It’s a hard-shelled life: calcium ion overload?

From, An Introduction To Geology", by William B. Scott

A couple of years ago, as I began exploring more broadly the role of toxicity in evolution, I struck up an email conversation with geologist John Saul. In a nice techno-age moment, we kept up the correspondence, me from my favorite coffee shop in Montague, and he from wherever he was in Paris.  Of course we’ve never met. I “found” John using the search terms “toxic and evolution.” He’d published a couple of papers hypothesizing about the role of toxic chemicals in the evolution of complex life writing about things like the possible role of oxygen in the evolution of multicellular life, and the role of mineral and ion availability (and the need to excrete) in the evolution of shells and other hard bits.  His ideas were intriguing and he came at them from the perspective of a geologist familiar with the constant shaping and reshaping of the earth’s surfaces, and the subsequent influence on seawater chemistry. Since life spent its childhood in the oceans and a good bit of its early adulthood (living things only seemed to have made landfall around 450 million years ago,) it makes sense that ocean chemistry would shape life.  Of the many important ions floating about, is calcium and as Saul noted in one paper entitled Did detoxification processes cause complex life to emerge? (see here for more)  that maybe life’s combined efforts to rid itself of toxic oxygen and excess calcium resulted in plenty of calcium carbonate to coat an army of tiny ocean creatures.  While this is just one of many examples set forth by Saul, in this case he cites the earlier work of Kenneth Simkiss who back in the 1980s, basically co-authored the book as they say , and who in 1977 pointed out that biomineralization may be the outcome of detoxification (he also pointed out that this had been pointed out earlier by others).  Even before calcium carbonate coatings and spicules became de rigueur means of detox,  writes RJP Williams in his comprehensive review  on the Evolution of the biochemistry of calcium  “procaryotic cells treated it as an intracellular poison rejecting it together with sodium and chloride ions.”  Eventually calcium ions, like oxygen worked their way from toxic to essential (yet still toxic in large amounts) for chemical message transmission – particularly important in multicellular animals and beyond that as a structural component.

Phew. So we’ve established the importance of calcium – both as a toxic ion and as an essential messenger. So what happened with calcium that may have spurred on evolution? A recent paper by Shanan Peters and Robert Gaines, on Formation of the ‘great unconformity’ as a trigger for the Cambrian explosion which has gained a good amount of press, suggests massive layers of the earth dissolved into the oceans just prior to the Cambrian, and as the oceans became mineral rich, living things which might once have made their peace with calcium ions, were facing even higher concentrations, prompting in turn, the evolution of mechanisms to rid themselves of the toxic stuff and in the process providing the raw materials for a spectacular explosion in the diversity of life.

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