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The biggest collection of ancient human fossils ever recovered from one site is causing scientists to reconsider the path of human evolution.

They reveal how Neanderthals, our sister species, developed their distinctive teeth before their brains, using them as a third hand for holding objects.

The new study adds to theories that the Neanderthals evolved their characteristic looks slowly, and intermittently, over hundreds of thousands of years.

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Neanderthals' trademark facial features took shape as a first step in their evolution, while their other defining features came along later, and not all at once, researchers have reported

The ancient skulls, dating back 400,000 years, show Neanderthal-like features in the face and teeth, while other parts of the skulls, including the brain case, resembled those of more primitive human ancestors.

This suggests the earliest Neanderthals used their jaws in a particular way for chewing as well as for holding objects.

The discovery at the Sima de los Huesos site in Spain's Atapuerca Mountains has allowed scientists to better understand pre-human evolution during the Middle Pleistocene period - a time in which the path of hominin evolution has been controversial and fiercely debated.

Juan-Luis Arsuaga, a paleontologist at the Complutense University of Madrid, claims the findings were consistent with a 'Game of Thrones' evolutionary saga.

The discovery of the skulls at the Sima de los Huesos site in Spain's Atapuerca Mountains has allowed scientists to better understand pre-human evolution during the Middle Pleistocene period - a time in which the path of hominin evolution has been controversial and fiercely debated

Juan-Luis Arsuaga, a paleontologist at the Complutense University of Madrid claims the findings were consistent with a 'Game of Thrones' evolutionary saga. A scene from the TV series is shown here

In this scenario communities of humans, equivalent to 'houses' in the sci-fi series, would compete for habitable areas more than 400,000 years ago.

Study lead author Juan-Luis Arsuaga, said: 'The Middle Pleistocene was a long period, of about half a million years, during which hominin evolution didn't proceed through a slow process of change with just one kind of hominin quietly evolving towards the classic Neanderthal.'

Co-author of the study published in Science, Ignacio Martinez, Professor of Paleontology at the University of Alcalá, added: 'With the skulls we found it was possible to characterise the cranial morphology of a human population of the European Middle Pleistocene for the first time.'

Around 400,000 to 500,000 years ago, archaic humans split off from other groups of that period living in Africa and East Asia, ultimately settling in Eurasia.

Once settled, they evolved characteristics that would come to define the Neanderthal lineage.

Several hundred thousand years later, modern humans - who had evolved in Africa - settled in Eurasia.

They interbred with the Neanderthals, but showed signs of reproductive incompatibility.

Because of this, modern humans eventually replaced Neanderthals.

The degree of divergence between Neanderthals and modern humans over such a short period of time has long puzzled scientists.

It has, until now, been difficult to fill in the gaps because the European fossil record is isolated and dispersed. However, samples at the Sima de los Huesos site are different.

Professor Arsuaga said: 'What makes the Sima de los Huesos site unique is the extraordinary and unprecedented accumulation of hominin fossils there; nothing quite so big has ever been discovered for any extinct hominin species-including Neanderthals.'

Professor Martinez added: 'This site has been excavated continuously since 1984.

Pictured is the discovery of one of the skulls in Spain's 'Cave of Bones'. Ignacio Martinez, Professor of Paleontology at the University of Alcalá, said: 'With the skulls we found it was possible to characterise the cranial morphology of a human population of the European Middle Pleistocene for the first time'

The ancient skulls (a specimen pictured) show a mix of Neanderthal and more primitive traits, suggesting Neanderthals evolved their defining characteristics in stages. This so-called 'mosaic pattern' of development supports a theory of Neanderthal evolution that suggests they developed their features separately

The Atapuerca Mountains, where the skulls were unearthed, is in the Spanish province of Burgos and near Atapuerca and Ibeas de Juarros

'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.'

The 17 skulls belong to a single population of a fossil hominin species. Some of have been studied before, but seven are presented anew here, and six are more complete than ever before.

With these intact samples at their fingertips, the researchers made progress characterising defining features.

The researchers' skull samples showed Neanderthal features present in the face and teeth, but not elsewhere.

Crucially, many of the Neanderthal-derived features the researchers observed were related to chewing, and possibly holding objects with their teeth as a 'third hand.'

Professor Arsuaga said: 'It seems these modifications had to do with an intensive use of the frontal teeth.

'The incisors show a great wear as if they had been used as a third hand,' typical of Neanderthals.'

He added: '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 - it's like finding a treasure.' 

On site: Prof Juan Luis Arsuaga excavating skull 17

Around 400 to 500 thousand years ago, archaic humans split off from other groups of that period living in Africa and East Asia, ultimately settling in Eurasia. Once settled, they evolved characteristics that would come to define the Neanderthal lineage. Pictured is what a Neanderthal may have looked like

A reconstruction of one of the skulls is shown here. The extraordinary collection includes 17 fragmentary skulls, many of which are almost complete

Pictured is australopithecus afarensis - an extinct hominid that lived between 3.9 and 2.9 million years ago. The Middle Pleistocene was a long period, of about half a million years, during which hominin evolution took place in different stages rather than being a gradual process