UNIVERSITY PARK, Pa. — In 2016, researchers in the Afar region of Ethiopia discovered a nearly complete cranium of an early human ancestor, Australopithecus anamensis, that dates to 3.8 million years ago. According to the international team of researchers, that fossil, designated MRD-VP-1/1, has revealed that A. anamensis and Australopithecus afarensis coexisted for about 100,000 years rather than at separate time periods as previously thought.

The digital reconstruction of the fossil that facilitated its analysis was initiated at Penn State. The results appear today (Aug. 29) in two papers in Nature.

"This is a game changer in our understanding of human evolution during the Pliocene (5.3 million to 2.6 million years ago)," said Yohannes Haile-Selassie, curator, Cleveland Museum of Natural History and adjunct professor, Case Western Reserve University.

Led by Haile-Selassie, paleoanthropologists conducted extensive analyses of MRD, and geologists determined the age and context of the fossil.

A. anamensis, the earliest species of the Australopith, was previously only known from tooth and jaw fragments, and these fragments made it possible for the researchers to identify the cranium. The completeness of the fossil also allowed the team to put a face on A. anamensis.

"MRD has a mix of primitive and derived facial and cranial features that I didn't expect to see on a single individual," said Haile-Selassie.

A collaboration between Timothy M. Ryan, interim head and associate professor of anthropology at Penn State, and Stefano Benazzi, professor of physical anthropology, and Antonino Vazzana, graduate student, both at the University of Bologna, Italy, allowed reconstruction of the fossils.

"We micro-CT (micro-computed tomography) scanned the fossils at Penn State in 2016 and I did the initial processing and 3D reconstruction from the scans," said Ryan. "I then passed those data over to our Italian colleagues to do a more complete reconstruction."

Digital reconstruction was necessary because the cranium was deformed and damaged during fossilization. To compare MRD to existing fossils, a more complete representation was necessary. The CT data were important not only for examining important details of the cranium, but also for creating an accurate reconstruction. Benazzi and Vazzana used computer modeling of the fossil, based on the CT scan data, to predict what MRD would have looked like. They created a virtual fossil reconstruction.

The researchers' identification and dating of MRD also allowed them to identify a partial cranium discovered in 1981 in Ethiopia that is referred to as the Belohdelie frontal. The researchers could now compare the A. anamensis facial features with those of the Belohdelie frontal. They found that the two were not the same, but that the Belohdelie frontal was much closer to A. afarensis.

Previously, paleoanthropologists thought that the transition from A. anamensis to A. afarensis was a serial process. A. anamensis existed through time and then A. afarensis existed, without overlap. However, if the Belohdelie frontal, which dated to 3.9 million years ago, is a member of the A. afarensis species, then A. afarensis existed during the same time that A. anamensis existed. In fact, the overlap was at least 100,000 years.

Also working on this project were Stephanie Melillo, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany; Beverly Saylor, Case Western Reserve University; Luis Gibert, Universitat de Barcelona; Alan Deino, Berkeley Geochronology Center, Berkeley, California; Mulugeta Alene, Addis Abba University, Ethiopia; Stanley A. Mertzman, Franklin and Marshall College; Naomi Levin, University of Michigan; Mark Peaple and Sarah Feakins, University of Southern California; and Benjamin Bourel, Doris Barboni, Alice Novello and Florence Sylvestre, Aix-Marseille University, Aix-en-Provence, France.

The National Science Foundation; the European Research Council; the Cleveland Museum of Natural History; W.J. and L. Hlavin; and T. and K. Leiden, supported this research.