A scientist this week proposed a new answer to that most-existential of questions: "How did life begin?" According to the "soup and sandwich" hypothesis," the compartments between layers of mica—a common, rather flaky mineral that cleaves into smooth, flat sheets—could have provided the shelter and protection needed for prehistoric molecules to organize into cells.
"I picture all the molecules of early life evolving and rearranging among mica sheets in a communal fashion for eons before budding off with cell membranes and spreading out to populate the world," said Helen Hansma, a University of California, Santa Barbara biologist who has spent her recent years eyeing molecules like cellular proteins and genetic material through an atomic force microscope.
Presenting her theory at a conference in Washington, D.C., Hansma supported her hypothesis by several lines of evidence, including the many chemical and physical similarities between a cell interior and the space between mica sheets. Both environments, for example, are loaded with potassium and pack a negative electrical charge, suggesting that mica "would have provided a very hospitable environment to the earliest biomolecules," she said.
The confined spaces between mica layers could have provided the isolation needed for Darwinian evolution, Hansma said, meaning that like Darwin’s famous Galapagos finches, pre-living groups of molecules would have had a secluded environmental niche in which to adapt and specialize. What's more, the expansion, contraction and movements of the mica sheets caused by temperature changes and ocean currents would have helped rearrange the corralled molecules and trigger the formation of bonds between and among them as required for life to originate.
Hansma came upon her mica hypothesis last spring when she was splitting some mica specimens she had collected in a mine in Connecticut under her dissecting microscope. The mica was covered with organic material. "As I was looking at the organic crud on the mica, it occurred to me that this would be a good place for life to originate—between these sheets that can move up and down in response to water currents, which would have provided the mechanical energy for making and breaking bonds," said Hansma.
Scientists generally agree life on Earth arose about 4 billion years ago when the planet’s oceans first appeared. Some hypothesize life originated in a prebiotic "soup" in the ancient seas. Others champion the "pizza crust" hypothesis, which suggests the earliest cells developed on terrestrial mineral surfaces.
"The primordial soup hypothesis was awesome 50 years ago," said Hansma. "But there are still a lot of questions. The mineral 'pizza' crust hypotheses have made progress explaining some things, but the molecules would still be quite unprotected in that scenario."
So, Hansma has added her primordial-soup-and-mica-sandwich theory to the geologic smorgasbord.
Finally, location is everything. A decade ago, scientists used radio-active dating to determine rocks recovered from Akilia Island off the coast of Greenland, were almost 4 billion years old—from a time when the Earth was cool enough to support water. The conditions suggest early life could have formed there.
It turns out rocks near the Greenland area also contain large amounts of mica, Hansma said.
Hansma's thing for mica started in the 1980s, when she began developing pioneering techniques in biological atomic-force microscopy—an imaging technique that generates a sort-of topographic map of a molecule, which can be used to identify or otherwise characterize it.
"We put our samples on mica, because it is so atomically flat, so flat that we can see even bare DNA molecules as little ridges on the mica surface," said Hansma.
Now the work begins, she says, as her and other research groups look for ways to test and prove the hypothesis.
—Leslie Fink, NSF
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.