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

Abstract

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.

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

“…This general assumption has commonly guided the interpretation of fossil discoveries with poor geological or stratigraphic context, including initial estimates for the age of the H. naledi fossils (Thackeray, 2016; Dembo et al, 2016). The new age estimates for H. naledi show that an approximate age for the hominin fossil fragments cannot be simply deduced from their morphology (Thackeray, 2016; Dembo et al, 2016). Detailed geological investigations are critical before any attempt to ascribe an age to the fossils is made, and even then great care must be taken in interpreting results, which may not always be conclusive (e.g.…”

“…U-Th dating is more precise in the <500 ka range than U-Pb dating, and is much less critically affected by detrital material. The initial tests with U-Th disequilibrium dating revealed that the fossils may be much younger than originally anticipated (e.g., Dembo et al, 2016; Thackeray, 2016), and mostly well within the range of the U-Th technique. Therefore, U-Pb dating was not pursued further.…”

“…Preliminary work was focussed on assessing the viability of U-series techniques for flowstone dating, using 14 C for dating bone fragments, and using OSL to test samples of quartz-bearing Unit 1 (Dirks et al, 2015). Initial tests were carried out at the University of Johannesburg (UJ) to assess suitability for U-Pb dating, which allows for the dating of older (>500 ka) flowstone material (e.g., Walker et al, 2006; Pickering et al, 2010; Pickering and Kramers, 2010), on the assumption that the H. naledi material could be older than 1 Ma based on its primitive morphology (Berger et al, 2015; Dembo et al, 2016; Hawks and Berger, 2016; Thackeray, 2016; Hawks et al, 2017). It was found that the older flowstones in the Dinaledi Chamber contained excessive common Pb caused by the inclusion of detrital material (mainly clays) making them unsuitable for U-Pb dating (Dirks et al, 2015).…”

Ribcage measurements indicate greater lung capacity in Neanderthals and Lower Pleistocene hominins compared to modern humans

“…Lower Pleistocene hominins from the Gran Dolina site (Burgos, Spain) are hypothetical ancestors of H. heidelbergensis (and thus Neanderthals) and are also thought to have large thoraces because of their long clavicles 6,30 . Whether H. antecessor is actually a species itself or represents an European branch of H. erectus / ergaster 34 , recent Bayesian analyses 58,59 suggest that H. antecessor belongs to a basal clade of modern human and Neanderthals, alongside other early Homo species such as H. erectus , ergaster and the recently discovered species named as H. naledi 60 . Therefore, H. antecessor could be used as an approach to test whether large bodied early Homo species already presented a large TLC.…”

An Early Pleistocene human pedal phalanx from Swartkrans, SKX 16699, and the antiquity of the human lateral forefoot

“…Although the recently discovered Dinaledi (Homo naledi) sample provides a distinctly human tarsometatarsal skeleton, its lateral pedal proximal phalanges have been described as having a mixture of australopith and later human features (Harcourt-Smith et al, 2015). Unfortunately, secure dates for this sample remain elusive (Dembo et al, 2016). It therefore remains unclear when the full morphology of the human lateral forefoot emerged.…”