Distinct mandibular premolar crown morphology in Homo naledi and its implications for the evolution of Homo species in southern Africa

Abstract

Homo naledi displays a combination of features across the skeleton not found in any other hominin taxon, which has hindered attempts to determine its placement within the hominin clade. Using geometric morphometrics, we assess the morphology of the mandibular premolars of the species at the enamel-dentine junction (EDJ). Comparing with specimens of Paranthropus, Australopithecus and Homo (n = 97), we find that the H. naledi premolars from the Dinaledi chamber consistently display a suite of traits (e.g., tall crown, well-developed P 3 and p 4 metaconid, strongly developed P 3 mesial marginal ridge, and a P 3 > p 4 size relationship) that distinguish them from known hominin groups. Premolars from a second locality, the Lesedi Chamber, are consistent with this morphology. We also find that two specimens from South Africa, SK 96 (usually attributed to Paranthropus) and Stw 80 (Homo sp.), show similarities to the species, and we discuss a potential evolutionary link between H. naledi and hominins from Sterkfontein and Swartkrans. Homo naledi is a hominin species first described in 2015 based on remains from the Dinaledi Chamber in the Rising Star cave system in South Africa 1 , and subsequently from a second chamber in the cave, the Lesedi Chamber 2. The species presents a combination of features not found in any other taxon, and attempts to interpret its phylogenetic position within the hominin clade have proved difficult. There are remarkably modern features such as the morphology of the foot 3 , as well as the morphology of the wrist and the relative length of the thumb 4. However, the cranial capacity is small, both absolutely and relative to body size 1,2 , and there are primitive Australopithecus-like traits in the fingers 4 , upper and lower limbs 5,6 and pelvis 7. Studies of the dental evidence have likewise revealed a unique combination of features. The permanent postcanine dentition is characterized by small teeth that retain principal molar cusps, but seemingly lack accessory crown traits common in other African hominin groups 1,8. However, the deciduous dentition shows a number of derived Paranthropus-like traits 9 , and molar root metrics find similarities between H. naledi and South African Homo specimens SK 15 and SK 45, as well as KNM-ER 1805 from Koobi Fora 10. Mandibular premolar morphology is particularly useful in studies of hominin taxonomy 11-15 , and initial descriptions suggest the P 3 of H. naledi is highly distinctive. The tooth is absolutely small in size, double rooted, fully bicuspid, and has a symmetrical occlusal outline, a combination of features suggested to be unique among the hominin fossil record 1. Recent morphometric studies of the mandibular premolar enamel-dentine junction (EDJ) demonstrate that this method has the potential to be a powerful tool in distinguishing between hominin taxa 16,17. We therefore aim to further investigate this distinctive premolar morphology at the EDJ.

Dental data challenge the ubiquitous presence of Homo in the Cradle of Humankind

“…It was suggested by Davies et al. ( 50 ) that SK 96 EDJ shares similarities with Paranthropus and also approximates that of Homo naledi . The taxonomic attribution of this specimen thus remains problematic.…”

“…If SK 96 belongs to Paranthropus , it differs from the morphology of P. robustus , and it raises questions regarding possible affinities with H. naledi . There is a chronological gap of more than 1 My between the Swartkrans member 2 specimen and the Rising Star Cave hominins, and phylogenetic relationships of H. naledi with other hominin taxa are still uncertain ( 50 , 65 ), and could be a focus of future studies. Our results also indicate that SK 15, holotype of the nomen oblitum T. capensis ( 15 ), as well as SK 18a that is supposedly associated with this mandible, actually represent an australopith taxon and not Homo .…”

“…Three-dimensional geometric morphometric (3D GM) analyses using landmarks and semilandmarks show that dentine horn height, crown height, and cervix shape can distinguish the postcanine teeth of Australopithecus , P . robustus , and Late Pleistocene to Holocene Homo ( 47 , 49 , 50 ). The recently developed diffeomorphic surface matching (DSM) method ( 51 ), which models the deformation between shapes, captures the geometric details of an anatomical structure and is now used as an advanced analytical tool in morphometrics ( 51 – 54 ).…”

Relative tooth size, Bayesian inference, and Homo naledi

“…As such, the initial intent here was to simply undertake an equivalent TSA analysis to dentally characterize H. naledi, estimate interspecific relationships, and assess its taxonomic classification. Earlier studies based on characters across the skeleton supported its inclusion in the genus Homo, but as a distinct member (Berger et al, 2015;Thackeray, 2015;Dembo et al, 2016;Irish et al, 2018; also see Holloway et al, 2018;Davies et al, 2020). A phenetic approach was deemed as most appropriate because continuous odontometric data do not lend themselves well to standard cladistic analyses; that is, they are typically reduced to a few ratios or crown areas qualitatively discretized into two or more states, along with other morphological characters (Berger et al, 2010;Strait & Grine, 2004).…”

“…The deciduous mandibular canine and the first molars in both isomeres share apparent derived traits with the latter genus, though features of the second deciduous molars are Homo-like(Bailey et al, 2019). In a geometric morphometric study of mandibular premolar enamel-dentine junctions (EDJ),Davies et al (2020) reported that the species is closest to P. robustus in a PCA ordination of the first two components (73.7% of variation) for LP3 shape. Homo habilis is plotted nearby, but other specimens in the genus, including H. erectus and, in particular, H. neanderthalensis and H. sapiens, are increasingly distinct.…”