The last theory I heard was that fish developed calcium deposits in order to move up river deltas into fresh water. Calcium is important for muscle function and you would have to bring a stable supply with you to survive.

The fact that mammals still cycle calcium in and out of their bones to manage blood chemistry was as I recall used to support that theory.

I was trying to read the article, but then I refreshed and it looks like the site is now suffering from the internet kiss of death. Resource limits reached.

It's also on Phys.org and that seems to be up ok https://phys.org/news/2017-08-oceans-alkalized-life-bones-sh...

This is something that I think about, we have CO2 contributing to acidity, but what are nature's ways of providing alcalinity??

calcium carbonate

which is CaO + CO2, so that doesn't answer the question too much