The skulls, dug up near a village called Herto, fill a major gap in the human fossil record, an era at the dawn of modern humans when the facial features and brain cases we recognize today as human first appeared.
The fossils date precisely from the time when biologists using genes to chart human evolution predicted that a genetic "Eve" lived somewhere in Africa and gave rise to all modern humans.
"We've lacked intermediate fossils between pre-humans and modern humans, between 100,000 and 300,000 years ago, and that's where the Herto fossils fit," said paleoanthropologist Tim White, professor of integrative biology at the University of California, Berkeley, and a co-leader of the team that excavated and analyzed the discovery site. "Now, the fossil record meshes with the molecular evidence."
"With these new crania," he added, "we can now see what our direct ancestors looked like."
"This set of fossils is stupendous," said team member F. Clark Howell, UC Berkeley professor emeritus of integrative biology and co-director with White of UC Berkeley's Laboratory for Human Evolutionary Studies. "This is a truly revolutionary scientific discovery."
Howell added that these anatomically modern humans pre-date most neanderthals, and therefore could not have descended from them, as some scientists have proposed.
The international team is led by White and his Ethiopian colleagues, Berhane Asfaw of the Rift Valley Research Service in Addis Ababa, Ethiopia, and Giday WoldeGabriel of Los Alamos National Laboratory in New Mexico. The results of the find will be reported in two papers in the June 12 issue of the journal Nature.