Human evolution in Europe may have been far messier than the tidy family tree in textbooks suggests. A fragment of a parietal bone — one of the bones forming the sides and top of the skull — discovered at the Ruidera site in central Spain, has become the first human fossil ever recovered from the locality, and its anatomy is puzzling paleoanthropologists.

Labeled RVH-1, the specimen was found embedded in rock breccia at the Middle Pleistocene locality of Ruidera, in the Ruidera Lakes Natural Park in Castilla-La Mancha. Using uranium-series dating and optically stimulated luminescence, the team estimated the fossil-bearing layer to be up to 224,000 years old.

"It was an incredible and fortunate surprise," said Dr. Mario Modesto-Mata, a paleoanthropologist at CENIEH and the International University of La Rioja. "We went through a range of emotions, from initial skepticism and disbelief to immense joy once it was confirmed that it could only be human."

Anatomically, RVH-1 stands out for its relative thickness and pronounced angle at the temporal line, a ridge on the side of the skull. Both measurements place it toward the upper end of the range seen in other Middle Pleistocene European fossils, similar to the archaic Ceprano cranium in Italy and the Caune de l'Arago fossils in France.

"The unusual anatomy of RVH-1 indicates that hominin populations with unexpected characteristics were still living in the Iberian Peninsula at that time," said Dr. Daniel García Martínez of the Complutense University of Madrid. "These traits had been inherited from ancestral lineages and persisted until relatively late in the Middle Pleistocene."

The team deliberately refrained from assigning the fragment to a specific species, classifying it simply as Homo sp. while noting morphological similarities to Homo heidelbergensis. "The discovery reinforces the idea that human evolution in Europe was not a linear process, but rather a complex scenario in which different populations with distinct evolutionary histories coexisted," said Dr. Carlos Palancar of the National Museum of Natural Sciences.

The paper was published in the Journal of Human Evolution.

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