The mandible of specimen KW 9000, an infant P. robustus. Credit: J. Braga

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Analysis of rare hominin infant fossils excavated in southern Africa reveals unexpected cranial features which add weight to the hypothesis that Paranthropus robustus is more closely related to the Homo genus than to other Australopithecines.

The Australopithecines are an extinct group of early hominins that lived from about 4.2 to 1.2 million years ago, and included the genus Paranthropus. Their members are known as “robust” Australopiths because of their large, heavily enamelled postcanine teeth set within broad jaws.

Paranthropus are thought to have lived alongside the earliest Homo species, but their relationship to other early hominins is a matter of debate. Some researchers argue that they belong to the genus Australopithecus because of similar skull features, but others maintain that they constitute a distinct genus.

The newly discovered fossils are skull fragments from four P. robustus infants, found at the Kromdraai and Drimolen sites by José Braga, of the University of Toulouse, and colleagues. The findings, reported in Science Advances, “suggest that Homo and Paranthropus share an as yet unknown common ancestor with more derived features,” says Braga. The analysis of the four specimens revealed that most of the features that characterise the robust Australopithecines appear late in craniofacial development.

Although the baby teeth are similar in size to those of other Australopithecine specimens, some features of the upper face appeared to stop growing at an earlier stage of development.

The skull exhibits growth patterns that would result in a larger and differently rotated occipital lobe of the brain than that in juvenile Australopithecine specimens such as the 'Taung child'. From this analysis, it appears that early development of the craniofacial skeleton in P. robustus was more similar to that of early Homo than to the Australopithecines.

“We will now use unpublished specimens to describe infant cranial development in specimens belonging to the genus Homo, which are of the same geological age as the Paranthropus specimens we described here,” says Braga .

“This will probably shed more light on the relationships between Paranthropus and other early hominins,” he adds