Homo naledi

The remains of at least 15 individuals were found in the Rising Star cave system in South Africa and announced as a new human species in 2015. The remains are the largest assemblage of a single hominin species yet discovered in Africa. Homo naledi combines primitive with modern features and is not a direct ancestor of modern humans.

Background of discovery

Age

The remains date to between about 335,000 and 236,000 years ago. This does not represent the timespan for this species, merely the age for a limited number of fossils. It is likely that this species first appeared much earlier, possibly as even 2 million years ago.

Different dating techniques were used to determine a time range. The minimum age of 236,000 years came from optically stimulated luminescence and uranium series dating on cave formations. The maximum age of 335,000 years came from uranium series and electron spin resonance dating on three teeth.

The dates were not announced in the original publication in 2015, but in a separate paper in 2017. The lack of easily dated sediments or other fossil remains in the cave made an initial assessment difficult. The team waited until after the skeletal analysis was completed because the dating process required material from teeth, hence damaging remains required for the original description. The dates were a relative surprise considering the many primitive characteristics of the skeleton.

Important fossil discoveries

In 2013, two recreational cavers Rick Hunter and Steven Tucker discovered bones in the Dinaledi cave of the Rising Star cave system, 50 kilometres northwest of Johannesburg. They were part of a ‘scouting’ team, reporting possible interesting finds to palaeoanthropologist Lee Berger of the University of the Witwatersrand, South Africa. Two short expeditions in 2013 and 2014 – carried out by small and very lean excavators with another above-ground team – recovered 1550 bones and teeth of 15 individuals, including juveniles, infants and adults.

Hunter and Tucker also found 131 remains of at least two adults and a juvenile in the nearby Lesedi cave in 2013 while exploring during the Dinaledi excavations. These finds were published in a separate paper in 2017.

Both Dinaledi and Lesedi caves are almost 100 metres from the entrance and almost 30 metres underground. They are only accessible through a series of extremely narrow passages. The Dinaledi cave takes about 30 minutes to reach from the entrance.

With the number of individuals, and the sexes and age groups represented, the find is the richest assemblage of associated fossil hominins ever discovered in Africa. As only small parts of both caves were excavated, many more bones remain for future expeditions to recover.

Attempts to extract DNA from the H. naledi remains have so far been unsuccessful as the DNA was too degraded. It was hoped that DNA extraction would be possible as the remains are only partially fossilised.

Key specimens:

• Dinaledi Hominin 1 (DH1); this is the holotype or type specimen and comprises the partial calvaria (skull cap), partial maxilla, and nearly complete mandible of a presumed male individual
• DH2 cranial paratype
• DH3 cranial paratype with partial mandible. Relatively old and probably female individual with extreme tooth wear
• DH4 cranial paratype
• DH5 cranial paratype
• Dinaledi Hand (H1): this nearly complete right hand was discovered in articulation and all bones are represented except for the pisiform.
• LES1: relatively complete skull found in Lesedi chamber of a male nicknamed Neo (‘gift’ in the Sesotho language). Brain size of about 610ml. Associated post-cranial remains from locality 102a probably belong to the same individual and form a partial skeleton with the skull.

Distribution

All known specimens have been found in Dinaledi and Lesedi chambers in the Rising Star cave system some 50 kilometres northwest of Johannesburg, South Africa.

What the name means

Homo is a Latin word meaning ‘human’ or ‘man’. This is the same genus as modern humans and shows the close relationship between this species and our own.

The word naledi means ‘star’ in the local Sotho language, in reference to the cave in which the remains were first found.

Relationships with other species

While acknowledged as belonging in the Homo family, it is difficult to position this species in relation to other family members because of the mosaic of similarities to and differences from known hominins. It is not considered a direct ancestor of modern humans.

Two scenarios have been proposed by the discoverers:

1. H. naledi belongs near the base of the Homo family. It has a number of primitive features in its anatomy and is most similar to early Homo species like H. habilis, H. rudolfensis and H. erectus. This scenario would mean that H. naledi evolved about 2 million years ago and survived, unchanged, for a long period. This is not unheard of (see Homo floresiensis).
2. H. naledi had a more modern ancestor and is more closely related to modern and archaic humans, making H. erectus a more divergent branch on the Homo family tree.

The lack of fossils from any other locations or time periods means the debate is unlikely to be resolved any time soon.

Given the relatively late date for the remains, it is possible that some archaic human populations existed in the same region as H. naledi – for instance, the Kabwe remains from Zambia and the Florisbad skull from South Africa. However, we can’t be sure they lived in close proximity or were in contact with each other. It is also possible that H. naledi met modern humans, who first appeared in southern and eastern Africa about 200,000 years ago. Perhaps modern humans even contributed to H. naledi’s extinction? This is an interesting scenario that requires more research and evidence.

Key physical features

H. naledi’s skeleton displays a unique mix of characteristics that were completely unexpected. The shoulders, hips and torso retain primitive features more like those of australopithecines. The lower body, cranium and teeth show more humanlike adaptations related to functions such as locomotion, hand-use and food processing. Several features are not known in any other hominin species.

Body size and shape

• adult males were about 150cm or 5 foot tall and weighed about 45kgs. Females were slightly shorter and lighter. Sexual dimorphism was limited.
• upright stance and bipedal locomotion
• the upper body was structurally more primitive than other Homo species and more australopith-like. For instance, the ribcage was relatively broad like that of A. afarensis.

Brain

• relatively small brain size only slightly larger than a chimpanzee’s – about 560cc for males and 465cc for females
• impressions suggest that despite its tiny size, H. naledi’s brain was similar in shape and structure to modern human brains, which are three times as large. The frontal lobe was more similar to a modern human’s than an ape’s. H. naledi had an asymmetrical brain, which is also seen in modern humans.

Skull

• the overall skull shape is unique but is most similar to early Homo species like H. erectus and H. habilis.
• some sagittal keeling (thickening of bone on the midline of the front part of the skull; not to be confused with sagittal crests) on the cranium. Keels occur in some species like H. erectus, and occasionally in H. heidelbergensis, but are very rare in modern humans, probably due to the trend of overall thinning of cranial bones.
• Homo features in the skull include a flat midface and little post-orbital constriction (narrowing of skull behind the eye sockets)
• well-developed supraorbital torus (brow ridge), although with a weak arch
• lacked the distinctive long and low cranial vault of H. erectus.
• the back of the skull was curved sharply, similar to a modern human’s

Jaws and teeth

• features of the jaws and teeth are mostly consistent with the genus Homo and have Homo adaptations for a high-quality diet
• lack derived features of later Homo
• premolar roots had primitive features
• primitive dental proportions in that the third molar is larger than the other molars, found in australopithecines and early Homo but not later Homo species
• teeth are small compared to early Homo species such as H. habilis, H. rudolfensis and H. erectus and similar in dimensions to later Homo species, including modern humans.
• molar crowns were small with five cusps, like modern humans
• incisors are similar in size to Australopithecus but canines, molars and pre-molars are notably smaller
• has smaller, but higher-crowned and more wear-resistant teeth than Australopithecus and Paranthropus.

Limbs

• primitive shoulder position configured largely like australopithecines that assists with climbing and hanging
• hands had a combination of features not seen in any other hominin, but overall more similar to modern human hands than was H. habilis.
• primitive fingers that were relatively long and curving and suited to climbing
• more modern features in the wrist, palms and thumbs that may have assisted with tool use and object manipulation
• relatively long thumb with unique features in the robust first metacarpal (thumb bone)
• lower limbs had a unique combination of primitive and derived features, and some features only found in this species.
• overall, leg bones were long and slender, typical of Homo, and had strong muscle attachments typical of modern bipedal walking
• the top of the femur was more australopith-like but unique in having a depression in the superior aspect (top part) of the femoral neck
• lower leg had a strong attachment of the pes anserinus tendon on the tibia, a feature unique to this species
• feet and ankles were similar to those of modern humans, with arches that indicate an efficient long-distance stride. The toe bones were moderately curved, more so than in modern humans

Pelvis

• widely flaring blades of the pelvis (ilia) were primitive and similar to australopithecines, but lower part of the pelvis (ischia) was more like a modern human’s
• hip bones were shorter front to back compared to modern humans

Lifestyle

Culture

These individuals were clearly not living in the cave in which their fossil remains were found as there were no signs of occupation such as tools, food or fire. This makes it difficult to determine cultural elements for H. naledi.

If the site wasn’t a living site, what were the remains doing there? The answer to this may have interesting implications for aspects of this species’ culture.

Burying the dead?

There is no evidence, such as marks on the bones, that individuals were dragged or dropped into the cave by predators. Nor is there evidence the remains were carried or relocated by water or gravity into the cave. One of the most distinctive aspects of the Rising Star site is that H. naledi is the only medium or large animal species found there. The individuals could have wondered into the caves and got trapped, but this is also unlikely.

It is possible – as the discovers suggest – that Homo naledi themselves entered the cave and used the chambers to deposit their dead. If this is the case, then these small-brained hominins were exhibiting repeated and deliberate behaviour. Any ritualistic context is difficult to determine as there is no suggestion if symbolism or any evidence the bones were deliberately placed as burials. Quite a few experts disagree with this interpretation and suggest there may have been a more accessible entrance in the past or one that allowed remains to be washed in. However, the discovery of H. naledi remains in two chambers, far apart from each other with no direct underground connection and difficult to reach from the surface, is compelling evidence for deliberate placement.

Using fire?

If H. naledi did repeatedly enter the Rising Star cave system, it is reasonable to assume they had controlled use of fire. However, there is no evidence of fire use in either the Dinaledi or Lesedi chambers. Other sites in South Africa older than one million years do have such evidence, including Swartkrans which is just 800 metres from the Rising Star caves.

Environment and diet

H. naledi’s teeth and lower jaw muscles are much smaller than those of most australopithecines, which suggests a diet that did not require heavy chewing or processing of foods such as grasses or sedges. Almost half of the teeth have one or more chips on the enamel surface, suggesting they regularly ate hard and abrasive items or environmental grit and dirt. Such foods may include underground storage organs, which, if unwashed, would contain large amounts of grit.

H. naledi fossils were found in a restricted site – there were few botanic or faunal remains recovered. This makes it hard to reconstruct the palaeoenvironment.

Extra reading:

Berger, Lee R; et al. (10 September 2015). ‘Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa.’ eLife.

Hawks et al (9 May 2017). ‘New fossil remains of Homo naledi from the Lesedi Chamber, South Africa.’ eLife; 6:e24232.

Berthaume, Michael A; Delezene, Lucas K; Kupczik, Kornelius (2018). ‘Dental topography and the diet of Homo naledi’. Journal of Human Evolution. 118: 14–26.

Berger, L. R; Hawks, J; Dirks, P. HGM; Elliott, M; Roberts, E. M. (9 May 2017). ‘Homo naledi and Pleistocene hominin evolution in subequatorial Africa’. eLife

Cofran, Zhongtao; Skinner, M. M; Walker, C. S (2016). ‘Dental development and life history in Homo naledi.’ American Journal of Physical Anthropology. 159: 3–346

Dirks, Paul H. G. M; et al. (10 September 2015). ‘Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa’. eLife.