Scattered on the floor of a cramped, claustrophobic cave chamber, they lay undisturbed for up to two million years in the pitch black before being discovered by a group of anthropologists.

Now the fossilised remains of a mysterious early human species called Homo naledi are causing a controversy over whether they had been deliberately placed within the cave.

If proven to be the case, it could have profound implications for the evolution of culture in early humans as burial of the dead was previously thought have emerged mainly in our own species.

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A recently discovered species of early human, Homo naledi (reconstruction pictured) may have buried its dead by dropping them into an inaccessible cave. However, new research has cast doubt on these claims and some researchers suggest the bones may have previously been in another area of the cave

Researchers who discovered Homo naledi in the Rising Star cave 30 miles (48km) from Johannesburg in South Africa, believe they may have disposed of their dead within the cave.

The alignment of some of the bones suggests they had fossilised where they had been lain just after death.

Scientists leading the excavation, which was published last September, suggested Homo naledi may have dropped their dead down a 'chute' from the surface into the cave.

WHAT WAS HOMO NALEDI? Analysis of a foot showed Homo naledi's feet share many features with a modern human foot, indicating it is well-adapted for standing and walking on two feet. However it differed in having more curved toe bones or proximal phalanges. Analysis of the hand revealed a unique combination of anatomy that has not been found in any other fossil human before. The wrist bones and thumb show anatomical features that are shared with Neanderthals and humans and suggest powerful grasping and the ability to use stone tools. However, the finger bones are more curved than most early fossil human species, such as Lucy's species Australopithecus afarensis, suggesting that H. naledi still used their hands for climbing in the trees. Advertisement

This would mean a primitive human – which stood a little under 5ft-tall (1.5 metres) and had a brain the size of an orange – may have had a far more advanced culture around death than believed possible.

But new research is suggesting the story may be even more complicated than the anthropologists could have imagined.

Analysis of the sediment and rock suggests there was never a direct opening to the underground fossil site from above.

Indeed, some of the bones found on top of those that were aligned as they would be in the body were far more jumbled, and most of the remains show signs of erosion.

A new theory published in the Journal of Human Evolution instead suggests the 1,550 bones found in the cave, belonging to at least 15 individuals, were brought into the cave through another entrance.

Dr Aurore Vale, an evolutionary biologist at the University of Witwatersrand in Johannesburg, South Africa, said the bones may have initially been in another part of the cave.

She suggests that water instead carried the bones and perhaps even body parts deeper into the cave to where they were found – known as the Dinaledi chamber.

She also points to modifications seen on the fossilised bones that have been attributed to beetles and snails such as the giant African land snail.

While the bones appear to have been damaged by these creatures, they would not have been found living so deep inside the caves as there was no vegetation there for them to live on.

The bones were located in the Rising Star cave system in South Africa's Gauteng province in a remote chamber that can only be accessed via several steep climbs and fissures (arrangement of fossils pictured)

Researchers said the bones showed signs they had been damaged and moved by snails and beetles, but these creatures would only have existed closer to an entrance of the cave. This suggests the bones may have once been closer to the surface and were buried in a collapse or swept there by water

HOMO NALEDI: WHAT WE DO - AND DON'T - KNOW ABOUT THE SPECIES Named Homo naledi, the species has been assigned to the genus Homo, to which modern humans also belong. It is not yet clear how more than a dozen H. naledi skeletons - ranging from babies to elderly individuals - ended up in a remote region of a large cave. The researchers have not ruled out the possibility of the bodies having been disposed of in the cave deliberately, or a catastrophic 'death trap' scenario in which the humans entered the cave and all died of an unknown cause. The age of the the skeletons are also yet to be determined. 'Some of Homo naledi's features, such as its hands, wrist and feet, are very similar to those of modern humans. 'On the other hand, the species' small brain and the shape of its upper body are more similar to a prehuman group called australopithecines'. Advertisement

Writing in the journal, she said: 'This raises serious questions about the veracity of the hypothesis that fresh, complete bodies were deliberately disposed into the Dinaledi Chamber.'

The fossils were originally discovered by cavers exploring the subterranean Rising Star caves when they stumbled across part of a skull and some bones in 2013.

Professor Lee Berger, a palaeoanthropologist at the University of Witwatersrand, then led a team to excavate the fossils.

However, the chamber in which they were found was so small and the entrance so tight that Professor Berger had to recruit a group of young women who were small enough to get inside.

Together they filmed, photographed and documented the whole process while Professor Berger directed from the surface.

In total they recovered 1,550 bones which they identified as an entirely new species.

Attempts to date the bones accurately have been difficult and estimates range from 20,000 to two million years old.

Homo naledi has been found to share many features similar to the early Australophithecus species that lived more than three million years ago.

But it also had some distinctive features seen in the Homo species.

Analysis of the bones (pictured) indicates the extinct species may have been uniquely adapted for both tree climbing and walking, while also being capable of precise manual manipulation

There are already some clues about how these early humans may have lived.

Researchers who worked on the excavation say Homo naledi appeared to have unique shoulder features.

Unlike other species of Homo naledi's shoulder blades appear to have been positioned low behind the chest, which would have been better suited to climbing trees than running, according to ScienceNews.

Homo naledi's hands were also built for climbing and gripping stones according to research published in October last years.

H. naledi's overall body plan is illustrated right and an how it compares to Homo species such as H. erectus (centre) and australopithecines such as Lucy (far left)

Researchers who worked on the excavation say Homo naledi appeared to have unique shoulder features. Scientists say that many more examples of the new species may lay uncovered in the remote cave system

Homo naledi may shed light on the transition from australopithecines (fossil pictured) - that emerged around four million years ago - to humans. Some of Homo naledi's features, such as its hands, are very similar to those of modern humans. But the species' small brain are more similar to a prehuman group

A reconstruction of Homo naledi's lower body has also allowed researchers to reconstruct how it would have walked.

In a paper presented at the Annual Meeting of the American Association of Physical Anthropologists in Atlanta earlier this month, Professor Berger and his colleagues said they would have had a bipedal stride much like humans.

Professor Berger said: 'From mid-thigh down it looks like a human - long legs and human like feet.

'It is really a combination we haven't seen before in the fossil record.'

Speaking at the American Association of Physical Anthropologists, his colleague Zach Throckmorton added: 'These characters suggest Homo naledi was well-adapted to a striding bipedal gait.

Peering inside the cave: The chamber, situated down a narrow 40ft (12 metre) chute, measures around 30 feet (9 metres) long and just a few feet wide, National Geographic reported

'Given the anatomy of the upper limb, Homo naledi demonstrates co-existence of both bipedalism and climbing adaptations in one hominin taxon.'

However, following their discovery, perhaps the most controversial claim made by Professor Berger and his colleagues was what Homo naledi were doing in the cave.

Speaking at the time of the announcement in September last year, Professor Berger said: 'This species of non-human hominin was deliberately disposing of its dead.

'Taking a dangerous journey into this deep chamber to place its dead or drop its dead into a place that was inaccessible.

'This is something that prior to this we thought was unique to humans and perhaps identified us but now it doesn't.'

However, many anthropologists expressed doubt at these claims.

Scientists said that the mixture of features in H. naledi (pictured) highlights the complexity of the human family tree and the need for further research to understand the history and ultimate origins of our species

Professor Francis Thackery, an evolutionary biologist also at the University of Witwatersran, told Talk Radio 702 that his research suggests there may have been another entrance to the cave.

He believes the bones may have been placed closer to this entrance and then became sealed in by a roof collapse or perhaps even living individuals became trapped by the rock fall.

He said: 'To carry these 15 carcasses in complete darkness through a very narrow route in complete darkness raises questions.

'I think in the past there could have been a second entrance.

The bones were located in the Rising Star cave system in South Africa's Gauteng province, part of the Cradle of Humankind World Heritage Site, in a remote chamber that can only be accessed via several steep climbs

'Dolermite rock that is quite soluble in geological time.

'There could have been a second opening and there was a collapse and these 15 individuals had been trapped.'

He added he had studied the bones themselves and found black spots on them that he believes are marks of manganese dioxide left by lichen that grew on them in the past.

This, he believes, suggests the bones may have been in a part of the cave that was closer to an entrance where there was light as lichen needs sunlight to grow.

However, speaking to MailOnline, Professor Berger said that regardless of whether there was another entrance to the cave, it still would have been difficult for the remains to get in there.

He said there are no other remains of animals in the cave, which means the Homo naledi were using it almost exclusively. He added the evidence for lichen growth was also still speculative.

He said: 'Whatever the entrance was, it would have to conform to the same restrictive criteria - only allowing one species of animal in, over time, while not allowing externally derived sediments into the cave and restricting access to predators and scavengers.

'So if there were another entrance, it would have to be nearly if not equally as restrictive as the one we use now.

'I can understand the discomfort - we have spent more than a hundred years of this field seeing the unique behaviours of humans as being due to a big brai.

'Its not going to be easy to convince everyone that complexity is not driven by a single organs size alone - despite the evidence.'