But geological timescales are a different matter entirely.

Over thousands of years, the same chemical processes that degrade other organic molecules attack DNA as well: the phosphodiester bonds that link the nucleotide building blocks of DNA strands are hydrolyzed by water; oxidative damage alters the chemical structure of the bases; the proteins that normally protect DNA in living cells are degraded by enzymatic activity.

The result, in most ancient specimens, is DNA that is highly fragmented — broken into short pieces that may be only a few dozen to a few hundred base pairs long, rather than the millions of base pairs that make up complete chromosomes in living cells.

The specific conditions that affect DNA preservation are, fortunately, better in some environments than others.