Where did life on Earth start? — this age-old question seems to have a new answer. Life may have originated between sheets of mica that were layered like the pages in a book, say scientists. A team at University of California has come up with the so-called "life between the sheets" mica hypothesis which is described in an upcoming issue of 'Journal of Theoretical Biology'. 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 progenitors to cells. Provided with the right physical and chemical environment in the structured compartments to survive and evolve, the molecules eventually reorganised into cells, while still sheltered between mica sheets. According to lead scientist Ms Helen Hansma, as 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 mica hypothesis is consistent with other well-known hypotheses that propose life originated as RNA, fatty vesicles or primitive metabolisms. "Mica world might have sheltered all the ancient metabolic and fat-vesicle and RNA 'worlds'," she said. Ms 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. Ms 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".