COMMENTARY

 

Commentary on Neves et al. (2024): Dental metrics of Sahelanthropus and other hominoids

 

 

Francis Thackeray

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

Correspondence

 

 

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ABSTRACT

SIGNIFICANCE:
Neves et al. (S Afr J Sci. 2024;120(7/8), Art. #16362) have presented a valuable dental database for a diversity of hominoids, which can be criticised for two reasons. Firstly, they make a taxonomic statement based only on a size-related first principal component. Secondly, they regard the second principal component as insignificant. Thus, they conclude on weak grounds that their results do not "preclude" Sahelanthropus from being a hominin. I used UPGMA (unweighted pair group method with arithmetic mean) analysis to infer that variability in upper dentition molars of the hominoid taxa under consideration is not inconsistent with the view that Sahelanthropus is a hominin.

Keywords: hominin, Sahelanthropus, Pan, Miocene, Pliocene, Pleistocene

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Introduction

Neves et al.¹ have undertaken an interesting "first of its kind" multivariate analysis of mesiodistal (MD) and buccolingual (BL) measurements of teeth of a diversity of hominoids (hominins and apes), including Sahelanthropus, circa 7 million years old from Chad as reported by Brunet et al.² From principal component (PC) analysis they conclude that their results are not inconsistent with the view that taxonomically Sahelanthropus is a hominin rather than an ape. Their conclusion is based on the first component (PC1) which they correctly recognise as being associated with size; but size alone is certainly not necessarily a reflection of taxonomic relationships. Secondly, they dismiss PC2 as having no relevance in their analysis. They explicitly state: "PC2 is residual in nature and does not allow for any interpretation."¹ However, this is not necessarily the case, as the second component is typically related to shape, associated with morphology, even if the amount of variance associated with Pc2 (or PC3) is relatively low.

These observations do not in any way diminish the value of their basic data set and the results of their PC analysis. What matters is the way in which the data are interpreted. In their graph upon which Figure 1 is based, PC2 values for Sahelanthropus, early Homo and Pan (chimpanzees) all fall within a limited range of about +1.5 to -1.5. Contrary to the view of Neves et al.¹ who stated that PC2 "does not allow for any interpretation", there is indeed significance in the sense that Paranthropus shows the greatest degree of variability in the first and second principal components.

 

 

MD/BL ratios independent of size

Recognising that size is strongly associated with PC1 which accounts for most of the variability of the multivariate analysis, it is appropriate to examine MD/BL ratios which are independent of size. Table 1 shows such ratios for upper first, second and third molars for nine hominoid taxa. These data are based only on mean MD/BL values published by Neves et al.¹ such that, as yet, it is not possible to demonstrate ranges of variation. Unfortunately, from the results available based on mean values (Table 1), independent of size, it is not possible to identify distinct taxonomic groups.

 

 

upGMA analysis

The results of an UPGMA (unweighted pair group method with arithmetic mean) analysis of the MD and BL measurements published by Neves et al.¹ for upper post-canine dentition are presented in Figure 2. Three groups are recognised. The y-axis reflects UPGMA distance on a scale not related to time.

The three groups based on UPGMA are shown schematically in Figure 3 with reference to relative dates for genera under consideration. Group 1 includes South African Australopithecus as well as East African and South African Paranthropus. Group 2 includes East African Australopithecus, East African early Homo, Orrorin and Sahelanthropus. Group 3 includes Pan. A conclusion which can be drawn is that Sahelanthropus relates more closely to Orrorin and later East African hominins, than it does to Pan.

 

Conclusion

Neves et al.¹ have presented a valuable database for MD and BL measurements for upper post-canine dentition of a diversity of hominoids, but they can be criticised for two reasons. Firstly, they make a taxonomic statement based only on a size-related PC1. Secondly, they regard PC2 as insignificant. Thus, they conclude on weak grounds that their results do not "preclude" Sahelanthropus from being a hominin. Based on an UPGMA, it can be inferred that the variability in upper post-canine dentition of the taxa under consideration is not inconsistent with the view that Sahelanthropus is a hominin (Figure 3).

 

Acknowledgements

I am grateful to Walter Neves for correspondence, and two anonymous readers for comments.

 

Declarations

I have no competing interests to declare. I have no AI or LLM use to declare.

 

References

1. Neves W, Valota L, Monteiro C. Dental metrics of Sahelanthropus tchadensis: A comparative analysis with apes and Plio-Pleistocene hominins. S Afr J Sci. 2024;120(7/8), Art. #16362. https://doi.org/10.17159/sajs.2024/16362        [ Links ]

2. Brunet M, Guy F, Pilbeam D, Mackaye HT, Likius A, Ahounta D, et al. A new hominid from the Upper Miocene of Chad, Central Africa. Nature. 2002; 418:145-151. https://doi.org/10.1038/nature00879        [ Links ]

 

 

Correspondence:
Francis Thackeray
Email: francis.thackeray@wits.ac.za

Published: 29 September 2025