An excavation in central Asia has unearthed a pair of 1.7-million-year-old fossil skulls, providing a glimpse of what may have been the first species of human ancestors to journey out of Africa.
The partial skulls resemble Homo ergaster, a contested fossil species dating to around the same time in eastern Africa, concludes a team led by anthropologist Leo Gabunia of the Republic of Georgia National Academy of Sciences in Tbilisi. The skulls showed fewer links to Homo erectus specimens in eastern Asia from as early as 1.6 million years ago.
"[H. ergaster] may represent the species that initially dispersed from Africa and from which the Asian branch of H. erectus was derived," Gabunia and his coworkers assert in the May 12 SCIENCE.
However, some scientists doubt the H. ergaster classification and instead place those fossils within H. erectus, which they view as the first human ancestor to have departed Africa.
A longstanding theory holds that H. erectus made the first move out of Africa more recently than 1 million years ago, after learning to make double-edged stone hand axes. In contrast, archaeologist Ofer Bar-Yosef of Harvard University champions the view that H. ergaster bearing simpler stone tools left Africa as early as 1.8 million years ago. Early migrants probably followed forested regions into Asia searching for larger hunting territories and relief from Africa's tropical diseases, he theorizes.
The newly discovered fossils, retrieved from ancient sediment beneath a medieval castle at Dmanisi in the Republic of Georgia, support Bar-Yosef's scenario, Gabunia and his colleagues say. Both Dmanisi skulls exhibit important similarities to H. ergaster craniums. These include large bony ridges above the eyes, a sharply angled braincase at the back of the head, and a smaller cranial volume--signifying a smaller brain--than H. erectus shows.