In brief - ageing is largely caused by free radicals produced by respiration. Oxygen kills us in the end. Iron in our cells is kept locked up to prevent it reacting with free oxygen radicals. Despite this and other protective measures our cells age and one effect is increased cross-linking of collagen. We get wrinkles!
At death the highly reactive iron is released and causes proteins and cell membranes to tie in knots - an action that mimics Formaldehyde thus preserving amino acids and therefore proteins.
It is a brilliant and fascinating discovery that allows investigation of samples that are millions of years old, previously thought impossible. It offers no comfort at all to Iron Age creation myths.
More information here...
See also Oxygen - The Molecule that made the world (chapter 9) by Nick Lane