It’s an archeological mystery 65 years in the making. In 1960, a villager was exploiting Petralona Cave about 22 miles southeast of Thessaloniki, Greece. Eventually, the villager spotted something unexpected—the nearly complete cranium of an unknown, ancient hominin protruding from a small chamber’s interior wall.
Although covered in calcite and missing its lower jaw, the Petralona skull has fascinated paleoarchaeologists since its discovery decades ago. The remains don’t match the anatomy of a Neanderthal or modern human, and even after decades of research, its exact place within the Homo genus remains unknown. Additionally, the environment in which the villager discovered the cranium has hindered efforts to date the skull, with previous studies estimating that the specimen is anywhere between 170,000 and 700,000 years old. Now, with advanced isotopic analysis, a team led by France’s Institut de Paléontologie Humaine has confidently established that our mysterious evolutionary relative is 286,000 years old at minimum.
The team’s findings published in the Journal of Human Evolution come from a technique called U-series dating. The process leverages the natural properties of uranium isotopes, including the precise half-life at which it breaks down into thorium. Comparing the ratio of uranium to thorium on the cranium’s calcite allowed the study’s authors to at least pinpoint when the isotopic decay began.
U-series dating isn’t possible for artifacts excavated from soil deposits because dirt is constantly adding uranium isotopes from decaying debris, erosion mixtures, and other factors. But a cave is a different situation—it loses its water-soluble minerals (and uranium) as moisture moves through the damp environment. However, any thorium present in the cave stays in place. And if that cave wall moisture eventually evaporates, all that’s left are residual layers of trace minerals and uranium isotopes, sans previously created thorium. The result is a closed environment that allows uranium isotopes to continue decaying into new thorium. This means that experts can pinpoint clear start dates for each layer of encrustation.
Researchers collected samples from the Petralona skull’s calcite coating and from various locations throughout the cave. Subsequent U-series testing finally shows that the cranium is at least 286,000 years’ old, give or take about 9,000 years. Compared with the additional sample dating, the team now believes the skull is likely 277,000 to 539,000 years old if sediment formed around it, or 410,000 to 277,000 if it was deposited there.
It remains to be seen if the Petralona skull will ever be linked to a clear hominin ancestor. That said, the narrower age range makes one thing clear:whoever they were, they likely lived alongside the evolving Neanderthal lineages of Europe’s Middle Pleistocene.
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