A New Answer to an Old Question
That age-old question, "where did life on Earth start?" now has a new answer. If the life between the mica
sheets hypothesis is correct, life would have originated between sheets of mica that were layered like the
pages in a book.
The so-called "life between the sheets" mica hypothesis was developed by Helen Hansma of the University of
California, Santa Barbara, with funding from the National Science Foundation (NSF). This hypothesis was originally
introduced by Hansma at the 2007 annual meeting of the American Society for Cell Biology, and is now fully described
by Hansma in the September 7, 2010 issue of Journal of Theoretical Biology.
"Life Between the Sheets" Hypothesis
According to the "life between the sheets" mica hypothesis, structured compartments that commonly form between
layers of mica--a common mineral that cleaves into smooth sheets--may have sheltered molecules that were the
progenitors to cells. Provided with the right physical and chemical environment in the structured compartments
to survive and evolve, the molecules eventually reorganized into cells, while still sheltered between mica sheets.
Mica chunks embedded in rocks could have provided the right physical and chemical environment for pre-life molecules
and developing cells because:
Mica compartments could have held, protected and sheltered molecules, and thereby promoted their survival. Also, mica
could have provided enough isolation for molecules to evolve without being disturbed and still allow molecules to migrate
towards one another and eventually bond together to form large organic molecules. And mica compartments may have provided
something akin to a template for the production of a life form composed of compartments, which are now known as cells.
The Possible Role of Potassium
Mica sheets are held together by potassium. If high levels of potassium were donated by mica sheets to developing cells,
the high levels of potassium found in mica sheets could account for the high levels of potassium currently found in human cells.
Mica chunks embedded in rocks that were sitting in an early ocean would have received an endless supply of energy from waves,
the sun, and the occasional sloshing of water into the spaces between the mica sheets. This energy could have pushed the
mica sheets into up-and-down motions that could have pushed together molecules sitting between mica sheets, thereby enabling
them to bond together.
Because mica surfaces are hospitable to living cells and to all the major classes of large biological molecules, including
proteins, nucleic acids, carbohydrates and fats, the "between the sheets" mica hypothesis is consistent with other well-known
hypotheses that propose that life originated as RNA, fatty vesicles or primitive metabolisms. Hansma says a "mica world" might
have sheltered all the ancient metabolic and fat-vesicle and RNA "worlds."
The Ideal Mineral Substrate
Hansma also says that mica would provide a better substrate for developing cells than other minerals that have been considered
for that role. Why? Because most other minerals would probably have tended to intermittently become either too wet or too dry
to support life. By contrast, the spaces between mica sheets would probably have undergone more limited wet/dry cycles that
would support life without reaching killing extremes. In addition, many clays that have been considered as potential surfaces
for life's origins respond to exposure to water by swelling. By contrast, mica resists swelling and would therefore provide a
relatively stable environment for developing cells and biological molecules, even when it did get wet.
Hansma sums up her hypothesis by observing that "mica would provide enough structure and shelter for molecules to evolve but
also accommodate the dynamic, ever-changing nature of life."
What's more, Hansma says that "mica is old." Some micas are estimated to be over 4 billion years old. And micas such as biotite
have been found in regions containing evidence of the earliest life-forms, which are believed to have existed about 3.8 million years ago.
Hansma's Research Background
Hansma's passion for mica evolved gradually--starting when she began conducting pioneering, NSF-funded research in former husband
Paul K. Hansma's AFM lab to develop techniques for imaging DNA and other biological molecules in the atomic force microscope
(AFM)--a high-resolution imaging technique that allows researchers to observe and manipulate molecular and atomic level features.
Says Helen Hansma, "Mica sheets are atomically flat, so we can see DNA molecules on the mica surface without having to cover the
DNA with something that makes it look bigger and easier to see. Sometimes we can even see DNA molecules swimming on the surface
of mica, under water, in the AFM. Mica sheets are so thin (one nanometer) that there are a million of them in a millimeter-thick piece of mica."
How the Idea Began
Hansma's "life between the sheets" hypothesis first struck her a few years ago, after she and family members had collected some
mica from a Connecticut mine. When she put water on a piece of the mica under her dissecting microscope, she noticed a greenish
organic 'crud' at some step edges in the mica. "It occurred to me that this might be a good place for the origins of life--sheltered
within these stacks of sheets that can move up and down in response to flowing water, which could have provided the mechanical energy
for making and breaking chemical bonds," says Hansma.
Hansma says that recent advancements in imaging techniques, including the AFM, made possible her recent research, leading to her
"between mica sheets" hypothesis. She adds that direct support for her hypothesis might be obtained from additional studies involving
mica sheets in an AFM, being subjected its push-and-pull forces while sitting in liquids resembling an early ocean.
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| Helen Hansma of the University at Santa Barbara discusses why the origin of life is an important topic and her new hypothesis that life started between mica sheets that were embedded in rocks that were sitting in an early ocean. Video by: University of California, Santa Barbara/National Science Foundation |
| Diagram of biomolecules between sheets of mica in a primitive ocean. The green lines depict mica sheets and the grey structures depict various ancient biological molecules and fatty vesicles. In the 'between the sheets' mica hypothesis, water may have moved in and out of the spaces between stacks of sheets, thereby forcing the sheets to move up and down. This kind of energy may have ultimately pushed biological molecules and/or fatty acids together to form cells. Image by Helen Greenwood Hansma, University of California, Santa Barbara. |
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