Homo naledi

 Homo naledi is a species of archaic human discovered in the Rising Star Cave, Cradle of Humankind, South Africa dating to the Middle Pleistocene 335,000–236,000 years ago. The initial discovery comprises 1,550 specimens, representing 737 different elements, and at least 15 different individuals. Despite this exceptionally high number of specimens, their classification with other "Homo" remains unclear.
Along with similarities to contemporary "Homo", they share several characteristics with the ancestral "Australopithecus" and early "Homo" as well (mosaic anatomy), most notably a small cranial capacity of 465–610 cm3 (28.4–37.2 cu in), compared to 1,270–1,330 cm3 (78–81 cu in) in modern humans. They are estimated to have averaged in height and in weight, yielding a small encephalization quotient of 4.5. Nonetheless, "H. naledi" brain anatomy seems to have been similar to contemporary "Homo", which could indicate equatable cognitive complexity. The persistence of small-brained humans for so long in the midst of bigger-brained contemporaries revises the previous conception that a larger brain would necessarily lead to an evolutionary advantage, and their mosaic anatomy greatly expands the known range of variation for the genus.
"H. naledi" anatomy indicates that, though they were capable of long distance travel with a humanlike stride and gait, they were more arboreal than other "Homo", better adapted to climbing and suspensory behaviour in trees than endurance running. Tooth anatomy suggests consumption of gritty foods covered in particulates such as dust or dirt. Though they have not been associated with stone tools or any indication of material culture, they appear to have been dextrous enough to produce and handle tools, and likely manufactured Early or Middle Stone Age industries. It has also been controversially postulated that these individuals were given funerary rites, and were carried into and placed in the chamber.
Taxonomy.
Discovery.
In October 2013, cavers Rick Hunter and Steve Tucker, under the direction of South African palaeoanthropologist Lee Rogers Berger, discovered the Dinaledi Chamber in Rising Star Cave in the Cradle of Humankind, South Africa, containing several hominin fossils. The chamber lies about from the entrance, and the main passage is about long and at its narrowest, and is at the bottom of a vertical drop. In total, more than 1,550 pieces of bone belonging to at least fifteen individuals (9 immature and 6 adults) have been recovered from the clay-rich sediments. Berger and colleagues published the findings in 2015.
The fossils represent 737 anatomical elements—including the parts of the skull, jaw, ribs, teeth, limbs, and inner ear bones—from old, adult, young, and infantile individuals. There are also some articulated or near-articulated elements, including the skull with the jaw bone, and nearly complete hands and feet. With the number of individuals of both sexes across several age demographics, it is the richest assemblage of associated fossil hominins discovered in Africa. Aside from the Sima de los Huesos collection and later Neanderthal and modern human samples, the excavation site has the most comprehensive representation of skeletal elements across the lifespan, and from multiple individuals, in the hominin fossil record.
The holotype specimen, DH1, comprises a male partial calvaria (top of the skull), partial maxilla, and nearly complete jawbone. The paratypes, DH2 through 5, all comprise partial calvaria. Berger and colleagues named the species "Homo naledi", the species name meaning "star" in the Sotho language, because the remains came from Rising Star Cave.
The remains of at least three additional individuals—two adults and a child—were reported in the Lesedi Chamber of the cave by John Hawks and colleagues in 2017.
Classification.
In 2017, the Dinaledi remains were dated to 335,000–236,000 years ago in the Middle Pleistocene, using electron spin resonance (ESR) and uranium–thorium (U-Th) dating on three teeth, and U-Th and paleomagnetic dating of the sediments they were deposited in. The fossils were previously thought to have dated to 1 to 2 million years ago because no similarly small-brained hominins had previously been known from such a recent date in Africa (the smaller-brained "Homo floresiensis" of Indonesia lived on an isolated island and apparently went extinct shortly after the arrival of modern humans.) The ability of such a small-brained hominin to have survived for so long in the midst of bigger-brained "Homo" greatly revises previous conceptions of human evolution and the notion that a larger brain would necessarily lead to an evolutionary advantage. Their mosaic anatomy also greatly expands the range of variation for the genus.
"H. naledi" is hypothesised to have branched off very early from contemporaneous "Homo". It is unclear whether they branched off at around the time of "H. habilis", "H. rudolfensis", and "A. sediba"; are a sister taxon to "H. erectus" and the contemporaneous large-brained "Homo"; or are a sister taxon to the descendants of "H. antecessor" (modern humans and Neanderthals). This would mean that they branched off from contemporary "Homo" at latest before 900,000 years ago, and possibly as early as the Pliocene. It is also possible their ancestors speciated after an interbreeding event between "Homo" and late australopithecines. Looking at the skull, "H. naledi" has the closest affinities to "H. erectus".
It is unclear if these "H. naledi" were an isolated population in the Cradle of Humankind, or if they ranged across Africa. If the latter, then several gracile hominin fossils across Africa which have traditionally been classified as late "H. erectus" could potentially represent "H. naledi" specimens.
Anatomy.
Skull.
Two male "H. naledi" skulls from the Dinaledi chamber had cranial volumes of about , and two female skulls . A male "H. naledi" skull from the Lesedi chamber had a cranial volume of . The Dinaledi specimens are more similar to the cranial capacity of australopithecines; for comparison, "H. erectus" averaged about , and modern humans for males and females respectively. However, the Lesedi specimen is within the range of "H. habilis" and "H. e. georgicus". The encephalization quotient of "H. naledi" was estimated at 4.5, which is the same as the pygmy "H. floresiensis", but notably smaller than all other "Homo" (contemporary "Homo" were all above 6). Nonetheless, the skull shape is more similar to "Homo", with a slenderer shape, the presence of temporal and occipital lobes of the brain, and reduced post-orbital constriction (the skull does not become narrower behind the eye-sockets). The frontal lobe morphology is more or less the same in all "Homo" brains despite size, which differs from "Australopithecus", and has been implicated in the production of tools, the development of language, and sociality. It is unclear if "H. naledi" inherited small brain size from the last common "Homo" ancestor, or, if it was evolved secondarily more recently.
Like modern humans, but unlike fossil hominins (including South African australopithecines, "H. erectus", and Neanderthals), the permanent 2nd molar erupted comparatively late in life, emerging alongside the premolars instead of before, which indicates a slower maturation unusually comparable to modern humans. The tooth formation rate of the front teeth is also most similar to modern humans. The overall size and shape of the molars most closely resemble those of three unidentified "Homo" specimens from the local Swartkrans and East African Koobi Fora Caves, and are similar in size (but not shape) to Pleistocene "H. sapiens". The necks of the molars are proportionally similar to those of "A. afarensis" and "Paranthropus". Unlike modern humans and contemporary "Homo", "H. naledi" lacks several accessory dental features, and has a high frequency of individuals who present main cusps, namely the metacone (midline on the tongue-side) and hypocone (to the right on the lip-side) on the 2nd and 3rd molars, and a Y-shaped hypoconulid (a ridge on the lip-side towards the cheek) on all 3 molars. Nonetheless, "H. naledi" also has many dental similarities with contemporary "Homo".
The anvil (a middle ear bone) more resembles those of chimps, gorillas, and "Paranthropus" than "Homo". Like "H. habilis" and "H. erectus", "H. naledi" has a well-developed brow-ridge with a fissure stretching across just above the ridge, and like "H. erectus" a pronounced occipital bun. "H. naledi" has some facial similarities with "H. rudolfensis".
Build.
The "H. naledi" specimens are estimated to have, on average, stood around and weighed . This body mass is intermediate between what is typically seen in "Australopithecus" and "Homo" species. Like other "Homo", male and female "H. naledi" were likely about the same size, males on average about 20% larger than females. A juvenile specimen, DH7, is skeletally consistent with a growth rate similar to the faster ape-like trajectories of MH1 ("A. sediba") and Turkana boy ("H. ergaster"). However, because dental development is so similar to that of modern humans, a slower maturation rate is not completely out of the question. Using the faster growth rate, DH7 would have died at 8–11 years old, but using the slower growth, DH7 would have died at 11–15 years old.
Concerning the spine, only the 10th and 11th thoracic vertebrae (in the chest region) are preserved from presumably a single individual, which are proportionally similar to those of contemporary "Homo", though are the smallest recorded of any hominin. The two transverse processes of the vertebra, which jut out diagonally, are most similar to those of Neanderthals. The neural canals within are proportionally large, similar to modern humans, Neanderthals, and "H. e. georgicus". The 11th rib is straight like that of "A. afarensis", and the 12th rib is robust in cross-section like that of Neanderthals. Like Neanderthals, the 12th rib appears to have supported strong intercostal muscles above, and a strong quadratus lumborum muscle below. However, unlike Neanderthals, there was weak attachment to the diaphragm. Overall, this "H. naledi" specimen appears to have been small-bodied compared to other "Homo", though it is unclear if this single specimen is representative of the species.
The shoulders are more similar to those of australopithecines, with the shoulder blade situated higher on the back and farther from the midline, short clavicles, and little or no humeral torsion. Elevated shoulder and clavicle bones indicate a narrow chest. The pelvis and legs have features reminiscent of "Australopithecus", including anterposteriorly compressed (from front to back) femoral necks, mediolaterally compressed (from left to right) tibiae, and a somewhat circular fibular neck; which indicate a wide abdomen. This combination would preclude efficient endurance running in "H. naledi", unlike "H. erectus" and descendants. Instead, "H. naledi" appears to have been more arboreal.
Limbs.
The metacarpal bone of the thumb was well-developed, which is used in holding and manipulating large objects, and had strong crests to support its opponens pollicis muscle used in precision-pinch gripping, and its thenar muscles. This is more similar to other "Homo" than "Australopithecus". "H. naledi" appears to have had strong flexor pollicis longus muscles like modern humans, with humanlike palm and finger pads, which are important in forceful gripping between the thumb and fingers. However, unlike "Homo", the "H. naledi" thumb metacarpal joint is comparably small relative to the thumb's length, and the thumb phalangeal joint is flattened. The distal thumb phalanx bone is robust, and proportionally more similar to those of "H. habilis" and "P. robustus".
The metacarpals of the other fingers share adaptations with modern humans and Neanderthals to cup and manipulate objects, and the wrist joint is overall comparable to that of modern humans and Neanderthals. Conversely, the proximal phalanges are curved and are almost identical to those of "A. afarensis" and "H. habilis", which is interpreted as an adaptation for climbing and suspensory behaviour. Such curvature is more pronounced in adults than juveniles, suggesting that adults climbed just as much or more so than juveniles, and this behaviour was commonly done. The fingers are also proportionally longer than those of any other fossil hominin (other than the arboreal "Ardipithecus ramidus" and a modern human specimen from Qafzeh cave, Israel) which is also consistent with climbing behaviour.
"H. naledi" was a biped and stood upright. Like other "Homo", they had strong insertion for the gluteus muscles, well-defined linea aspera (a ridge running down the back of the femur), thick patellae, long tibiae, and gracile fibulae. These indicate that they were capable of long distance travel. The "H. naledi" foot was similar to that of modern humans and other "Homo", with adaptations for bipedalism and a humanlike gait. However, the heel bone has a low orientation, comparable to those of non-human great apes, and the ankle bone has a low declination, which possibly indicate the foot would have been subtly stiffer during the stance phase of walking before the foot pushed off the ground.
Pathology.
The adult right mandible U.W. 101-1142 has a bony lesion, suggestive of a benign tumour. The individual would have experienced some swelling and localised discomfort, but the tumour's position near the medial pterygoid muscle may have impeded function of the muscle, and changed elevation of the right side of the jaw, and caused discomfort on the right temperomandibular joint (connecting the jaw with the skull).
Dental defects in "H. naledi" specimens during 1.6–2.8 and 4.3–7.6 months of development were most likely caused by seasonal stressors. This may have been due to extreme summer and winter temperatures causing food scarcity. Also, minimum winter temperatures of the area average about , and can drop below freezing; staying warm for an infant of the small-bodied "H. naledi" would have been difficult, and winters likely increased susceptibility to respiratory diseases. Environmental stressors is consistent with present-day flu seasons in South Africa peaking during winter, and paediatric diarrhoea hospitalisation being most frequent at the height of the rainy season in summer.
Local hominins were likely preyed upon by large carnivores, such as lions, leopards, and hyaenas. However, there seems to be a distinct paucity of large carnivore remains from the northern end of the Cradle of Humankind, where Rising Star Cave is located, possibly because carnivores preferred the Blaaubank River to the south which may have offered better hunting grounds with a greater abundance of large prey items. Alternatively, because many more sites are known in the south than the north, carnivore spatial patterns may not be well-represented by the fossil record.
Culture.
Food.
Dental chipping and wearing indicates the habitual consumption of small hard objects, such as dirt and dust, and cup-shaped wearing on the back teeth may have stemmed from gritty particles. These could have originated from unwashed roots and tubers. Alternatively, aridity could have stirred up particulates onto food items, coating food in dust. It is possible that they commonly ate larger hard items, such as seeds and nuts, but these were processed into smaller pieces before consumption. Whatever the case, "H. naledi" occupied a seemingly unique niche from previous South African hominins, including "Australopithecus" and "Paranthropus". However, the teeth of all 3 species indicate that they needed to exert high shearing force to chew through perhaps plant or muscle fibres. The teeth of other "Homo" cannot produce such high forces perhaps due to the use of some food processing techniques, such as cooking.
Technology.
Though "H. naledi" remains are not associated with any stone tools, it is likely they produced Early Stone Age (Acheulean and possibly the earlier Oldawan) or Middle Stone Age industries because they have the same adaptations to the hand as other human species which are implicated in tool production. "H. naledi" is the only identified human species to have existed during the early Middle Stone Age of the Highveld region, South Africa, possibly indicating that this species manufactured and maintained this tradition at least during this time period. Such industries and stone cutting techniques likely evolved independently several times among different "Homo" species and populations, or were transported over long distances by the inventors or apprentices and taught.
Funerals.
In 2015, archaeologist Paul Dirks, Berger, and colleagues concluded that the bodies had to have been deliberately carried and placed into the chamber by people because they appear to have been intact when they were first deposited in the chamber (no evidence of trauma by being dropped into the chamber nor of predation, and exceptional preservation), the chamber is inaccessible to large predators, the chamber appears to be an isolated system and has never been flooded (that is, natural forces were not at play), there is no hidden shaft by which people could have accidentally fallen in through, and there is no evidence of some catastrophe which killed all the individuals inside the chamber. They said it is also possible that the bodies were dropped down a chute and fell slowly due to irregularity and narrowness of the path down, or a soft mud cushion to land on. In both scenarios, the morticians would have required artificial light to navigate the cave; and the site was used repeatedly for burials as the bodies were not all deposited at the same time.
In 2016, palaeoanthropologist Aurore Val countered that such preservation may have been due to mummification rather than careful burial, and the absence of long bone heads is reminiscent of predation, and she believes that discounting natural forces such as flooding for depositing the bodies is unjustified. Further, there is evidence of damage done by beetles, beetle larvae, and snails (which facilitate decomposition); but, the chamber does not present ideal conditions for snails, nor does it contain snail shells, which would indicate decomposition actually initiated before deposition in the chamber.
Nonetheless, in 2017, Dirks, Berger, and colleagues reaffirmed that there is no evidence of water flow into the cave, and that it is more likely that these "H. naledi" were buried in the chamber. They also said it is possible that they were buried by contemporary "Homo", such as the ancestors of modern humans, rather than other "H. naledi", but, nonetheless, that the cultural behaviour of funerary practises is not impossible for "H. naledi", and burial in the chamber may have been done to remove decaying bodies from a settlement, prevent scavengers, or due to social bonding and grief.
In 2018, anthropologist Charles Egeland and colleagues echoed Val's sentiments, and stated that there is insufficient evidence to conclude that human species had developed a concept of the afterlife so early in time. They also said that the preservation of the Dinaledi individuals is similar to those of baboon carcasses which accumulate in caves (either by natural death of cave-dwelling baboons or by a leopard dragging in carcasses).