The use of mineralized structures, such as shells and bones, is widespread among the multicellular animals; even something as basic as a sponge makes some mineral materials. The origin of these structures appears to have followed quickly upon the origin of multicellular animals themselves. The first hints of biomineralization appear in the Ediacaran era (which is when the first indications of multicellular animal life appeared) and the diversity of mineral forms exploded along with life itself during the Cambrian. Two papers being released by Science today peer into the distant past and suggest that environment and genetics interacted to produce the mineral toolkit in use today.
The first paper looks at things from the perspective of genes. All biominerals used by animals involve some form of calcium carbonate compound, but the history of carbonate (CO3) use is complex. Modern animals create CO3 in order to regulate acidity, remove waste, and pull in carbon from the atmosphere, among other functions. Did carbonate-based mineralization arise separately from these other functions, or was there a single, ancestral genetic toolkit?
The authors look at an ancient lineage of sponge, the ancestors of which can be identified in the Cambrian. They isolated and sequenced proteins from their mineralized structures and found that all of them belonged to a group of three closely-related CO3-generating enzymes. Sequence comparisons show that this enzyme group is ancient and is now widespread among multicellular animals. The authors conclude that either all animals adopted descendants of a single ancestral protein for biomineralization, or the last common ancestor of all animals was already using it to make skeletal structures.
The second paper goes a long way towards suggesting that the first of these two options is the best explanation. It notes that there are different types of calcium carbonate, aragonite and calcite, that are structurally distinct. Researchers have determined that, once a phylogenetic group of animals starts out using one or the other structure, it sticks with it: the fossilized ancestors of modern aragonite users can be shown to contain deposits of aragonite.
The research team traced different aragonite- and calcite-using groups back to their first appearance in the fossil record to estimate a date when they started making skeletal elements. They compared those dates to independent measures of the oceanic chemistry, made based on deposits of evaporated sea water. The two lined up perfectly. In the early Cambrian and before, ocean water favored aragonite, and all of the animals that originated in this period appear to have used it. From the mid-Cambrian on, ocean chemistry favored calcite, and all the groups arising during this time wound up using that instead.
This would argue that skeletal elements evolved multiple times. Although animals took advantage of the same genes to produce CO3, what they did with it was dictated by the chemistry of the environment they were living in.