When a dead creature is fossilized, its organic remains are replaced by a mineral. Many fossils preserve the structure of the original organism with microscopic fidelity, as if the tissue had been replaced molecule by molecule by precipitated mineral — silica, calcium carbonate, or whatever. The dead organism is presumably permeated by a slightly supersaturated solution of the mineral. As each organic molecule is decomposed to soluble matter, its place is taken by deposited mineral. The process goes so extremely slowly that Daedalus sees it as an example of thermodynamic reversibility. Each organic molecule exchanges with an equivalent volume of mineral only when the process can go with no rise in entropy. The deposited mineral will adopt a crystal habit or microstructure whose strain-energy or grain-boundary density corresponds to, and therefore encodes perfectly, the free energy of the dissolving organic.

So Daedalus wants to reverse the process. Imagine, he says, a silica or carbonate fossil immersed in water quite free of mineral, but saturated with a wide range of biological substances: fats, sugars, proteins, DNA, and so on. The mineral will dissolve very slightly in the water. Its place will be taken by whatever biological molecules are best fitted to undertake the exchange deposition. These, of course, will be the ones originally replaced by the mineral. Keep the solution circulating round the fossil, remove the mineral from it as fast as it dissolves, top up the biochemicals as fast as they are deposited; and the whole process of fossilization will be gradually reversed. It may take years; but ultimately the original specimen will be exactly reconstituted.

This audacious process will require extreme biochemical subtlety. If the right substances are not present in solution, the defossilization mechanism will accept the closest available but incorrect match — especially if the process is hurried, thus losing reversibility. The next layer of organic, influenced by this mismatch, will be more mismatched still, and the reconstruction will deviate more and more from the original. Many fossils of the same type will have to be sacrificed upon the learning curve before a flawless specimen is finally recovered. But Daedalus dreams of the day when a perfectly reconstituted ancient frog or crab or fish opens it eyes in wonder, wiggles its body, and swims off into the solution.