**SCIENTIFIC CORRESPONDENCE**

**Estimating the age and affinities of Homo naledi**

**J. Francis Thackeray**

Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa

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**Keywords:**Rising Star; Plio-Pleistocene; least squares linear regression; morphometric analysis

Recent discoveries of more than 1500 hominin fossils from the site of Rising Star in the South African Cradle of Humankind, attributed to a new species *(Homo naledi), ^{1}*have attracted global interest. As yet no secure date for this extraordinary material has been obtained, and the relationship of this species to other Plio-Pleistocene taxa has been greatly debated in the media. Here I report results of morphometric analyses that may facilitate an assessment of the age and affinities of crania attributed to

*H. naledi.*

The method is based on a least squares linear regression analysis of mean values of measurements for crania of 12 hominin species (Table 1), as published by Berger et al.^{1} The analyses were performed to obtain standard errors of m-coefficients (se_{m}) in regression equations of the form *y = mx + c, *based on pairwise comparisons of cranial data, as described elsewhere for pairs of hominin specimens,^{2,3} taking into account criticisms raised by Gordon and Wood^{4}.

The degree of scatter around a regression line of pairwise comparisons is quantified by the se_{m} statistic. Log transformed se_{m} values for conspecific pairs of modern vertebrates (as well as invertebrates) display a normal distribution with a mean value of -1.61,^{5} which has been considered to be an approximation of a biological species constant (T) with a standard deviation of circa 0.1,^{3} and which has been used to facilitate a mathematical (probabilistic) definition of a species^{5}.

*H. naledi*to those of other species listed in Table 1, using data published by Berger et al.

^{1}Log se

_{m}Hn (x axis) values refer to results of pairwise comparisons when the measurements for

*H. naledi*are on the

*x*axis, and measurements for other taxa are on the

*y*axis in regression analyses. Log se

_{m}Hn (y axis) values refer to results of pairwise comparisons when the measurements for

*H. naledi*are on the

*y*axis, and measurements for other taxa are on the

*x*axis. 'Log se

_{m}mean' refers to the mean of these two values, and 'delta log se

_{m}' refers to the difference between the two values, which can also be used to assess degrees of similarity or dissimilarity in the context of log se

_{m}values.

^{3}

In the context of results reported for conspecific pairs of modern taxa, it is relevant to report two results from the current study. Firstly, *H. naledi *appears to be significantly different (dissimilar) from other species listed in Table 1, because all of the mean log se_{m} values listed in Table 1 are outside the 95% confidence limits around the mean value of -1.61 +/- 0.1 for conspecifics. Secondly, *H. naledi *appears to be most similar to specimens attributed to early *Homo, *notably *H. habilis, *and (to a lesser extent) *H. rudolfensis *and *H. erectus *(see numbers listed in bold in Table 1).

A conclusion from this analysis is that the claim that *H. naledi *represents a distinct species appears to be warranted, at least from cranial data. Without assuming that log se_{m} values can provide accurate dates, the results presented in Table 1 may be used to provide an estimate for the age of *H. naledi, *here considered to be in the order of 2 million years (+/- 0.5 years), recognising that the maximum age for *H. rudolfensis *is circa 2.5 mya, the age for African *H. erectus *in this comparative study is circa 1.5 mya, and the age for *H. habilis *from sites such as Olduvai Gorge in Tanzania is circa 1.8 mya.

**Acknowledgements**

This research was supported by the National Research Foundation (South Africa) and the A.W. Mellon Foundation. I am grateful to Sue Dykes for assistance and discussion.

**References**

1. Berger LR, Hawks J, De Ruiter DJ, Churchill SE, Schmid P Delezene LK, et al. *Homo naledi, *a new species of the genus *Homo *from the Dinaledi Chamber, South Africa. eLife. 2015;4:e09560. http://dx.doi.org/10.7554/eLife.09560 [ Links ]

2. Thackeray JF. *Homo habilis *and *Australopithecus africanus, *in the context of a chronospecies and climatic change. In: Runge J, editor. Changing climates, ecosystems and environments within arid southern Africa and adjoining regions: Palaeoecology of Africa 33. Forthcoming 2015. [ Links ]

3. Thackeray JF, Dykes S. Morphometric analyses of hominoid crania, probabilities of conspecificity and an approximation of a biological species constant. HOMO J Comp Hum Biol. Forthcoming 2015. [ Links ]

4. Gordon AD, Wood BA. Evaluating the use of pairwise dissimilarity metrics in paleoanthropology. J Hum Evol. 2013;65:465-477. http://dx.doi.org/10.1016/j.jhevol.2013.08.002 [ Links ]

5. Thackeray JF. Approximation of a biological species constant? S Afr J Sci. 2007;103:489. [ Links ]

**Correspondence**:

J. Francis Thackeray ]]>
Evolutionary Studies Institute

Private Bag 3, Wits 2050

South Africa

Email:Francis.thackeray@wits.ac.za