Homo naledi, your recently discovered human relative

The discovery of hundreds of Homo naledi fossils was the largest such find ever made on the African continent. The fossils display a unique mix of modern and archaic traits and are shaking up our understanding of the origins and diversity of our human lineage.

Homo naledi highlights, once again, that we can't think of human evolution in terms of ape-like ancestors gradually evolving more modern features in a linear fashion. Instead, multiple human species evolved in parallel and coexisted, sometimes side-by-side.

Many mysteries surround Homo naledi, including how the remains got into the caves, what its tools were like, and how it survived alongside bigger-brained species.

Where has Homo naledi been found?

So far Homo naledi fossils have only been found in South Africa's Cradle of Humankind World Heritage Site, about 40 kilometres from Johannesburg.

Homo naledi discovery: excavations deep within a cave

The first Homo naledi finds were discovered by cavers in a remote, almost inaccessible chamber deep within the Rising Star cave system.

Palaeoanthropologist Lee Berger assembled a team of excavators who proceeded to recover more than 1,500 fossil bones belonging to at least 15 individuals, ranging from infants to elderly adults.

This video by Wits University (featuring recreational caver Rick Hunter who discovered the chamber) highlights just how difficult to reach the fossils were, and why the initial team sent to search for fossils was made up of smaller women:

A subsequent search of a second chamber more than 100 metres away in the same cave system yielded a further 130 fossils.

Puzzling combination of features

Homo naledi displayed a unique combination of human and non-human traits throughout its skeleton.

Prof Stringer explains, 'Some of Homo naledi's features, such as its hands, wrist and feet, are very similar to those of modern humans and Neanderthals.

'Other characteristics are much more primitive. The species' small brain and the shape of its upper body are more similar to the pre-human australopithecines and the very early human species Homo habilis.'

The creature's curved fingers and hip joint also more closely resembled those of australopithecines and H. habilis.

The teeth increase in size towards the back of the mouth, a primitive characteristic, but they are relatively small and simple and set in light jawbones.

'The mixture of traits of Homo naledi highlights once again the complexity of the human family tree,' says Prof Stringer.

Together, H. naledi's anatomy suggests it walked on two legs with a modern gait and an efficient long-distance stride. Its shoulder position and shape of its fingers would have helped it climb and hang from trees and could be traits retained from a more ape-like ancestor.

Where did Homo naledi live?

Because H. naledi is currently only known from two sites within a single cave system, it is unclear whether the species lived only in southern Africa or was more widespread.

If it was more widespread, scientists might need to re-examine other diminutive fossils from across Africa that have often (and perhaps wrongly) been attributed to a small form of Homo erectus, another early human species.

The surprisingly young age of Homo naledi

It took a while for the Homo naledi remains to be dated. The age published in 2017 took scientists by surprise: they were between 236,000 and 335,000 years old.

'This is astonishingly young for a species that still displays primitive characteristics found in fossils about two million years old,' says Prof Stringer.

Despite the young date for the H. naledi fossils, their anatomy suggests that in evolutionary terms they could lie close to the origin of the genus Homo, suggesting that this is a relic species, retaining many primitive traits.

Importantly, the young age means Homo naledi was living on the African continent with a number of other bigger-brained humans, including our own species Homo sapiens.

How did the bones get into the cave?

How Homo naledi remains got to their final resting place is a matter of active debate.

The Dinaledi Chamber where they were found is located deep within the cave system - around 80 metres from any known entrance or opening to the surface - and must have always been in in darkness. 

The discovery of at least three more individuals in the Lesedi Chamber, equally remote and dark and over 100 metres away, further adds to the mystery.

There is no evidence to suggest the humans ever lived so deep in the caves. So how did the bones get there - were they deposited there by natural means or other humans?

The authors of the first Homo naledi studies rejected scenarios such as water carrying the bones there or predators dragging them there. Instead, they favour the idea that other members of the species disposed of bodies in the caves intentionally and repeatedly.

Prof Stringer isn't convinced: 'Many experts, including me, consider such complex behaviour unlikely for a creature with a brain size close to that of a gorilla, particularly when a requirement for the controlled use of fire, for lighting, probably has to be added in.

'I think alternatively it's possible that, now and again, individuals sought refuge deep in the cave to escape from predators or other humans. Without light and climbing equipment, once deep in the cave, there may have been no way back. Over hundreds or thousands of years, this behaviour could have led to an accumulation of bodies deep in the cave.'

What kind of tools did Homo naledi make?

This is another of the mysteries associated with this species.

Homo naledi's hands and wrists were well-suited to tool making but no stone tools have yet been found associated with its fossils. However, Prof Stringer thinks it is 'highly likely that naledi's handiwork is already present in the archaeological record we have for southern Africa, but it is currently unrecognised or attributed to another human species'.

How did Homo naledi survive against bigger-brained competition?

The teeth and lower body skeleton suggest Homo naledi may have had a lifestyle and diet similar to other hunter-gatherers present elsewhere in Africa at the same time, such as late Homo heidelbergensis and early Homo sapiens.

Although we do have another example of a small-brained species surviving until relatively recently - the diminutive Homo floresiensis in Indonesia - that probably survived because it was isolated on an island.

'Perhaps Homo naledi was relatively geographically isolated for much of its evolutionary history,' says Prof Stringer.

'Or did it avoid competition by continuing a different niche for feeding and lifestyle, allowing it to survive longer? It could have spent more time feeding in and around trees, as might be indicated by its upper body skeleton and curved fingers.'

Small but human-like brain

Recent research, published in the journal PNAS in 2018, examined imprints Homo naledi's brain made on its skull. The findings, say the authors, indicate that the evolution of brain size in humans was not a simple pattern of gradual increase over time.

The team found that, despite the difference in size, the structure of Homo naledi's frontal lobe was similar to that of other human species rather than that of non-human hominins or great apes.

Other parts of the brain also showed human-like changes compared to more primitive australopithecines with a similar brain size.

The authors point out that the areas of the brain found to be similar to other humans have been implicated in the evolution of tool use, language and social behaviour and may mean Homo naledi shared some behaviours present in other humans, despite a much smaller brain size.

This article includes information from Our Human Story by Dr Louise Humphrey and Prof Chris Stringer.