Asia was the first continent that early humans explored on their trek out of Africa. But exactly when they took that leap has long mystified paleoanthropologists. Now, a study dating stone tools found near an ancient lakebed in northern China supports the notion that the exodus may have occurred very early in history.
So far, the first signs of human presence in Asia are Homo erectus fossils dated to between 1.7 million and 1.9 million years ago in Dmanisi, Georgia, on Asia's western edge, and in Java, Southeast Asia (Science, 12 May 2000, p. 948). But doubts linger about some of those dates, and other traces of ancient Asians are questionable until about 1 million years ago.
To gather more evidence, a Chinese and American team took a new look at a set of stone tools--simple flakes, cores, and scrapers--found 21 years ago in the badlands of the Nihewan Basin, 150 kilometers west of Beijing. Researchers had suspected that the artifacts were more than 1 million years old, but dating sediments in China has been notoriously difficult because there is no volcanic tuff for radiometric methods.
So the team used high-resolution paleomagnetic dating, relying on known, ancient shifts in Earth's magnetic field to tie the tools to a particular period, says Rixiang Zhu, a geophysicist at the Institute of Geology and Geophysics at the Chinese Academy of Sciences in Beijing. In a paper published in the 27 September issue of Nature, they conclude that the tools are at least 1.36 million years old.
"Very early on, Homo had the capability to spread out of Africa and to move significantly northward across long distances with relatively simple tool kits," concludes archaeologist Kathy Schick of Indiana University in Bloomington. The team's dating efforts are "very nice, clean" adds geologist Frank Brown of the University of Utah in Salt Lake City.
Although no human fossils have been found in the basin, the tools' antiquity shows that early humans had already managed to adapt to life at 40 degrees north, says co-author Rick Potts, a paleoanthropologist at the Smithsonian Institution in Washington, D.C. Potts speculates that the climate may have been relatively warm at the time and that the toolmakers had to adapt to life in the north because the massive Qinling Mountains blocked them from migrating south.