“Morphological clock” age estimate of 912 ka for Homo naledi

The evolutionary relationships and age of Homo naledi: An assessment using dated Bayesian phylogenetic methods

Mana Dembo, Davorka Radovčić, Heather M. Garvin, Myra F. Laird, Lauren Schroeder, Jill E. Scott, Juliet Brophy, Rebecca R. Ackermann, Chares M. Musiba, Darryl J. de Ruiter, Arne Ø. Mooers, Mark Collard

Journal of Human Evolution Vol. 97, August 2016: 17–26; doi:10.1016/j.jhevol.2016.04.008

Homo naledi is a recently discovered species of fossil hominin from South Africa. A considerable amount is already known about H. naledi but some important questions remain unanswered. Here we report a study that addressed two of them: “Where does H. naledi fit in the hominin evolutionary tree?” and “How old is it?” We used a large supermatrix of craniodental characters for both early and late hominin species and Bayesian phylogenetic techniques to carry out three analyses. First, we performed a dated Bayesian analysis to generate estimates of the evolutionary relationships of fossil hominins including H. naledi. Then we employed Bayes factor tests to compare the strength of support for hypotheses about the relationships of H. naledi suggested by the best-estimate trees. Lastly, we carried out a resampling analysis to assess the accuracy of the age estimate for H. naledi yielded by the dated Bayesian analysis. The analyses strongly supported the hypothesis that H. naledi forms a clade with the other Homo species and Australopithecus sediba. The analyses were more ambiguous regarding the position of H. naledi within the (Homo, Au. sediba) clade. A number of hypotheses were rejected, but several others were not. Based on the available craniodental data, Homo antecessor, Asian Homo erectus, Homo habilis, Homo floresiensis, Homo sapiens, and Au. sediba could all be the sister taxon of H. naledi. According to the dated Bayesian analysis, the most likely age for H. naledi is 912 ka. This age estimate was supported by the resampling analysis. Our findings have a number of implications. Most notably, they support the assignment of the new specimens to Homo, cast doubt on the claim that H. naledi is simply a variant of H. erectus, and suggest H. naledi is younger than has been previously proposed.

Dembo_et_al_2016_Fig_3Fig. 3: A plot of the geological dates associated with the fossil taxa compared with the dates estimated with the morphological clock. Points above the dotted line of equality represent species with dates that are overestimated by the morphological clock while points beneath the dotted line are species whose dates are underestimated by the morphological clock.

Live report by Michael Brass from the ongoing 23rd biennial Society of Africanist Archaeologists Meeting in Toulouse:

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The talk in question:

(Session 18 — “The Dead as Witnesses to the African Past”, Communication 3)

1. University of the Witwatersrand (Johannesburg, South Africa)
2. Department of Anthropology, Sewell Social Science Building, University of Wisconsin (Madison, USA)

Abstract – The Dinaledi Chamber of the Rising Star cave system (Cradle of Humankind, South Africa) has yielded more than 1550 fossil specimens representing a minimum number of 15 individuals, all of which have been attributed to the taxon Homo naledi. Multi-disciplinary lines of evidence (from structural geology, sedimentology, palaeo- and forensic taphonomy) exclude conventional depositional models seen in other caves in the Cradle of Humankind, including carnivore predation, scavenging, death trap, water transport, mudflow, and mass fatality. We have proposed that the remains of Homo naledi may have been deliberately introduced into the chamber by conspecifics through the practice of funerary caching. This paper discusses the evolution of primary forms of mortuary practice – structured abandonment, funerary caching, and formal burial – highlighting the evolutionary, ethological and cultural contexts in which such mortuary behaviours may have developed. In doing so, we draw on comparative anthropology and primatology to investigate the relationships between non-human primate reactions to death, and the origins of complex ritualised mortuary behaviours in hominins. We consider the evidence from the Dinaledi Chamber in relation to these behaviours, and examine the implications for the African archaeological record of the possible cultural transmission of ritualised behaviours.