DNA damage can reveal a sample’s age
Ancient DNA presents a paradox: the damage that makes it difficult to read can also help show that it really is ancient. After an organism dies, chemical reactions gradually alter its DNA. One common change affects cytosine, one of DNA’s four building blocks: it loses part of its chemical structure and becomes uracil.
That change matters when scientists copy and read the DNA. The copying machinery tends to treat uracil like thymine, so a damaged strand can appear to contain a different letter than it originally did. Because the ends of old DNA fragments are especially prone to this damage, researchers can look for an excess of apparent C-to-T changes near those ends.
A molecular pattern, not a certificate
Modern human DNA can enter an ancient sample during excavation, handling, or laboratory work. It usually has not spent millennia accumulating the same pattern of chemical decay. Researchers therefore examine whether the DNA fragments are short and whether their damage appears in the expected places—not just whether a sequence resembles a Neanderthal’s.
This pattern is evidence of age and helps distinguish ancient genetic material from recent contamination. It is not a magic test that makes every result genuine on its own: scientists also assess the sample and the genetic data using other checks. But predictable damage gives them a way to test whether the DNA’s history fits the age of the remains.
From damaged fragments to ancestry
Svante Pääbo and other researchers used such patterns as part of the work that made it possible to study Neanderthal genomes. The practical payoff was substantial: instead of dismissing degraded fragments as unreadable noise, scientists could interpret them while accounting for the chemical changes that had accumulated.
The result is a striking reversal. A molecular scar that obscures the original DNA can also help establish that the sequence came from an ancient individual—making decay part of the evidence for studying human ancestry.
