Virtual reconstruction of the Aroeira 3 cranium as seen from the front (A), back (B), top (C), and side (D). Image Credit: Zilhão et al. in PNAS, 2017

When archaeologists were digging up the dirt floor at the Aroeira cave in central Portugal in 2014, they were already well on their way to understanding the lifestyle of hunter-gatherers who took shelter there during the Middle Pleistocene.

The researchers had found Acheulean handaxes—the most advanced technology of the day—that were used for cutting, scraping, and butchering. They also found burnt deer bones, suggesting that whoever camped out at this cave 400,000 years ago had the know-how to make fires—then a relatively new human achievement.

Then, at the end of the field season, the excavators unexpectedly struck paleoanthropological gold. They were using demolition hammers to break through the cave’s hard fragmented rock when they chipped a piece of bone.

“Immediately we knew we were dealing with a human fossil,” said University of Barcelona archaeologist João Zilhão, the director of the excavation. His team removed a block of the sediment surrounding the fossil, and preparators in Madrid spent two years carefully revealing a human cranium encased inside. Their findings are reported in the current issue of the Proceedings of the National Academy of Sciences.

It’s exceptionally rare to find human remains from this period. It’s even more rare to find them in a good archaeological context, among artifacts like tools and animal bones. So the discovery of this skull—the oldest ever found in Portugal—is exciting for some paleoanthropologists.

Below, you can see the researchers' discovery, removal, cleaning, and finally restoration of the ancient cranium.

Zilhão et al. in PNAS

The prehistoric skull from Aroeira shares some similarities with human fossils of the same era found in Spain, France, and Italy—including some fossils thought to be early Neanderthals, which were just starting to emerge in Europe, and some fossils that had been attributed to another species, Homo heidelbergensis. But the new specimen doesn’t fit neatly into a species category, and could help researchers argue for a diversity of human populations in Europe before the rise of modern humans.

Mental_floss spoke to a trio of experts in human evolution for their take on the current study, which they were not involved in. “This find adds more complexity to the European picture, as we are also seeing in Africa and Asia,” Chris Stringer, a paleoanthropologist at London’s Natural History Museum, says.

University College London paleoanthropologist Maria Martinón-Torres sees the skull as a potential ancestor to the Neanderthal. “I think this finding obliges us to abandon the idea that Neanderthal evolution is a single and lineal process,” she says.

She noted that there are several fossils from similar mid-Pleistocene sites, such as Sima de los Huesos in Atapuerca, Spain and Arago in France, that share some Neanderthal traits but have very different combinations of those traits.

“We see now that there is no linearity in the combination of these traits,” Martinón-Torres says. “This is probably due to the fragmentation and isolation of these populations due to the hard climatic conditions in Europe at that time.” She said that when climatic conditions are harsh, populations can get isolated and evolve their own particular distinctive features. Then, when conditions get better, these groups meet and recombine again. “So for a long period we will find a Neanderthal-like but highly variable population,” she says.

Bence Viola, a paleoanthropologist at the University of Toronto, notes that with the addition of the Aroeira skull we now have a few different humans fossils that look quite different from one another despite being from the same period. He says the new skull seems more primitive than the fossils found nearby at Sima de los Huesos, which, ancient DNA results have shown, represent early Neanderthals. “This cranium makes this scenario of having several different populations in Europe in the middle Pleistocene seem likely,” says Viola, who adds that he would be very interested to see what this specimen’s DNA looks like.

Stringer points out that the skull would be even more illuminating if it weren't missing some key parts, including the back of the cranium: “Unfortunately, this new fossil, interesting as it is, doesn’t have the most useful bits to put it into this puzzle."