Skulls uncovered in a Spanish cave dubbed the "Pit of Bones" may have been those of early ancestors of Neanderthals and may yield clues to how early humans evolved, researchers say.
Believed to be around 430,000 years old, the skulls display both Neanderthal-like features and physical traits usually found in more primitive types of humans, paleontologists say.
The finding suggests Neanderthals might have "developed their defining characteristics separately, and at different times -- not all at once," the researchers report in the journal Science.
It may provide answers to questions about human-like primate creatures known as hominids in the Middle Pleistocene era between 400,000 and 500,000 years ago, they said.
"We think that a 'Game of Thrones' scenario probably describes hominid evolution in Eurasia and Africa," researcher leader Juan-Luis Arsuaga of Madrid's Complutense University said. "As in the famous serial, there was never a unified and uniform European Middle Pleistocene kingdom but a number of 'houses,' living in different regions and often competing for land.
"Some of the big houses were closely related. All of the houses were much more differently related," he said.
The generally accepted evolutionary path assumes some early human groups split off from others in Africa and in East Asia and moved into Eurasia, where they evolved into the Neanderthals.
They were followed into Eurasia hundreds of thousands of years afterward by modern humans, who had evolved while still in Africa.
While the separate species -- Neanderthal and modern humans -- interacted and even successfully mated, modern humans would eventually dominate as the Neaderthals died out.
However, the Neanderthals left a genetic legacy still found in modern peoples of the world.
The Spanish skulls exhibit typical Neanderthal features in their jaws and teeth and in their facial structures but with brain cases that look more primitive, like earlier hominids.
The suggestion is evolution may have happened with this species in separate and distinct phases, with bigger brows and jaws first, followed by bigger brains later.
And while they are apparently in the Neanderthal lineage they may not be direct ancestors of that particular hominid range, the researchers suggest.
The Spanish site known as Sima de los Hueses (Pit of Bones) in the country's Atapuerca Mountains has been excavated since 1984 and has yielded some 7,000 human fossilized bones.
At least 28 individual hominids have been identified among the fossil collections, researchers say.
Most were young adults when they died, they say, and the pit may have served as a burial site with the bodies thrown into it, possibly with ceremony but possibly not.
Of the 17 skulls from the site analyzed for the study, some had been examined before, but seven were being looked at for the first time, the researchers said.