Bone
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.
Biomineralization
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.
Martian Minerals
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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