The skull of an ancient human-like species, discovered 65 years ago in a Greek cave, has perplexed decades of researchers, but progress finally seems to be under way. The mystery of the Petralona Cave skull, a grotto about 22 miles southeast of Thessaloniki, northeastern Greece, has centred around two unanswered questions. Firstly, while it is clearly of the Homo genus, it is distinctly different from both Neanderthals and modern humans. Secondly, dating the skull has remained incredibly difficult to narrow down, with previous estimates spanning about 170,000 to 700,000 years old.
However, research led by the Institut de Paléontologie Humaine finally seems to have provided a minimum age for the nearly complete cranium. In the study “New U-series dates on the Petralona cranium, a key fossil in European human evolution,” researchers presented new U-series dates performed on the calcite that grew directly on the skull to provide crucial information on the fossil’s age. U-series dating takes advantage of the natural properties of uranium isotopes, which break down into thorium at a precise half-life.
By measuring the ratio of uranium to thorium, it is possible to calculate when the process began, giving an atomically calibrated start date. If moisture collects on a cave wall, it can evaporate, leaving behind the trace of minerals and uranium isotopes without thorium, forming a layer of crusty residue, explained phys.org. Within layers of these crusty deposits, the uranium isotopes continue to break down into thorium, now locked into place by the mineral deposit in a closed system, giving each layer a set date at which the process began.
U-series dating then analyses the uranium-to-thorium ratios of the innermost deposit layers to determine when they first appeared, giving a minimum starting point for anything found beneath them.
The test results provided a finite minimum age of 286,000, plus or minus 9,000 years. If the skull initially remained dry or covered, it could have been in the cave much longer.
Researchers concluded that the Petralona hominin forms part of a distinct and more primitive group than Homo sapiens and Neanderthals. Their results support the coexistence of such populations alongside an evolving Neanderthal lineage in the later Middle Pleistocene of Europe.
The Petralona Cave skull was discovered by a villager, Christos Sariannidis, in 1960, stuck to the wall of a small cavern within the cave. Its lower jaw is missing, but the cranium is almost complete. It is similar to specimens discovered at Arago (France), Bodo (Ethiopia) and Kabwe (Zambia). All of these combine Homo erectus-like traits – prominent browridges, a ridge along the rear of the skull, and thick braincase bones – with other characteristics, including a somewhat larger brain of later Homo species, like Neanderthals and modern humans.
The fossil has been preserved at the Geology School of the Aristotle University of Thessaloniki since its discovery.
