12 May 2000

New Fossils May Be First Human Ancestors Out Of Africa

by Kate Melville

A nearly complete fossil cranium and another skullcap, representing the earliest known human ancestors from Eurasia, may also belong to the first hominid species to journey out of Africa. A team of Georgian, German, French and U.S. researchers describe these "First Eurasians" from the site of Dmanisi in the Republic of Georgia in the 12 May issue of Science.

The Science authors say that these 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.

Most scientists think that 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 the African cradle.

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 the course of 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, according to 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. "It was a very nice surprise to find these skulls," says co-author David Lordkipanidze of the Republic of Georgia State Museum, who notes that the well-preserved crania provided enough diagnostic detail for the researchers to compare them with other fossil human species. Their analysis showed that the Dmanisi fossils shared extensive similarities with the African species Homo ergaster from the well-known site of Koobi Fora in Kenya.

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 around 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 1000 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'," explains co-author Susan Antón of the University of Florida in Gainesville. "With the appearance of Homo , we see bigger bodies that require more energy to run, and therefore need these higher quality sources of protein as fuel." Antón notes that 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.