Seventeen skulls unearthed in a Middle Pleistocene cave in Spain showcase a mix of traits: some Neanderthal features, some more primitive ones. The findings suggest that Neanderthals evolved their defining characteristics in stages, rather than all at once -- starting with their face and teeth early on. 

About 500,000 years ago, a group of ancient Homo split off from others living in Africa and East Asia, eventually settling in Eurasia. Once there, their populations evolved characteristics that would later define the Neanderthals. Several hundred thousand years later, modern humans (who arose in Africa) also settled in Eurasia. The details of this time remain controversial, though ultimately, modern humans replaced Neanderthals by around 30,000 years ago.

"What I have been telling people is that it was like Game of Thrones," says Juan-Luis Arsuaga of Joint Center UCM-ISCIII Evolution and Human Behavior in Madrid. "There were a few spread-out populations, some related, some not, emigrating or going extinct over time. And winter was always coming."

Excavated from a deep pit of bones at the Sima de los Huesos site in Atapuerca, Spain, these 17 intact skulls -- including seven newly described specimens -- belong to a single species dating back 430,000 years ago. They appear to have Neanderthal face and teeth, but everything else was associated with primitive human ancestors (such as Homo heidelbergensis), including a small braincase. 

But the fossils don’t belong to a common ancestor of theirs: They’re too young to be H. heidelbergensis, and they’re far too Neanderthal-like. And although Arsuaga and colleagues haven’t assigned them to a particular species yet, they conclude that these fossils are the oldest reliably dated proto-Neanderthals. 

The Sima population appears to have been part of an early European lineage that includes Neanderthals, but is more primitive than the later Pleistocene variety that we’re more familiar with. "We think based on the morphology that the Sima people were part of the Neanderthal clade," Arsuaga says in a news release, "although not necessarily direct ancestors to the classic Neanderthals." And definitely not an ancestor of ours. 

Judging from these skulls, facial modification was the first step in Neanderthal evolution. Their trademark features -- thick brow ridges with a double arch, robust lower jaw, small molars at the rear, and distinct teeth cusp patterns -- took shape first, then their other defining features came along later. Classic Neanderthals had braincases as big as, and sometimes bigger than, humans.

It’s important to note that the Neanderthal-derived features were related to chewing. "It seems these modifications had to do with an intensive use of the frontal teeth," Arsuaga explains. "The incisors show a great wear as if they had been used as a ‘third hand,’ typical of Neanderthals." They may gripped objects (like meat) with their teeth, allowing them to use one free hand to steady the object and the other to cut it with a tool. 

The findings point to a mosaic pattern of evolution, with different features evolving separately at different rates. The "Neanderthalization" process was more of an accretion, rather than a single linear sweep. 

Researchers have been exploring the 12-meter pit since the 1970s. With thousands of bones belonging to at least 28 individuals, it’s the largest trove of hominin fossils ever. 

The work was published Science this week. 

[AAAS via Science, New Scientist]

Images: Javier Trueba / Madrid Scientific Films