Thursday, December 6, 2007

life began between the sheets

A new hypothesis has been brought forth to help shed light on the origin of life on Earth. Dr. Helene Hasma of UC Santa Barbara proposed on Tuesday that life's first biomolecules may have developed and evolved between sheets of mica. Though we do not yet know if these biomolecules were RNA or protein, the spaces between the layers of mica would have provided an environment exceptionally conducive to both biochemical reactions and subsequent evolutionary steps.

Hansma, a biophysicist, was collecting samples in a mica mine when she noticed organic material growing on the mica, and so an idea was born. There is a remarkable poetry in this hypothesis. Individual layers of mica are perfectly flat and thin, and would have acted as cell membranes for the first biomolecules, providing support, protection, and the isolation necessary for Darwinian selection and differentiation. RNAs and some proteins and lipids are negatively charged, just like mica. The distance between RNA phosphate groups is the same as the spacing between negative charges of mica. Mica is held together with potassium, which is found at the same concentration in our cells. The heating and cooling expansion of the mica layers, as well as the movement of the ocean would have provided a mechanical energy for the breaking and forming of bonds in the earliest biochemical reactions.

Granted all of this is just conjecture at this point. But this one struck me as a particularly harmonious hypothesis. I just love science!


Carrie said...

This is so cool! But wouldn't it make more sense if the mica and cellular molecules had opposite charges? I've got to read more about this.

Paulina said...

Carrie dear,
You bring up a good point. Since the ASCB meeting only happened this week I cannot find a copy of the proceedings, but here is the abstract from Dr. Hansma's presentation:

"Life may have originated between mica sheets, which would have provided many many confined spaces with surprising similarities to cells. Mica's hydrophilic 2-nm-thick mineral sheets have a hexagonal array of anionic sites spaced 0.5 nm apart, with potassium ions bridging the anionic sites on adjacent sheets. Like cells, the spaces between mica sheets are potassium-rich, while the external environment would have been sodium-rich sea water. Like cells, the spaces between mica sheets are negatively charged and interact with a variety of inorganic cations. Single-stranded nucleic acids such as RNA have an anionic phosphate spacing of 0.5 nm, like the periodicity of mica. Mica provides a possible explanation for the chirality of biopolymers and presence of only ribose in RNA: the steric hindrance of confinement between mica sheets would constrain the pathways of synthetic chemistry in ways not found when synthetic chemistry occurs on surfaces or in solution. Mica has small substitutions of elements such as Mg, Ca, and Fe, which may have provided catalytic sites for chemical syntheses. Mica provides many many adjacent compartments - the spaces between the sheets. These adjacent compartments provided hospitable environments that might have served at least two functions: (1) Ribozymes would have enough space to evolve and to avoid extinction caused by 'selfish' ribozymes [e.g., Szabo, et. al. Nature 2002 420:340]. (2) Complementary metabolic processes could have co-evolved in adjacent compartments [cf. DeLong, Science 2007 317:327]. Finally, mica provides a possible source of energy - mechanical energy from the movement of mica sheets in response to ocean currents and temperature changes - that might have facilitated mechanochemistry."

They certainly mentioned the physical constraints of the environment to explain the chirality of the ribose, but not how the charges would have impacted the choice of bonds. I do not know what incentive they would have to mimic their environment (mica), other than if the mica substrate initially functioned IN PLACE of the phosphate groups.

I would be glad to hear your thoughts!