【化石】アウストラロピテクス・アナメンシスの頭蓋骨と向き合う

A nearly complete 3.8-million-year-old hominin skull discovered in Ethiopia is described in two papers published in Nature this week. The skull, which the authors assign to the species Australopithecus anamensis, provides new insights into the earliest australopiths and their origins.

The earliest members of the hominin genus Australopithecus have remained poorly understood, owing to the near absence of cranial remains older than 3.5 million years. Specimens of A. anamensis, the oldest known members of the genus, date to 3.9 - 4.2 million years ago and consist primarily of jaws and teeth, although multiple skulls are known for younger species, dating to 2.0 - 3.5 million years ago.

Yohannes Haile-Selassie and colleagues report a nearly complete cranium from Woranso-Mille, Ethiopia, which they assign to A. anamensis on the basis of its teeth and jaw. The specimen is likely to be an adult male despite its small size. However, the primitive cranial morphology links this fossil to even more ancient hominins, such as Sahelanthropus and Ardipithecus, and casts doubt on previous assumptions about a direct link to the younger Australopithecus afarensis (represented by the famous ‘Lucy’ fossil). In particular, the findings suggest that A. anamensis and A. afarensis lineages may have overlapped for at least 100,000 years (‘cladogenesis’) rather than the former preceding the latter in a single evolving lineage (‘anagenesis’).

A second paper describes the age and context of the cranium, and suggests that the hominin lived in predominantly dry shrubland, with varying proportions of grassland, wetland and riverside forest.

“This cranium looks set to become another celebrated icon of human evolution,” writes Fred Spoor in an accompanying News & Views article. He concludes that the discovery will “substantially affect our thinking […] on the evolutionary family tree of early hominins”.