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guiding what in many cases appears to be essentially diagenetic crystal growth from
amorphous precursors (Gotliv et al. 2003). Thus, many animals and protists with no
skeleton-forming common ancestors nonetheless share the underlying capacity to
synthesize and localize carbonate-inducing biomolecules, although homologies among
these molecules remain uncertain.
Biochemical similarities extend, as well, to the molecules that inhibit CaCO
3
mineralization. Marin et al. (1996, 2000) proposed that organisms sculpt mineralized
skeletons via “anti-calcifying” macromolecules that locally inhibit crystal growth. Because
spontaneous calcification of cell and tissue surface may have been a problem in highly
oversaturated Proterozoic oceans (Grotzinger 1989; Knoll et al. 1993), Marin et al. (1996)
reasoned that molecular inhibitors evolved early as anti-calcification defenses and were
later recruited for the physiological control of skeleton growth. [Even today, molecular
inhibitors are necessary to retard spontaneous calcification of internal tissues in vertebrates;
Luo et al. (1997).] Immunological comparison of anti-calcifying molecules in mollusks
and cnidarians supports the hypothesis that this biochemistry already existed in the last
common ancestor of cnidarians and bilaterian animals, if not before that.
Thus, in the view of Westbroek and Marin (1998), the many origins of calcareous
skeletons reflect multiple independent cooptations of molecular and physiological
processes that are widely shared among eukaryotic organisms. Skeletons that are not
homologous as
structures
share underlying physiological pathways that are. This
powerful idea has to be correct at some level. The details—how much biochemical
innovation and diversity underlies carbonate skeletonization across clades—await an
expanded program of comparative biological research.
Silica skeletons
Skeletons of opaline silica also enjoy a wide distribution among eukaryotes,
occurring in five or (depending on the still debated phylogenetic relationships of
ebridians) six of the eight great eukaryotic clades. Biologically, however, SiO
2
skeletons
differ in distribution from those made of CaCO
3
. Silica biomineralization is limited to
intracellular precipitation. Thus, microscopic scales and skeletons are abundant in
cercozoans (radiolarians and their relatives, as well as the siliceous scales of euglyphid
amoebans), ebridians, and heterokonts (reaching their apogee in the exquisite frustules of
diatoms). In animals, however, siliceous skeletons are limited to sponge spicules and
minor occurrences such as the opalized mandibular blades of boreal copepods (Sullivan
et al. 1975) and the micron-scale silica tablets formed intracellularly in the epidermis of
some brachiopod larvae (Williams et al. 2001). Within Plantae, the carbonate
biomineralization of marine and freshwater algae is replaced by silica phytolith
mineralization in the epidermis of some vascular plants, especially grasses, sedges, and
the sphenopsid genus
Equisetum
(Rapp and Mulholland 1992). Accepting that
choanoflagellates and sponges could share a common ancestor that precipitated silica, we
can estimate that silica skeletons evolved at least eight to ten times in eukaryotic
organisms. Much has been learned in recent years about silica precipitation by diatoms
(e.g., KrĘger and Sumper 1998; KrĘger et al. 2000, 2002; Zurzolo and Bowler 2001).
Frustule formation requires active silica transport to local sites of precipitation (silica
deposition vesicles), where opal deposition is mediated by enzymes. The vesicular spaces
for mineral formation appear to be modifications of the Golgi-vesicle apparatus shared
almost universally by eukaryotes. On the other hand, little is known about intracellular
silica transport. Various molecules have been shown to promote the polymerization of
silicic acid molecules—polyamines and silaffins in diatoms (KrĘger et al. 2000),
silicatein and collagen in sponges (Krasko et al. 2000). To date, however, we do not
know enough about the comparative biology of these molecular processes to know the
