Chin morphology and sexual dimorphism in the fossil hominid mandible sample from Klasies River Mouth

Abstract

The site of Klasies River Mouth (KRM) in South Africa has produced a small sample of early Upper Pleistocene hominid remains that have been a focus for discussions of the origins of modern humans. Despite certain primitive characteristics exhibited by these fossils, proponents of a single recent origin have attributed them to early modern humans. Critics of this hypothesis have emphasized the significance of the archaic features evident in this sample, including the absence of pronounced chins among the mandibular specimens. This study compares the size range and chin morphology exhibited by the KRM mandibles with that of Neandertals, Upper Pleistocene humans, and recent humans. The extreme sexual dimorphism documented among the KRM fossils reflects the presence of a very small individual, and previous efforts to classify the KRM sample as archaic on the basis of their robusticity have failed to address the significance of this diminutive hominid. While each KRM fossil falls within the 95% envelope of variability established for chin development in a comparative modern sample, a similarly low frequency of pronounced chins is very unlikely to be found in any recent human population. The morphological pattern of the KRM mandibles is clearly distinct from that of Neandertals and of recent humans.

Mandibular Symphysis of Large-Bodied Hominoids

“…, the frequent preservation of mandibular specimens in the mammalian fossil record (White and Johanson 1982;Beecher 1983;Daegling and Grine 1991;Tobias 1991;Ward 1991;Wood 1991;Ravosa and Simons 1994;Brown 1997;Ravosa 1996Ravosa , 2000Takai et al 2000), and, finally, an anthropocentric interest in the unique morphology of the human chin (Lam et al 1996;Schwartz and Tattersall 2000;Dobson and Trinkaus 2002). Here, we study the two-dimensional symphyseal morphology of extant large-bodied hominoids in an attempt to identify factors that influence variation in mandibular shape.…”

The enduring puzzle of the human chin

“…4 Detailed investigations of the anterior symphyseal surface morphology of Neanderthal mandibles are needed to clarify this issue. However, some, on the basis of surface contours, have argued that certain Neanderthal specimens, including Zafarraya 2, Guattari 3, and Spy 2 (pictured here), have "incipient" chins, 4,[111][112][113] although they still lack significant lowerborder protrusion. Paradoxically, signs of incipient chins are sometimes interpreted as evidence of modern human-Neanderthal interbreeding 114,115 a well-confirmed phenomenon, 116,117 which has (rightfully) muddied the Neanderthal chin debate.…”

Technological, elemental and colorimetric analysis of an engraved ochre fragment from the Middle Stone Age levels of Klasies River Cave 1, South Africa

“…This also relates to the human remains recovered from Cave 1. Layers 14e15 yielded numerous fragmentary human remains representing more than 10 individuals bearing both modern and archaic morphological features (Grün et al, 1990;Rightmire and Deacon, 1991;Deacon, 1992;Lam et al, 1996;Klein, 2001). Some remains also represent individuals that appear to have been dismembered, either as a result of cannibalism (Deacon, 1993(Deacon, , 2008Deacon and Wurz, 2001) or the post-mortem 'ritualised' treatment of the dead (see White, 1986;Clark et al, 2003;White et al, 2003).…”