💀 The mystery of the Petralona skull impossible to date

A cave in northern Greece has preserved an ancient skull for several hundred thousand years, directly attached to a wall. Its discovery, almost miraculous, raises questions about the first European inhabitants.

Discovered in 1960 by a local in the Petralona cave, this fossil intrigued from the start due to its atypical morphology. Without a precise stratigraphic context, its age and affiliation remained uncertain, sparking scientific debates for over six decades.

The mystery of the skull and its initial analyses

The fossil presents human characteristics, but ones that are difficult to attribute to a specific species. According to different studies, it could be Homo erectus, Neanderthal, or an archaic Homo sapiens. This ambiguity is accentuated by the absence of sedimentary layers allowing for reliable dating.

Indeed, most fossils are usually found in layers of sediment (soil, sand, mineral deposits) that accumulate over time and allow for estimating the fossil's age, which is not possible in this case because the skull was directly attached to the cave wall.

Initial dating attempts, focusing on the bones and the calcite covering the skull, produced highly variable results, ranging between 160,000 and 700,000 years. These discrepancies show the difficulty of studying fossils in karst environments.

The site itself complicates the analysis: the skull is cemented, with no associated skeleton, in a cave shaped by millennia of erosion and mineral deposits. This configuration makes any direct correlation with artifacts or animal remains impossible.

Uranium/thorium dating and new perspectives

Researchers recently applied uranium/thorium dating to the calcite covering the skull. This method measures the radioactive transformation of uranium into thorium in stable mineral deposits. It thus provides a reliable chronology in the closed environment of a cave, unlike sediments in open environments.

The results of dating the oldest layer covering the skull indicate an age of 286,000 years, with a margin of error of 9,000 years. This is a minimum age, indicating that the individual lived before this period. By analyzing other geological elements, scientists estimate that the skull could date from 277,000 to 410,000 years. This dating places it among the oldest known human remains in Europe.

By combining morphology and dating, researchers suggest that the skull could belong to Homo heidelbergensis, a primitive human group likely to be the common ancestor of Neanderthals and Denisovans.

The Petralona skull confirms that a distinct human population coexisted with Neanderthals during the Middle Pleistocene. It also illustrates the diversity of human groups present in Europe before the appearance of Homo sapiens.

To go further: how does uranium/thorium dating work?

The method relies on the radioactive decay of uranium into thorium in minerals like calcite. Thorium does not move with water, unlike uranium, allowing for a reliable "chronometer." This technique is particularly suited to cave deposits.

By measuring the U/Th ratio, scientists can estimate the age of the deposit since its formation. U/Th dating has allowed the Petralona skull to be placed in a coherent chronology, offering a clearer vision of human evolution in Europe.