for the origin of life,” says Robert Hazen, a geophysicist at the Carnegie Institution for Science in Washington, D.C. “They tend to have charged surfaces that attract small organic molecules and nice regular surface structures that allow those molecules to be concentrated and organized,” he says.
Hansma proposes that organic components were drawn out of the primordial soup and into the mica sandwich by capillary action — the same force that draws water into a sponge — and that charged attraction held them there.
Gustaf Arrhenius, a professor of biogeochemistry at the Scripps Institution of Oceanography in La Jolla, Calif., says that he is not convinced. “[Mica’s] silicate sheets are strongly glued together by ionic forces,” he says. He points out that there is a significant amount of literature that “shows that organic molecules approaching this solid bed will not be let between the sheets.”
James Ferris, a chemist at the Rensselaer Polytechnic Institute in Troy, N.Y., says the mica hypothesis is plausible, but adds, “I’m not sure I’d get too excited about this one until she can provide some experimental data.” That’s the next step, Hansma says.