Homo naledi

Homo naledi (from the Sesotho word naledi, meaning "star," in reference to the Rising Star Cave system) is an extinct species of archaic hominin recovered from the dolomitic karst caves of the Cradle of Humankind World Heritage Site in Gauteng Province, South Africa, approximately 50 km northwest of Johannesburg. Discovered in September 2013 by recreational cavers Rick Hunter and Steven Tucker inside the remote Dinaledi Chamber—accessible only through a vertical 12-meter limestone chute narrowing to 18 cm ("Superman's Crawl" and the "Dragon's Back")—and excavated by a team of six specialized "underground astronauts" directed by Lee R. Berger (University of the Witwatersrand), Homo naledi was formally described in eLife in September 2015 from an assemblage of more than 1,550 fossil elements representing at least 15 individuals, making it the largest single-species hominin fossil sample ever recovered on the African continent. In 2017, combined uranium-series and electron spin resonance (US-ESR) geochronology revealed that Homo naledi lived not at the base of the Pleistocene 2 million years ago as its small 465–610 cm³ braincase initially suggested, but in the late Middle Pleistocene between 335,000 and 236,000 years ago—making this small-brained hominin a contemporary of early anatomically modern Homo sapiens (such as Jebel Irhoud and Florisbad) in Africa.

Following the 2013–2014 excavations in the Dinaledi Chamber (situated ~80 meters inside the cave entrance and 30 meters below the surface in total darkness), Lee Berger's team announced a second, independent chamber in 2017—the Lesedi Chamber (145 meters from the Dinaledi Chamber)—which yielded an additional 131 hominin fossils from at least three individuals, including the notably complete adult male skeleton LES1 ("Neo"), preserving a complete skull, mandible, clavicle, humerus, femur, tibia, vertebrae, and hand bones. Across the Dinaledi, Lesedi, and adjacent Hill Antechamber deposits, fossils of infants, juveniles, prime-age adults, and senescent old adults have been recovered from unconsolidated clay-rich mudstone sediments lacking any non-hominin macro-mammalian fauna (aside from a few recent owl pellets and rodent bones).

Because the Dinaledi clays lacked volcanic ash tuffs for ⁴⁰Ar/³⁹Ar dating and the carbonate flowstones suffered from variable detrital thorium contamination, Paul Dirks, Eric Roberts, and a blind multi-laboratory consortium (2017) applied six independent radiometric methods to constrain the age of the fossils:

1. Direct Combined Uranium-Series and Electron Spin Resonance (US-ESR): Applied directly to three Homo naledi tooth enamel fragments (two from Unit 3 and one from the surface), yielding a maximum age of 335,000 years ago (253⁺⁸²_-70 ka).
2. Uranium–Thorium (U–Th) disequilibrium dating on Flowstone 1a: A clean laminated calcium carbonate sheet that grew directly over articulated Homo naledi hand and foot bones, establishing a strict minimum age (terminus ante quem) of 236,000 years ago (242 ± 5 ka).
3. Paleomagnetism and Optically Stimulated Luminescence (OSL): Confirmed normal Brunhes polarity (<780 ka) and bracketed the underlying Unit 1–2 clays prior to Unit 3 deposition.

This 335–236 ka age range fundamentally altered interpretations of the African Middle Stone Age archaeological record: archaeologists can no longer automatically assume that every stone tool assemblage in sub-Saharan Africa between 500,000 and 200,000 years ago was manufactured by large-brained Homo sapiens or Homo heidelbergensis (H. rhodesiensis).

The skull of Homo naledi (represented by DH1–DH5 and LES1) combines a diminutive neurocranium with derived Homo masticatory reductions:

• Cranial Vault and Supraorbital Torus: Endocranial volume ranges from 465 cm³ in the small presumed-female cranium DH3 to 560 cm³ in DH1 and 610 cm³ in the large male LES1 ("Neo")—overlapping Australopithecus and the smallest Homo habilis specimens (KNM-ER 1813). Yet unlike australopithecines, the cranium exhibits minor postorbital constriction, parasagittal keeling, an angled occipital torus, a flattened articular eminence of the temporomandibular joint, and a well-defined supratoral sulcus behind a double-arched supraorbital torus—resembling miniature early Homo erectus skulls from Dmanisi (such as D2280 and D4500).
• Derived Human-Like Inferior Frontal Gyrus (Endocast): Despite its small 465–610 cm³ volume, high-resolution laser and CT endocasts analyzed by Ralph Holloway and Shawn Hurst (2018) show that the inferior frontal gyrus (Broca's cap: Brodmann areas 44, 45, and 47) and the lateral orbital frontal cortex of Homo naledi lack the ape-like fronto-orbital sulcus of Australopithecus and instead display a derived, modern-Homo-like sulcal organization associated with complex tool motor planning and social vocalization.
• Reduced Postcanine Dentition and MolarWear Resistant Enamel: The premolars and molars of Homo naledi are notably small—smaller than those of Homo habilis, Homo rudolfensis, and many Homo erectus individuals—and set in a parabolic dental arcade with simple premolar roots, lacking third-molar (M₃) megadontia. At the same time, the molars exhibit an evolutionary increase in cusp height and steepness alongside deep chipped enamel fractures caused by chewing hard, grit-laden underground resources in dry terrestrial habitats.

Because the Dinaledi Chamber preserved an articulated right hand (Hand 1: 26 of 27 bones in anatomical position) and an articulated right foot (Foot 1: 107 foot bones total across the chamber), the postcranial functional anatomy of Homo naledi is known in exceptional detail, revealing a stark partition between a primitive upper body and a derived lower body:

1. Primitive Australopith-Like Thorax, Shoulder, Pelvis, and Curved Fingers:
   • Shoulder & Ribcage: The clavicle is short and the scapular glenoid fossa is angled cranially (~120° axilloglenoid angle), positioning the shoulder high for overhead reaching, while the lower ribs (11th and 12th) show a wide, funnel-shaped australopith-like lower thorax.
   • Pelvis & Femoral Neck: The iliac blades of the pelvis are flared laterally like Australopithecus, and the femur possesses a long, anteroposteriorly compressed femoral neck with two internal cortical pillars on the superior neck surface.
   • Curved Manual Phalanges: Although the wrist and thumb of Hand 1 are fully derived and modern-human-like—possessing a long, robust first metacarpal with expanded opponens pollicis muscle flanges and a boot-shaped trapezoid capable of forceful precision stone-tool manipulation—the proximal and intermediate phalanges of digits II–V are more strongly curved than in Australopithecus afarensis! This manual curvature demonstrates that Homo naledi continued to use its hands for powerful suspensory grasping and vertical climbing on rock faces or trees throughout its life.
2. Derived Modern-Human-Like Leg and Foot:
   • In contrast to the primitive pelvis and curved fingers, the tibia and fibula are elongated and slender, and Foot 1 is virtually indistinguishable from the foot of an anatomically modern Homo sapiens or Neanderthal: it possesses a fully adducted, non-grasping hallux, short straight pedal phalanges, a calcaneocuboid locking joint, and both longitudinal and transverse plantar arches optimized for long-distance striding bipedalism.

The taphonomic origin of the Dinaledi and Lesedi hominin assemblages has sparked one of the most vigorous methodological debates in contemporary archaeology:

• Absence of Carnivore, Scavenger, or Fluvial Transport: Microscopic inspection of >1,550 bones by Marina Elliott, Ashley Kruger, and Patrick Randolph-Quinney revealed zero carnivore tooth punctures, gnaw scores, digestive acid etching, porcupine gnawing, stone-tool cut marks, or high-energy water-rolling abrasion. Furthermore, fragile articulated hands, feet, and axial segments were preserved intact inside fine-grained autochthonous cave muds, ruling out acute flash floods washing carcasses in from the surface.
• Intentional Mortuary Caching vs. Natural Death-Trap / Slumping Models: Lee Berger, Paul Dirks, and colleagues proposed that Homo naledi intentionally carried and dropped the bodies of deceased group members down the dark vertical chute into the Dinaledi and Lesedi chambers as a deliberate mortuary practice. Critics (including María Martinón-Torres, Aurore Val, and Michael Petraglia) have countered that natural karst openings existing in the Middle Pleistocene (since sealed by roof collapse or flowstone) or slow gravitational sediment slumping of hominins trapped inside the cave system must be fully tested before attributing symbolic funerary rites and cave engravings to a hominin with a 500 cm³ brain.

Regardless of its mortuary taphonomy, Homo naledi proves that hominin evolution in Pleistocene Africa was not a linear ladder (Australopithecus → H. habilis → H. erectus → H. sapiens), but a braided bush in which archaic, small-brained lineages coexisted alongside large-brained Homo for over a million years.

1. Berger, L. R., Hawks, J., de Ruiter, D. J., Churchill, S. E., Schmid, P., Delezene, L. K., ... & Zipfel, B. (2015). Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife, 4, e09560. https://doi.org/10.7554/eLife.09560
2. Kivell, T. L., Deane, A. S., Tocheri, M. W., Orr, C. M., Schmid, P., Hawks, J., ... & Churchill, S. E. (2015). The hand of Homo naledi. Nature Communications, 6(1), 8431. https://doi.org/10.1038/ncomms9431
3. Dirks, P. H. G. M., Roberts, E. M., Hilbert-Wolf, H., Kramers, J. D., Hawks, J., Dosseto, A., ... & Berger, L. R. (2017). The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa. eLife, 6, e24231. https://doi.org/10.7554/eLife.24231
4. Holloway, R. L., Hurst, S. D., Garvin, H. M., Schoenemann, P. T., Venter, R. G., ... & Hawks, J. (2018). Endocast morphology of Homo naledi from the Dinaledi Chamber, South Africa. Proceedings of the National Academy of Sciences, 115(22), 5738–5743. https://doi.org/10.1073/pnas.1720842115