Fossils Hint at Who Left Africa First

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.

The skulls appeared in sediment that had already yielded the fossil jaw of an undetermined Homo species (SN: 2/11/95, p. 85).

More than 1,000 stone artifacts found with the fossils feature single edges and other signs of simple tool construction.

Three lines of evidence narrowed the age estimate for the fossil finds to 1.7 million years. These consisted of magnetic reversals in the soil just above the skulls, measurements of the decay of the element argon in fossil-bearing soil, and the bones of extinct rodents and other ancient creatures found with the skulls.

H. ergaster in Africa was taller than earlier Homo species and as tall as modern people. This size increase required high-quality protein sources, such as meat, suggest the researchers. An expansion of home ranges to follow the movements of prey may have led to migrations out of Africa, they propose.

The new finds look little like the earliest Homo skulls in western Europe, which date to about 1 million years ago. Colonization of western Europe from Africa or central Asia may have first occurred at that later date, the scientists say.

“The Dmanisi skulls are a most extraordinary discovery,” comments anthropologist Bernard Wood of George Washington (D.C.) University. “It’s the best evidence to date that Homo ergaster existed outside Africa.”

Harvard anthropologist David Pilbeam agrees. H. ergaster, which had a “stunningly” small brain for its body size, apparently left Africa for reasons that had nothing to do with burgeoning intelligence or toolmaking skills, he says.

Although the Dmanisi braincases are surprisingly small, they nevertheless represent a regional variation of H. erectus, argues anthropologist G. Philip Rightmire of the State University of New York at Binghamton. Some H. erectus skulls in Java and China only slightly exceed the Dmanisi finds in cranial volume, he says.