How Ancient DNA Works

In 1997, Svante Pääbo’s team published 379 base pairs of mitochondrial DNA from the arm bone of the original Neanderthal specimen found in the Neander Valley in 1856. It was enough to show Neanderthals sat outside modern human variation. Thirteen years later they published a draft whole Neanderthal genome, and revealed that most living people carry a piece of it. Pääbo received the Nobel Prize in Physiology or Medicine in 2022.

None of this should be easy. DNA is a fragile molecule that begins degrading within hours of death. Here is how it is actually done.

What happens to DNA after death

Three processes destroy genetic material, and understanding them is what makes the field possible:

• Fragmentation. Enzymes and water break the DNA backbone. Living DNA comes in chromosomes millions of base pairs long; ancient DNA typically survives in pieces under 100 base pairs, often 30–60.
• Chemical damage. Cytosine bases lose an amino group and become uracil, which sequencers read as thymine. This produces a characteristic excess of C-to-T substitutions concentrated at fragment ends.
• Contamination. A bone that has been handled, washed and stored in a museum drawer for a century is coated in modern human DNA, plus vast quantities of soil bacteria. In a typical ancient bone, less than 1% of recovered DNA is from the individual; the rest is microbial.

The second of these is a gift. That C-to-T damage pattern is a signature of authenticity that modern contaminating DNA does not have. If a sequence lacks it, it is suspect.

How old can DNA get?

A 2012 study of 158 radiocarbon-dated moa bones from New Zealand estimated a DNA half-life of about 521 years for a 242-base-pair fragment under those burial conditions — meaning half the bonds break every 521 years. Extrapolated, readable DNA should be gone by roughly 1.5 million years even in ideal cold conditions.

Reality has run close to that ceiling:

• ~430,000 years — the oldest hominin nuclear DNA, from the Sima de los Huesos cave in Atapuerca, Spain. It showed those hominins were early Neanderthals, not Homo heidelbergensis in the traditional sense.
• ~1.2 million years — mammoth DNA from Siberian permafrost, published in 2021.
• ~2 million years — environmental DNA from frozen sediments at Kap København, Greenland, reconstructing an entire vanished ecosystem including mastodons, reindeer and poplar trees.

Temperature is the dominant variable. Cold, dry, stable conditions preserve DNA; heat and humidity destroy it. That is why we have abundant Siberian genomes and almost nothing from tropical sites — and why Homo luzonensis and Homo floresiensis remain genetically silent.

The petrous bone revolution

Before 2015, ancient DNA work had a brutal success rate. Then a team led by Ron Pinhasi showed that the petrous portion of the temporal bone — the dense pyramid of bone housing the inner ear, the hardest bone in the body — can yield up to 100 times more endogenous DNA than teeth or long bones from the same individual.

This single finding transformed the field from a trickle of heroic individual genomes into an industry producing thousands. Almost every large ancient population study since — including the datasets behind the AADR Genome Atlas — rests on petrous bone sampling. Cementum, the tissue anchoring tooth roots, is a good second choice.

Inside the clean room

Ancient DNA labs are built like semiconductor fabs run backwards: instead of keeping dust out of the product, they keep modern DNA out of the sample.

• Positive air pressure, HEPA filtration, physical separation from any lab that handles modern DNA or PCR products.
• Full body suits, face masks, double gloves, dedicated shoes.
• Surfaces bleached and UV-irradiated between sessions; UV lamps run overnight.
• Bone surfaces removed by drilling or abrasion before sampling the interior.
• Blank controls processed alongside every sample, at every step, to catch contamination.
• Independent replication in a second laboratory for major claims.

Excavators increasingly help by handling finds with gloves and freezing them immediately, rather than washing and cataloguing them by hand as was standard for a century.

From fragments to a genome

The extracted fragments are converted into a sequencing library: short synthetic adapters are attached to both ends of every fragment so it can be amplified and read. Because most of what is in the tube is bacterial, researchers often use hybridisation capture — synthetic DNA “baits” matching about 1.24 million informative human positions (the widely used “1240K” panel) that fish out human fragments and discard the rest, cutting sequencing costs enormously.

The reads are then mapped against a reference human genome. Each fragment on its own is meaningless; millions of overlapping fragments, aligned by a computer, reconstruct a genome. A high-coverage ancient genome may have every position read 30 or more times over.

DNA without bones

Two developments have pushed past the need for a skeleton entirely.

Sediment DNA. In 2017, Viviane Slon and colleagues recovered Neanderthal and Denisovan mitochondrial DNA directly from cave floor sediments containing no visible bone — from urine, faeces, skin cells and decayed tissue bound to mineral grains. It is now possible to determine who occupied a cave layer without finding a single fossil, and to track occupation across layers.

Palaeoproteomics. Proteins survive longer than DNA. Sequences from tooth enamel have placed Homo antecessor at 800,000 years on the hominin tree, identified the Xiahe mandible as Denisovan, and resolved the position of Gigantopithecus at 1.9 million years. Proteins carry far less information than a genome — but they are readable when DNA is long gone, including in the tropics.

What ancient DNA has actually told us

• We interbred. Most people outside sub-Saharan Africa carry roughly 1–2% Neanderthal ancestry; people in New Guinea and Australia carry 4–6% Denisovan.
• Denisovans existed at all. They were discovered from a fingertip bone in 2010, purely genetically, and only got a face in 2025 with the Harbin cranium.
• Denny. Denisova 11, a bone splinter from the same cave, turned out to be a teenage girl with a Neanderthal mother and a Denisovan father — a first-generation hybrid.
• Prehistoric Europe was made by migration. Successive waves — Anatolian farmers around 8,000 years ago, Yamnaya steppe pastoralists around 5,000 — each transformed the continent’s ancestry.

The honest limits

Ancient DNA is not a universal solvent. It works best in cold climates, which biases the record toward Eurasia and against Africa, where our species originated. It gives population histories, not individual life stories. And it can be over-read: a genome tells you who someone’s ancestors were, not what language they spoke, what they believed, or how they thought of themselves.