It interacts with humic acids and other organic compounds from the soil environment in ways that can introduce modern or near-modern carbon into the sample, making it appear younger than it actually is.

Formic acid extraction, the method used for the osteocalcin fraction, does not remove this contamination effectively.

The collagen fraction, extracted using the more demanding XAD purification method, was a different matter.

XAD purification — named for the Amberlite XAD resins used in the process — removes a wider range of contaminants, including the humic acids that cause problems with osteocalcin.

The collagen that survived this process was more likely to represent the original biological material.

The researchers also examined the amino acid composition of the collagen — its molecular building blocks — and compared it to what was expected from well-preserved ancient collagen of known characteristics.