A new study, published in the Proceedings of the National Academy of Science, confirms close relationship of Ardipithecus ramidus – a species of hominid that lived in the east of the African continent around 4.4 million years ago – to the subsequent Australopithecus and humans.

Though Ardipithecus ramidus had an ape-size brain and a grasping big toe used for clambering in the trees, it walked on two feet and had diamond-shaped upper canines, not the v-shaped ones chimps use to chomp.

Anthropologists disagree about where this mixture of features positions the ancient hominid on the tree of human and ape relationships.

Now new research has revealed a pattern of similarity that links Ardipithecus ramidus to Australopithecus and modern humans and but not to apes.

The hominid shares with Australopithecus and Homo a relatively short, broad central cranial base and related modifications of the tympanic, petrous, and basioccipital elements.

“Given the very tiny size of the Ardipithecus ramidus’ skull, the similarity of its cranial base to a human’s is astonishing,” said lead author Dr William Kimbel of Arizona State University.

The cranial base is a valuable resource for studying phylogenetic, or natural evolutionary relationships, because its anatomical complexity and association with the brain, posture, and chewing system have provided numerous opportunities for adaptive evolution over time. The human cranial base, accordingly, differs profoundly from that of apes and other primates.

In humans, the structures marking the articulation of the spine with the skull are more forwardly located than in apes, the base is shorter from front to back, and the openings on each side for passage of blood vessels and nerves are more widely separated. These shape differences affect the way the bones are arranged on the skull base such that it is fairly easy to tell apart even isolated fragments of ape and human basicrania.

The cranial base of Ardipithecus ramidus shows the distinguishing features that separate humans and Australopithecus from the apes.

Earlier research had shown that these human peculiarities were present in the earliest known Australopithecus skulls by 3.4 million years ago. The new study expands the catalogue of anatomical similarities linking humans, Australopithecus, and Ardipithecus ramidus on the tree of life and shows that the human cranial base pattern is at least a million years older than Australopithecus afarensis.

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Kimbel WH et al. Ardipithecus ramidus and the evolution of the human cranial base. PNAS, published online on January 06, 2014; doi: 10.1073/pnas.1322639111