Saturday, 28 July 2018

Australopithecus afarensis: An early Hominin foot fossil from Ethiopia.

Whilst extensive searches of likely deposits in Southern and East Africa have provided us with a large number of specimens of early Hominins, most of these are known only from their skulls, and the larger bones of the limbs and torso, with smaller skeletal elements such as the hands and feet being extremely rare. This is unfortunate, as one of the things which most clearly differentiates Humans from their nearest Ape relatives is their upright stance and locomotion, making the transition between the two conditions of great interest to archaeoanthropologists, and while some deductions about this can be made by examining the long bones of the legs, access to the bones of the feet is potentially much more useful.

In a paper published in the journal Science Advances on 4 July 2018, Jeremy DeSilva of the Department of Anthropology at Dartmouth College, Corey Gill of the Department of Anthropology at Boston University, the Department of Radiology at Massachusetts General Hospital and Harvard Medical School, and the Department of Medicine at Icahn School of Medicine at Mount Sinai, Thomas Prang of the Center for the Study of Human Origins at New York University, and the New York Consortium in Evolutionary Anthropology, Miriam Bredella, also of the Department of Radiology at Massachusetts General Hospital and Harvard Medical School, and Zeresenay Alemseged, of the Department of Organismal Biology and Anatomy at the University of Chicago, desctibe a fossil foot from a juvenile specimen of the Hominin Australopithecus afarensis from Dikika in the Afar Region of Ethiopia.

The specimen described DIK-1-1f, was found in association with the previously described DIK-1-1 skeleton in January 2002, however, while its discovery was recorded, the specimen was not properly described as it was embedded in the matrix and it took considerable time and effort to expose it. The specimen is 54.6 mm in length, and comprises the hind part of the foot, to the bases of the metatarsals, though the forward parts of the metatarsals and the phalanges are missing.

Australopithecus afarensis juvenile foot DIK-1-1f shown in (clockwise from top left) medial, dorsal, and lateral views. DaSilva et al. (2018).

This is the first described juvenile foot of Australopithecus afarensis, and with an estimated age of 3.3 million years old is the oldest described foot of any juvenile Hominin. However, a foot of an adult of the same species has previously been described, enabling a comparison of the ontology (growth progress) of Australopithecus afarensis, compared to Apes and Modern Humans. 

The feet of young Apes are typically gracile (slender) and adapted for gripping as well as locomotion, a trait which often becomes more extreme as they age. The feet of young Humans, in contrast, tend to be robust and adapted to load bearing, which again becomes more extreme as we age. The feet of adult Australopithecus afarensis are Human-like, robust and suited to load-bearing during bipedal locomotion, but not for gripping. The foot of DIK-1-1f, however, is more slender and apparently capable of gripping, indicating that the change in lifestyle of adult Hominins occurred before that of juveniles. DaSilva et al. suggest that, like many other Primate species, juvenile Australopithecus afarensis may have been carried by their mothers for an extended period before becoming independent, and that an ability to grip onto their mothers with their feet would have facilitated this.

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