Homo naledi

Discovery

On September 13, 2013, while exploring the Rising Star cave system looking for an extension, recreational cavers Rick Hunter and Steven Tucker of the Speleological Exploration Club (SEC) of South Africa found a narrow, vertically oriented "chimney" or "chute" measuring 12 m (39 ft) long with an average width of 20 cm (7.9 in) in northwest of Johannesburg. This chute led to a room 30 m (98 ft) underground (Site UW-101, the Dinaledi Chamber), the surface of which was littered with fossil bones. Before exploring the cave that day, the cavers had been asked by fellow caver and geologist Pedro Boshoff to let him know if they came across any fossils. On October 1, 2013, photos were shown to Boshoff who recognized their significance and delivered them to Lee Berger.

Excavation

In November 2013, the National Geographic Society and the University of the Witwatersrand funded an expedition called Rising Star Expedition for a twenty-one day excavation at the cave, followed by a second expedition in March 2014 for a 4-week excavation in the Dinaledi Chamber. In total, over 1,550 pieces of bone belonging to at least fifteen individuals have been recovered from the clay-rich sediments. The layered distribution of the bones suggests that they had been deposited over a long time, perhaps centuries. Only one square meter of the cave chamber has been excavated; other remains might still be there. Around 300 bone fragments were collected from the surface of the Dinaledi Chamber, and ∼1250 fossil specimens were recovered by excavation. The fossils include skulls, jaws, ribs, teeth, bones of an almost complete foot, of a hand, and of an inner ear. The bones of old, young and infants were found. Although much of the fossil material is disarticulated (separated at joints), the deposit contains articulated or near-articulated examples such as the maxilla and mandible of single individuals and the bones of a nearly complete hand and foot.

The description of the new species was announced at a press conference on September 10, 2015 held at Maropeng, Cradle of Humankind, Johannesburg, South Africa. A display case of the fossils was unveiled during the ceremony and was subsequently on display to the public at Maropeng throughout September and October 2015.

Ownership

The University of the Witwatersrand is the curator of the fossils. The fossils are owned by the Republic of South Africa and will likely stay there, in accord with a 1998 resolution by the International Association for the Study of Human Paleontology – also approved by a South African permanent council member of the organisation – "strongly recommending that original hominid fossils not be transported beyond the boundaries of the country of origin, unless there are compelling scientific reasons which must include the demonstration that the proposed investigations cannot proceed in the forseeable [sic] future in the country of origin".

Morphology

The physical characteristics of H. naledi are described as having traits similar to the genus Australopithecus, mixed with traits more characteristic of the genus Homo, and traits not known in other hominin species. The skeletal anatomy displays plesiomorphic ("ancestral") features found in the australopithecines and more apomorphic ("derived," or traits arising separately from the ancestral state) features known from later hominins.

Adult males are estimated to have stood around 150 cm (5 ft) tall and weighed around 45 kg (100 lb), while females would likely have been a little shorter and weighed a little less. An analysis of H. naledi's skeleton suggests it stood upright and was bipedal. Its hip mechanics, the flared shape of the pelvis are similar to australopithecines, but its legs, feet and ankles are more similar to the genus Homo.

The hands of H. naledi appear to have been better suited for object manipulation than those of australopithecines. Some of the bones resemble modern human bones, and other bones are more primitive than Australopithecus, an early ancestor of humans. The thumb, wrist and palm bones are modern-like while the fingers are curved, more australopithecine, and useful for climbing. The shoulders are configured largely like those of australopithecines. The vertebrae are most similar to Pleistocene members of the genus Homo, whereas the ribcage is wide distally like A. afarensis. The arm has an Australopithecus-similar shoulder and fingers and a Homo-similar wrist and palm. The structure of the upper body seems to have been more primitive than that of other members of the genus Homo, even apelike. In evolutionary biology, such a mixture of features is known as an anatomical mosaic.

Four skulls were discovered, thought to be two females and two males, with a cranial volume of 560 cm³ (34 cu in) for the males and 465 cm³ (28.4 cu in) for females, approximately half the volume of modern human skulls; average Homo erectus skulls are 900 cm³ (55 cu in). The H. naledi skulls are closer in cranial volume to australopithecine skulls. Nonetheless, the cranial structure is described as more similar to those found in the genus Homo than to australopithecines, particularly in its slender features, and the presence of temporal and occipital bossing, and the fact that the skulls do not narrow in behind the eye-sockets. The species' brains were markedly smaller than modern Homo sapiens, measuring between 450 and 550 cm³ (27–34 cu in). The teeth and mandible musculature are much smaller than those of most australopithecines, which suggests a diet that did not require heavy mastication. The teeth are small, similar to modern humans, but the third molar is larger than the other molars, similar to australopithecines. The teeth have both primitive and derived dental development.

The overall anatomical structure of the species has prompted the investigating scientists to classify the species within the genus Homo, rather than within the genus Australopithecus. The H. naledi skeletons indicate that the origins of the genus Homo were complex and may be polyphyletic (hybrid), and that the species may have evolved separately in different parts of Africa.

A reconstruction of a model of a H. naledi head was made by measuring the bones of the head, the eye sockets, and where the jaw muscles insert to the skull. The measurements were used to make the model, including skin, eyes, and hair.

Dating

The fossils have not been dated. After three dating methods have been tried with inconclusive results, local scientists are working on adapting another dating technique which they expect will be performed in 2017. The discovery team waited until after the research article was published before trying radiocarbon dating of the fossils because radiocarbon dating will have to destroy parts of the fossils. Radiocarbon dating can only date fossils which are less than 50,000 years old. Geologists estimate that the cave in which the fossils were discovered is no older than three million years. As of April 2016, they are attempting five different dating techniques at seven different labs.

The bones were found lying on the cave floor or buried in its claylike sediments ("mud clast breccias") interspersed with layers of deposited calcite flowstone. As of 2016, they are working on determining the geological age of the fossil deposit, applying techniques that can be used to date the site's flowstones. Berger declared that the anatomy of H. naledi suggests it originated at or near the start of the Homo genus, around 2.5 million to 2.8 million years ago, but the actual excavated bones may be younger.

Francis Thackeray, of the University of the Witwatersrand, suggests that H. naledi lived about 2 million years ago (±0.5 million years), based on the skulls' similarities to H. rudolfensis, H. erectus, and H. habilis, species that existed around 1.5, 2.5, and 1.8 million years ago, respectively.

Opinions

The research team proposes the bones represent a new species, H. naledi in the genus Homo; other experts contend further analyses are needed to support this classification. Paleoanthropologist Tim D. White observed that the significance of this discovery is unknown until dating has been completed and additional anatomical comparison with previously known fossils has been done.

According to Rick Potts, without an age there is no way to judge the evolutionary significance of this find. He stated that "it's hard to know without a date whether it's from that period, as one of those experiments that then went nowhere, or whether it's in fact much less than one million years old. In that case, we could be talking about something that also didn't go anywhere and was just an isolated, probably very small population that persisted for a long time in splendid isolation."

New York University anthropologist Susan Anton stated that even after dating, experts will likely spend many years striving to put these fossils in the proper context because there is no consensus in paleoanthropology about exactly how such comparisons are used to define the genus Homo. "Some would argue that striding bipedalism is a defining feature, so that being Homo means using a specific way of moving around the environment. Other scholars may look more to cranial characteristics as Homo family features."

Bernard Wood, a paleoanthropologist at the George Washington University, agrees the remains represent a new species, but thinks the bones may represent a relict population that may have evolved in near isolation in South Africa, similar to another relict population, a small-brained species of Homo floresiensis from the island of Flores in Indonesia. With the number of individuals, and the sexes and age groups represented, scientists consider the find to be the richest assemblage of associated fossil hominins ever discovered in Africa, and 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.

Jeffrey H. Schwartz, an evolutionary biologist at the University of Pittsburgh in Pennsylvania, argues that the material is too varied to represent a single species.

Donald Johanson doubts that the individuals were subject to ritualistic burials citing the small brain capacity (450 cc) and the probability of limited cultural practices.

Comparisons to H. erectus

Paleoanthopologist Tim D. White thinks that, based on the published descriptions, the fossils belong to a primitive Homo erectus. Anthropologist Chris Stringer also stated that the fossils look most similar to the small-bodied examples of Homo erectus from Dmanisi in Georgia, which have been dated at ∼1.8 million years old. Berger rejected the possibility of the fossils representing H. erectus at the announcement news conference. S. V. Drobyshevskiy identifies remains of H. naledi with Telanthropus (Telanthropus capensis) from Swartkrans.

Deliberate placement of bodies hypotheses

There are some indications the individuals may have been deliberately placed in the cave near the time of their death, and experts state more evidence is needed to support this hypothesis.

Anthropologist John D. Hawks, from the University of Wisconsin-Madison who was a member of the team, stated that the scientific facts are that all the bones recovered are hominid, except for those of one owl; there are no signs of predation, and there is no predator that accumulates only hominids this way; the bones did not accumulate there all at once. There is no evidence of rocks or sediment having dropped into the cave from any opening in the surface; no evidence of water flowing into the cave carrying the bones into the cave. Hawks concluded that the best hypothesis is that the bodies were deliberately placed in the cave after death, by other members of the species.

Dirks et al. say that "Mono-specific assemblages have been described from Tertiary and Mesozoic vertebrate fossil sites (...), linked to catastrophic events (...) Among deposits of non H. sapiens hominins, where evidence of catastrophic events is lacking, mono-specific assemblages have been associated typically with deliberate cultural deposition or burial". They stated that there is no evidence a catastrophe placed the bodies in the cave, and that the bodies were deliberately placed in cave.

William Jungers, anatomical scientist at Stony Brook University, does not dispute that the H. naledi bones belong in the genus Homo and were likely deposited deliberately, but he cautions against trying to argue for "complex social organization and symbolic behaviors." He suggests that "Dumping conspecifics down a hole may be better than letting them decay around you." He speculates that in the past there may have been another, easier, way to access the chamber where the bones were found. However, Dirks et al. mentions that there is insufficient evidence to suggest the existence of an alternate and easier access route from the surface to the Denaledi Chamber.

Carol Ward, professor of pathology and anatomical sciences at the University of Missouri, is also skeptical of the intentional burial explanation and asked, "If it’s really that hard to get to the cave, how do you get to that long dark cave carrying your dead grandmother?"

Berger thinks that deliberate disposal of bodies within the intricate cave system would have required the species members to find their way through total darkness and back again, and he speculates that this would have required light in the form of torches or fires lit at intervals. Aurore Val argues the team has yet to make a convincing case on their deliberate placement hypothesis. Berger at one point even thought that Homo naledi might have been exploring, and when they came to the end of the cave, they got lost and could not find their way out. Therefore, they remained there.

Martha Tappen of the University of Minnesota thinks that hiding from menacing predators is a likely scenario.

Ritual hypotheses

Berger et al. suggest that "these individuals were capable of ritual behaviour". They speculate the placing of dead bodies in the cave was a ritualistic behaviour, a sign of symbolic thought. "Ritual" here means an intentional and repeated practice (disposing of dead bodies in the cave), and not implying any type of religious ritual. Ritualistic behavior has been generally considered to have emerged among Homo sapiens and Homo neanderthalensis. The oldest confirmed Neanderthal burial is 100,000 years ago.

Rick Potts noted:

There is no evidence of material culture, like tools, or any evidence any kind of symbolic ritual that we almost always associated with burial....These bodies seem to have simply been dropped down a hole and disposed of."

Research article Dirks et al. (2015) states:

Every previously known case of cultural deposition has been attributed to species of the genus Homo with cranial capacities (brain size) near the modern human range, and unlike the Dinaledi assemblage, each of these hominin associated occurrences also contains at least some medium- to large-sized, non-hominin fauna.

William Jungers has raised similar concerns regarding the hypothesis. Science writer Michael Shermer suggests considering homicide, war, and even sacrifice as the cause of death, but John D. Hawks, one of the scientists who categorized and analyzed the fossils, notes that there is no evidence for a violent death among the bodies.

Methods

Palaeontologists Tim White and Christoph Zollikofer think that the fossils were excavated too fast to protect them from damage, in a desire to get publicity, and that the findings were not examined and peer-reviewed sufficiently before publishing. Lee Berger disputes these opinions and considers that the openness of the excavation, the analysis, publishing and availability of the fossils used valid methods.

Documentaries

A PBS NOVA National Geographic documentary Dawn of Humanity, describing the discovery of H. naledi, was posted online on 10 September 2015, and broadcast nationwide in the United States on 16 September 2015. According to archeologist K. Kris Hirst, the Dawn of Humanity documentary film provides "a rich context for the discovery [of the fossils of Homo naledi], setting the historical and evolutionary background so that viewers can understand the significance of the discovery."

The National Geographic Society has videos on its website describing, explaining and showing different phases of the discovery, the scientists, the six women researchers, excavation of the fossils during a two-year period, and the process of making a model of a head of H. naledi from the fossils.

Lee Berger introduces Homo naledi in a Wits University video.