Evolution

Sowing the Seeds of Spherulites

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Science  16 Apr 2010:
Vol. 328, Issue 5976, pp. 286
DOI: 10.1126/science.328.5976.286-b

The evolution of hard calcified structures such as shells and skeletons gave their bearers a selection advantage over older soft-bodied organisms. Most organisms equipped with such hard structures synthesize enzymes or other biomolecules to serve as sites for controlled crystal growth. The modern sponge Astrosclera willeyana—a living fossil related to some of the most primitive sponges—may form its simple calcite spheres via another mechanism. Within the organic matrix of these spherulites, Jackson et al. found biomarkers exclusive to the biomolecules of bacteria. This finding suggests that A. willeyana degrades the bacteria that enter its much larger cells and then uses the degradation products, rather than its own biomolecules, to nucleate calcite crystals. Bacteria on their own can form large calcified structures such as stromatolites, and did so billions of years ago; however, if sponge-like organisms ∼300 million years ago did not have the full suite of genetic machinery to direct biocalcification themselves, the harvesting of bacteria to jump-start the process would have served as an efficient evolutionary shortcut. The appearance of more sophisticated mineralization pathways, which allowed species related to modern corals to outcompete the ancient sponges that previously dominated ocean reefs, may have occurred much later.

Geobiology 8, 10.1111/j.1472-4669.2010.00236.x (2010).

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