Ancient skulls recovered from a deep cave in northern Spain are the oldest known remains to show clear signs of Neanderthal facial features, researchers claim.

Scientists reconstructed 17 skulls from pieces of bone found in the mud at Sima de los Huesos, or the ‘Pit of Bones’, in the Atapuerca mountains. The skulls had some Neanderthal-like features, but their appearance was otherwise far more primitive.

Juan Luis Arsuaga, professor of palaeontology at the Complutense University of Madrid, said the remains belonged to a “missing link” population that fell somewhere between the Neanderthals and a more archaic group of human forerunners.

The term “missing link” has fallen out of favour with many researchers, in part because it implies a simple, step-wise progression from one species to another. But the phrase is still used at times to describe species that bridge a divide between distinct ancestors and descendants.

The skulls come from a haul of bones that belong to at least 28 individuals who came to rest in a chamber at the bottom of a 14-metre-deep cave shaft. The bodies are thought to have been washed into the pit after they died elsewhere in the cave system.

Measurements of the bones, which are around 430,000 years old, suggest that trademark features of Neanderthals did not emerge at the same rate, but that some evolved much earlier than others.

The skulls at Sima de los Huesos have Neanderthal-like teeth and jaw structures, and other similarities in the brow ridges and nasal apertures, or channels. But their braincases are small, unlike the elongated crania of the big-brained Neanderthals. Of the 17 skulls reported in Science, seven have not been studied before.

The Sima population, as they are known, probably developed Neanderthal-like jaws and teeth from chewing and the heavy use of their front teeth and incisors for other tasks. “We think it’s related to the use of their mouths as a ‘third hand’, or as part of their behaviour to grasp and to pull things with the front teeth,” Prof. Arsuaga said.

“We can’t say they are the direct ancestors of Neanderthals. All we can say is that the population are members of the Neanderthal lineage. They are a ‘missing link’ between the Neanderthals and a population that was much more primitive,” he added.

The Spanish team believe the more primitive population could be an ancient human species called Homo antecessor, which lived in Europe around one million years ago. “They could be the stem group before the split between Neanderthals and modern humans,” Prof. Arsuaga said.

Neanderthals emerged around 400,000 years ago, and lived in Europe and Asia until around 35,000 years ago. They were replaced — though not before some interbreeding — by modern humans that evolved in Africa and colonised Eurasia 50,000 years ago.

In previous reports, the Spanish researchers had claimed the Sima de los Huesos remains were much older, around 600,000 years old, and that they belonged to an ancient group called Homo heidelbergensis. The latest study changes both of those interpretations.

“They now agree that the fossils belong to the Neanderthal lineage but not to the species Homo heidelbergensis. And they have revised the dating of the fossils to about 430,000 years, giving much more substantial agreement between our views,” said Chris Stringer, head of human origins at the Natural History Museum in central London.

“The rich Sima de los Huesos material, with every part of the skeleton beautifully preserved, will continue to inform us about human evolution 400,000 years ago as research continues on this astonishing, and even beautiful, collection of human fossils,” Mr. Stringer said.

© Guardian News & Media 2014

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