Skulls found in a Spanish cave exhibit both Neanderthal and primitive human features, according to a new study published in the journal Science.

The discovery provides clues about when the common ancestor of modern humans and Neanderthals lived, what happened after the two groups diverged, and how the two became so different over a relatively short period of time.

There is consensus about the ending of the story: Modern humans and Neanderthals  interbred, and Neanderthal DNA is still present in people of European and Asian ancestry. But the thousands of years before they connected, however, have been a mystery.

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That’s where the Spanish excavation site comes in. Sima de los Huesos in Atapuerca holds the largest collection of ancient human fossils recovered from a single site.

“What makes the Sima de los Huesos site unique is the extraordinary and unprecedented accumulation of hominin (human family tree) fossils there,”  lead author Juan-Luis Arsuaga, a professor of paleontology at the Complutense University of Madrid, said in a press release. “Nothing quite so big has ever been discovered for any extinct hominin species — including Neanderthals.”

“This site has been excavated continuously since 1984,” added co-author Ignacio Martínez. “After 30 years, we have recovered nearly 7,000 human fossils corresponding to all skeletal regions of at least 28 individuals. This extraordinary collection includes 17 fragmentary skulls, many of which are very complete.”

Using multiple, independent dating techniques, the researchers determined that the common ancestor of modern humans and Neanderthals lived more than 430,000 years ago. This alone is significant, the authors write: “With this new age, the SH hominins are now the oldest reliably dated hominins to show clear Neandertal (features).”

Other studies support that between 400,000 to 500,000 years ago, ancient humans lived in Africa and East Asia. Some split off, ultimately settling in Europe and Asia. Others remained in Africa.

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The separation between the groups resulted in key physical differences, which the researchers believe evolved separately, and at different times — as opposed to all at once, as some other scientists have proposed. Neanderthals found at other locations, for example, have a shorter and stockier build, angled cheekbones, prominent brow ridges and wide noses.

The skulls from Spain suggest that these individuals had brains comparable to those of more primitive humans, but evolution led to changes in their face and teeth. The observed features were mostly related to chewing.

“It seems these modifications had to do with an intensive use of the frontal teeth,” Arsuaga said. “The incisors show a great wear as if they had been used as a ‘third hand,’ typical of Neanderthals.”

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Even today, we often use our teeth to cut or manipulate objects. These early humans frequently used their teeth in this manner, and it affected their evolution.

Another fascinating twist is the diversity of the skulls. As Arsuaga said, “One thing that surprised me about the skulls we analyzed is how similar the different individuals were. The other fossils of the same geological period are different and don´t fit in the Sima pattern. This means that there was a lot of diversity among different populations in the Middle Pleistocene (around 400,000 years ago).”

This Spanish fossil trove could help answer questions about Neanderthal and other early human anatomy, behaviors, interactions with Homo sapiens, and how all of these help to explain human populations today.

“Finding a single tooth is a great success in any other site of comparable age, so imagine what it is like to painstakingly reconstruct 17 skulls,” Arsuaga said. “It’s like finding a treasure.”

Photo: Skull from the Sima de los Huesos site. Credit: Javier Trueba, Madrid Scientific Films