The Frozen Ledger

Bones fossilize. This is well understood — mineral replaces organic matrix over geological time, preserving shape while erasing chemistry. DNA degrades within a million years at best. Proteins last longer but fragment. The assumption has been that fossilization is a one-way door: structure survives, biochemistry does not.

Bromage et al. (Nature, 2025) tested this assumption by running mass spectrometry on fossilized animal bones from sites in Tanzania, Malawi, and South Africa, ranging from 1.3 to 3 million years old. Modern mouse bones yielded roughly 2,200 identifiable metabolites. The fossil bones — rodents, antelope, pig, elephant — yielded thousands as well. Not fragments of structural proteins. Metabolites: the small molecules produced by active cellular metabolism. Amino acid breakdown products. Carbohydrate intermediates. Vitamins. Minerals in their biologically processed forms.

The bone had fossilized. Its metabolic ledger had not.

In one specimen — a ground squirrel from Olduvai Gorge, 1.8 million years old — the researchers identified a metabolite unique to Trypanosoma brucei, the parasite that causes sleeping sickness in humans. Alongside it, markers of the animal’s anti-inflammatory response. They had diagnosed a disease across 1.8 million years from the chemistry trapped in bone mineral.

Plant metabolites identified aloe and asparagus species, which in turn constrained temperature, rainfall, and canopy cover — the environments were warmer and wetter than today.

The structural insight is about what survives and why. DNA is a long polymer, vulnerable to hydrolysis. Proteins are folded chains, vulnerable to denaturation. But metabolites are small, thermodynamically stable molecules that lodge in the mineral matrix like insects in amber. The metabolism stops. Its products remain. The body is gone. The books are still open.