I was wondering if anyone has any information on the biochemical evolution of the exoskeleton. A kid asked in bio class how organisms evolved from soft bodied to shelled. I am aware of how modern mollusks form their shells but I could not find any information on the possible biochemical pathways involved and the gene mutations necessary for the change. Do we simply not know the answer yet?

Also, do we know the mutations that were necessary for the evolution of the endoskeleton? Again, I am aware how bone cells secrete the minerals into the extracellular matrix, but I can find few details on its evolution.

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I tend to go here for some evolution info when I need it, not sure if it will help but it might point you in the right direction.

http://evolution.berkeley.edu/evolibrary/home.php
There are several ways that organisms produce hardened skins and shells, so there is no single answer.

Chitin is used by fungi and many invertebrates; it is a cellulose-like polymer of N-acetylglucosamine (note the glucose in it).

Calcium carbonate is the favorite mineral used by invertebrates and protists, though some use silica.

Vertebrate bone and teeth are calcium hydroxide phosphate (hydroxyapatite).

Plants make lots of cellulose, and dead parts of them, like tree bark and seed/nut shells, can protect living parts of them (tree-trunk cambium and seeds).

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