Three-dimensional geometric morphometric analysis of Homo erectus lumbar vertebrae, with a focus on the Dmanisi...

Hominin lumbar vertebrae are characterized by adaptations for bipedal locomotion, which are mostly known from adults. However, vertebrae of Homo erectus are exclusively known from immature individuals. As such, understanding the ontogeny of lumbar vertebrae is warranted. Here, we use three-dimensional geometric morphometrics to quantify the morphology of three H. erectus upper lumbar vertebrae, D2672 from Dmanisi, Georgia; KNM-WT 15000 from Nariokotome, Kenya; and SK 853 from Swartkrans, South Africa, as well as those of two Australopithecus sediba U.W.88-92 and U.W.88-232 from Malapa, South Africa, in a comparative developmental context to assess the likely vertebral positions of D2672 and SK 853. The result suggests that the vertebral position of D2672 cannot be distinguished between the first and second lumbar vertebrae, while SK 853 most likely represents a first lumbar vertebra. We also quantified the ontogenetic variation in the first and second lumbar vertebrae of modern humans, from late infancy to full adulthood, and compared these with H. erectus. In modern humans, the lumbar vertebrae increase in size and undergo a series of shape changes across ontogeny. In particular, the vertebral canal relatively decreases in dorsoventral length but increases in mediolateral length with age, thus juveniles possess relatively narrower vertebral canals than adults. All H. erectus individuals fall within the range of modern humans at a comparable developmental stage in the morphospace, despite their smaller size, suggesting that the vertebral traits associated with humanlike bipedal locomotion were established relatively early in ontogeny and in the evolutionary process.