Skulls offer clue to man's first steps out of Africa
LOS ANGELES Two fossil skulls found beneath a medieval castle in the former Soviet republic of Georgia may be the remains of the first human species to journey out of Africa, an international research team said Thursday.
Reliably dated to 1.7 million years ago, the bones offer the earliest known anatomical evidence of the direct link between humanity's ancestral birthplace in Africa and the primitive human forebears who colonized much of the rest of the world. They provide concrete evidence for the widely accepted theory that the human species originated in Africa, then migrated elsewhere, the researchers said.
The fossils predate what had been the first known appearance of humans in Europe by about 500,000 years.
The fragmented skulls belong to a young male and a female from a species called Homo ergaster. They were relatively tall, agile tool users who lived among the elephants, hyenas and rhinos along the lush riverbanks of prehistoric Dmanisi, about 90 km southwest of the Georgian capital of Tbilisi.
Most scientists think that Homo erectus were the first to leave the African continent, due to their development of comparatively sophisticated Acheulean tools, such as hand axes, that enabled them to explore unknown environments.
But numerous tools and stone objects found at Dmanisi are of an earlier, less sophisticated type, and the site is older than any known Acheulean tools, the researchers say.
All the clues "argue for early pre-Acheulean migrations out of Africa," according to the researchers.
The fossils and the many simple stone tools discovered with them reveal not only when humans first ventured from the place where they evolved, but also offer a suggestion as to why they might have left Africa, several experts said.
Many experts have speculated that the invention of more advanced stone tools, such as double-edged hand axes, gave primitive people a new ability to brave the challenges of new homelands. At Dmanisi, however, only the crudest stone choppers, flakes and scrapers have been discovered.
That suggests to some experts that it perhaps was hunger, not new technology that opened the world to the primitive hominids. Experts estimate that Homo ergaster were about one-third larger than earlier hominid species and might have needed at least 40 percent more energy to maintain themselves.
Under the spur of their voracious appetites, they may have foraged ever more widely to sustain the high-protein energy demands of their large bodies and evolving brains, eventually ranging into Asia.
The discovery by a team of Georgian, German, French and American scientists led by the Georgia National Academy of Sciences is documented in research published Friday in Science.
"This is one of the most important finds in a long time," said Carl Swisher, a dating expert at the Berkeley Geochronology Center who helped determine the fossils' age.
"It looks like we have representatives of that first early migration out of Africa."
Palaeoanthropologist Susan Anton at the University of Florida in Gainesville, a member of the research group, said there was no mistaking the fossils' distinctive features or their kinship with other hominids in Africa from the same period. "The anatomy is very clear," Anton said. "And the anatomy is telling us that this is something that came from Africa. We are talking about the very first time that hominids actually left the African continent to go into other parts of the world.
"With the appearance of Homo, we see bigger bodies that require more energy to run, and therefore need ... higher quality sources of protein as fuel," she said.
"Basically the argument that we're making is that during that time in Africa, the Savanna is expanding and there is a greater availability of 'protein on the hoof,'" Anton said.
Thus it is possible that the first hominids integrated more and more animal protein in their diet and migrated in pursuit of game.
Anthropologist Clark Howell at the University of California, Berkeley, called the discovery "a miracle" and said, "I think this is the beginning of a real revolution in human evolutionary studies."
The bones were discovered last May by a team excavating the grounds of a castle set high on a promontory overlooking two 80-meter-deep gorges carved by the Masavera and Pinezaouri rivers.
The skull fragments, along with hundreds of stone tools, were preserved in ancient river and lake sediments underlying the courtyards of the castle and its surrounding village walls. They were only a meter away from the spot where, in 1991, researchers had found a jawbone from an unidentified hominid species.
The researchers uncovered two partial skulls, crushed and distorted by the inexorable pressure of geologic burial. The larger skull has a braincase of about 775 cc, about half that of a modern human. The smaller and more complete skull has a cranium of about 650 cc.
The researchers dated the fossil deposits by precise argon isotope measurements of lava flows directly underneath the site, the paleomagnetic signature of the surrounding rocks, and by analyzing the fossil plants and animals found with the bones.
The bones closely resemble other early human fossils discovered in East Africa. And the ergaster species is almost identical to Homo erectus hominids found throughout ancient Asia during the same period, showing the full extent of the journey that the creatures and their descendants made.
Based on the few partial skeletons discovered elsewhere, they may have weighed about 45 kg on average and were about 1.7 meters tall.