Bone, an extraordinary composite material, features complex intergrowths of minerals (primarily hydroxyapatite) and protein (notably osteocalcin). In the Oct. 30, 2003, issue of Nature, Quyen Hoang and colleagues document the complex structure of osteocalcin and illustrate an amino acid sequence with three glutamic acid molecules that bind strongly to five adjacent calcium ions of hydroxyapatite. This binding not only provides strength and flexibility to bone, but it protects osteocalcin from the rapid decay experienced by most other proteins.
Bone's strong interaction between mineral and protein has at least two major implications for understanding the history of life. In the December 2002 issue of Geology, Christine Nielsen-Marsh and co-workers describe how they exploit this feature to extract and sequence osteocalcin from fossil bison bones more than 50,000 years old. Over time, osteocalcin undergoes slight mutations in its amino acid sequence. Comparison of small differences in this sequence among fossils of various species and ages thus reveals patterns of mammalian evolution.
The osteocalcin work also reveals that binding of organic molecules onto mineral surfaces may stabilize organic molecules that would otherwise decompose. A recurrent difficulty in origin-of-life scenarios is the relative fragility of many key biomolecules, especially under hydrothermal conditions. Minerals may thus have played a vital role in the concentration, stabilization and assembly of macromolecules essential to the origin of life.
The idea that minerals promoted life's origin mirrors another key natural process biomineralization by which living cells promote the growth of minerals. Two recent studies, in particular, highlight the remarkable ability of organisms to mediate mineral growth.
Mollusks and other organisms rely on proteins to make tough calcium carbonate shells and other hard parts. In the Oct. 10, 2003, issue of Science, Söllner and colleagues describe a previously unknown protein that normally deposits aragonite "otoliths," microscopic structures that aid in the balance and hearing of some fish. They find that a reduction in the protein concentration raises pH and triggers a shift from aragonite to calcite precipitation,a change that drastically affects the size and shape of otoliths.
In a similar vein, reporting in the Feb. 6, 2004, issue of Science, Jinwook Kim and co-workers describe the remarkable ability of microbes to promote a phase transition from the layer silicate smectite to illite. This reaction had been thought to require several months at elevated temperature and pressure, but cultures of the microbe Shewanella oneidensis mediated this reaction in 14 days at room conditions by dissolving smectite and precipitating illite.
Few mineralogical discoveries in history have brought the prime-time, front page publicity enjoyed by NASA's Mars rover, Opportunity, whose findings underscore the intimate association of life-forming processes and minerals. Life has yet to be documented on the red planet, but an abundance of new mineralogical evidence proves beyond any reasonable doubt that Mars was once rich in water, the essential medium of all known organisms. To bolster its claims for a once-wet Mars, the NASA science team reported several lines of visual and spectroscopic evidence for calcium, magnesium and iron sulfates (the minerals gypsum, epsomite and jarosite, respectively) that must have been deposited from an evaporating sea. Even more striking was the discovery of martian "blueberries," centimeter-diameter spheres of iron oxide hematite that were precipitated in water. After decades of tantalizing evidence for martian water, minerals at last have provided the smoking gun.
Martian blueberries, courtesy of NASA/JPL.
Given the rapid pace of discovery on Earth and beyond, it is evident that we are only beginning to recognize the intimate connections between minerals and life.
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