Who’s That Hominin? The mystery of Homo naledi

Homo sapiens (also known as us!) are currently the only extant species in the genus Homo. But, only 100,000 years ago, many different species of Homo roamed the plant. Paleoanthropologists lump together this collection of different species into one group known as ‘hominins’. Not to be confused with hominids which include primates such as chimpanzees, gorillas, and orangutans, hominins consist exclusively of humans, both modern and extinct from the genera Homo, Australopithecus, Paranthropus and Ardipithecus (1)Homo naledi, first recognized in October of 2013, elicited excitement and controversy within the global paleoanthropological community. Discovered approximately 30 meters (about 98 feet) underground at the Cradle of Humankind World Heritage Site in South Africa, H. naledi was the first of its kind (2). The discovery of H. naledi at an almost impossible to reach location, included at least 1,550 hominin specimens, with 15 individuals exhibiting both Homo and Australopithecus traits, resulting in the upheaval of the already established understanding of our hominin ancestry.

H. naledi was discovered in the Dinaledi Chamber, as a result of speleological surveys throughout the Rising Star Cave system. Accessing the chamber is not an easy feat. In order to reach the Dinaledi Chamber researchers “must be skinny and preferably small…They must not be claustrophobic, they must be fit, they should have some caving experience”- Lee Burger site director. To reach the chamber, researchers must shuffle their way through the cave system’s tight tunnel about 80 meters (260ft) from the entrance to the ante-chamber. Once in the ante-chamber, they must nimbly climb up an exposed vertical face about 15 meters (50ft) to reach a small opening known as the Dragons Back. Researchers must then climb through a narrow, vertically orientated ‘chute’ measuring 12 meters (39ft) long with an average width of 20 cm (7.9in) in order to descend into the Dinaledi Chamber (6). Now that’s a climb! H. naledi is a strange looking hominin. With traits from both the genus Homo and Australopithecus, as well as traits not linked to any other hominin, it’s no wonder that paleoanthropologists were mesmerized by it. And, while there are hominins that exhibit traits from both the genus Homo and Australopithecus, they do not show as confusing morphology as H. naledi. Cranial, lower limb, wrist, and fingertip morphology all are relatively more derived Homo-like traits (2). Whereas brain size, pelvis, fingers, shoulders and ribcage morphology resemble ancestral traits found in Australopithecus more than Homo traits (3 & 4).

H. naledi traits similar to H. erectus

  • thin cranial vault bones becoming thicker in the occipital region
  • presence of temporal and occipital bossing
  • small post-canine teeth
  • vertebrae resemblance
  • homo-like foot and ankle

H. naledi traits similar with Australopithecus

  • Endocranial volume of 465-560 cc
  • Ilia on pelvis is short from top to bottom and flare out broadly side to side (Australopithecus afarensis)
  • Proximal and intermediate phalanges (middle part of your finger) curved
  • Arboreally adapted scapulae (shoulders)
  • Distally wide ribcage

Overall the morphology indicates that H. naledi had an enhanced locomotor performance for a striding (walking) gait, enhanced climbing ability relative to Australopithecus and possible enhanced object manipulation. This strange combination of skeletal and behavioral features contradicts many of the preconceived notions regarding the single evolutionary package in the hominin lineage. This package included an increase in tool manipulation, body size and brain size, smaller dentition, and greater commitment to terrestrial long-distance walking or running (10). So, why does H. naledi not fall under the classical evolutionary package? Well, first of all it is important to note that the H. naledi sample includes many bones that have only been marginally researched in other early hominins (H. rudolfensis, H. habilis and H. erectus). As a result, it can be difficult to determine the amount of morphological similarity between these species (6). Also, the individual mix of primitive and derived characteristics in various Homo fossils may indicate that the genus is polyphyletic, where some members of the genus have originated independently in different regions of Africa (6). If this turns out to be true, it would mean that all species currently placed within the genus Homo would need to be reconsidered. The material, recovered during two different field expeditions (November 2013 and March 2014) to the Dinaledi Chamber, presents a morphologically homogeneous sample that cannot be linked to any other known hominin species (2 & 3). And, while one bird and a few fragmentary rodent remains were discovered alongside the 1,550 hominin specimens, no other faunal evidence has been located. This lack of fauna is unusual and can hold certain implications as to how these hominins arrived in the cave.

There are many different speculations offering explanations of how these hominins arrived in the Dinaledi Chamber, however only two are currently thought to be likely- through a catastrophic event or deliberate disposal. Throughout the hominin record when mono-specific assemblages have been discovered, they are typically associated with a catastrophic event in which a group of animals were trapped. However, this scenario does not explain why these hominins chose to progress so far into the cave system dark zone, and away from all entrance and exit points (7 & 8). The other possible and more controversial scenario is the deliberate disposal of a body. Since none of the fossils to date show any green fractures, or disarticulation, linking to either a fall or predation, some researchers have suggested that H. naledi actively cared for its dead. It has been speculated that since the entrance is quite irregular and narrow, and there is a soft pile of muddy sediment below, bodies could have been dropped down the entrance of the chamber (9). However, this scenario has its flaws, as a long, hard climb through the dark zone is necessary to reach the disposal site, and the intentional disposal of dead bodies is considered a complex behavior for a hominin with a brain no larger than a gorilla, (6 & 9). So, while both of these hypotheses are considered the most plausible compared to others, additional investigation is still required. Aptly put by Mr. Sherlock Holmes “It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts”- A Scandal in Bohemia

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