GAINESVILLE, Fla. --- A University of Florida anthropologist is part of an international team whose discovery of early human skull remains in the Republic of Georgia represent the earliest known human ancestors from Eurasia and also may belong to the first hominid species to journey out of Africa. The findings will appear Friday in the journal Science.
The team of researchers from Georgia, Germany, France and the United States the nearly complete fossil cranium and another skullcap in Dmanisi.
The Science authors, who include UF's Susan Antón, say the 1.7-million-year-old fossils are the first fossils discovered outside of Africa to show clear signs of African ancestry. The age and skeletal characteristics of the Dmanisi skulls link them to the early human species Homo ergaster, a species that some researchers believe is the African version of Homo erectus.
"The age of the site confirms our work in Indonesia that suggests that human ancestors left Africa fairly early -- about 1.7 million or 1.8 million years ago, shortly after the evolution of relatively larger bodies and bigger brains but before significant advances in stone tool technology," Antón said.
Most scientists think Homo erectus was the first hominid species to leave the African continent, although the exact identity of these ancestral travelers and the timing of their departure has been hotly debated for decades. Under the classic scenario, Homo erectus, armed with an advanced tool kit called the Acheulean or hand-ax tradition, became the first human species capable of braving an array of challenging environments outside of Africa.
The Dmanisi fossils, however, may undermine this tale of the technologically triumphant hominid. Stone tools found with the two skulls are of the less sophisticated "pebble-chopper" type that preceded the Acheulean in Africa, and the site itself is older than any known Acheulean tools. The tools, along with details of the fossils' anatomy and the age of the site, "argue for early, pre-Acheulean migrations out of Africa," say the authors.
The fossils were retrieved during archaeological investigations of a medieval castle at Dmanisi. Meticulous geological investigation confirms that the human fossils, accompanying animal bones and tools come from sediment-filled, irregularly-shaped "burrows," scooped out of the ancient strata by the flow of groundwater, said co-author Reid Ferring of the University of North Texas.
The two skulls were collected from the same layer and excavation pit as a hominid jawbone that was found at the site in 1991 and whose species identity was debated.
The research team discovered that the Dmanisi and Koobi Fora fossils overlap in age as well. Dmanisi contains a jackpot of chronological clues, from the isotope dates on the layer of basalt rock running beneath the site, to the paleomagnetic signature and contemporaneous animal fossils in overlying deposits.
Isotope analysis of the basalt places the age of the site at about 1.77 million years old, but the paleomagnetic signature of the sediment burrows themselves encompasses a period from 1.77 million to a little over a million years ago. Since the European faunal record is already well dated, the associated animal fossils at Dmanisi became "essential for understanding the timing of the site. Small rodents known to have lived more than 1.7 million years ago occur with the hominids," says co-author Carl C. Swisher III of the Berkeley Geochronology Center. In this case, the faunal evidence tipped the scales in favor of an earlier date.
More than 1,000 stone artifacts have been recovered from the Dmanisi fossil layers, providing further support to the 1.7-million-year-old date for the site. Despite the ready availability of raw material suitable for making Acheulean tools, the authors say, all of the Dmanisi artifacts are of a pre-Acheulean type that appeared in Africa as early as 2.4 million years ago.
If superior technology didn't lead the way out of Africa, as the Dmanisi evidence suggests, what other factors may have prompted these early humans to leave the continent? The Science authors speculate that the move might have been appetite-driven. "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',"Antón said. "With the appearance of Homo ergaster, we see bigger bodies that require more energy to run, and therefore need these higher quality sources of protein as fuel."
She said as early humans shifted their diets to include larger amounts of animal protein, there probably was a corresponding expansion in their home range to match the ranges of these animals.
Along with Ferring, Swisher and Antón, the Dmanisi research group includes director Leo Gabunia and Abesalom Vekua of the Republic of Georgia National Academy of Sciences; Antje Justus, Gerhard Bosinski, David Lordkipanidze and Olaf Jöris of the Romisch-Germanisches Zentralmuseum; Merab Tvalchrelidze, Givi Majsuradze, Aleksander Mouskhelishvili and Media Nioradze of the Republic of Georgia State Museum; and Marie-A. de Lumley of the Laboratoire Muséum National d'Histoire Naturelle, CNRS.
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