A skull found in the Petralona Cave near Thessaloniki, Greece, that has kept researchers guessing for decades, has been dated as circa 300,000 years old and classified as neither early human nor Neanderthal.
A study published in the Journal of Human Evolution used uranium-series dating to show that calcite on the Petralona skull began forming 286,000 ± 9,000 years ago, setting a firm minimum age and narrowing earlier estimates that ranged from about 170,000 to 700,000 years; the team from the Institut de Paléontologie Humaine in France added that the skull could be older than 300,000 years if calcite growth started later and said there was high confidence the calcite was at least 277,000 years old, reported Live Science.
Researchers dated the calcite by measuring the decay of uranium isotopes into thorium, a method suited to caves where seepage leaves calcite deposits that contain uranium but initially no thorium; the uranium-to-thorium ratio then yields an age because uranium decays at a fixed half-life.
For the Petralona work, scientists collected samples from the calcitic layer on the cranium and from several cave structures, including a hall called the Tomb; records of the discovery were poor, but evidence indicated the skull was stuck to a wall by calcite encrustations in the Mausoleum Chamber. The dating marked only when calcite began to form, so if the skull initially remained dry or covered, it might have lain in the cave longer before the first calcite crust formed. “It likely didn’t take long for a first crust-like layer to form,” said Chris Stringer, a paleoanthropologist at the Natural History Museum in London and co-author of the study.
The fossil was discovered in 1960 when a villager was exploiting Petralona Cave about 22 miles southeast of Thessaloniki. The nearly complete cranium was found protruding from a chamber wall, embedded in rock, covered in calcite, and missing its lower jaw—factors that complicated classification and earlier dating efforts.
Morphologically, the skull showed traits from both Neanderthals and early Homo Sapiens but did not fit either group and may belong to an unknown species, possibly Homo Heidelbergensis. “The Petralona fossil is distinct from H. Sapiens and Neanderthals, and the new age estimate supports the persistence and coexistence of this population alongside the evolving Neanderthal lineage in the later Middle Pleistocene of Europe,” said Stringer.
Referring to the Kabwe skull from Zambia, Stringer said “that fossil is closely comparable to the Petralona one, and I would classify them both as Homo heidelbergensis”. He added that the skull’s size and robustness indicated it was almost certainly male, and its teeth showed moderate wear consistent with a young adult.
“Assigning an age to the Petralona cranium is of outstanding importance because this fossil has a key position in European human evolution,” the study authors wrote, according to LadBible.
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