Southern exodus. A trail of stone tools and fossil bones suggests that early humans left Africa 1.8 million years ago. Some headed north to Dmanisi, Georgia; others may have taken a southern route into China and Java, Indonesia.

NASA/TerraMetrics/Human Origins Program, Smithsonian

Speed-Walking Across Asia

Over a million years ago, a band of early humans left their stone tools and two front teeth in a hilly basin in southwest China. For decades, the precise age of the fossils has remained a mystery, leaving open a central question in paleontology: How quickly did our human ancestors reach China after leaving Africa? Now, thanks to advanced dating techniques, scientists may finally have the answer.

Chinese paleontologists discovered the two incisors in 1965 and the relatively simple stone tools in 1973 in the Yuanmou Basin. The teeth came from a hominin, the group that includes humans and our exclusive ancestors, and might be from the species Homo erectus, a direct ancestor of humans that may have been the first human to leave Africa about 1.8 million years ago. Scientists have gotten mixed results for the age of the site because there were no volcanic crystals in the soils for reliable radiometric dating. Lacking solid dates, researchers thought until a decade ago that the earliest humans didn't reach Asia until 1 million years ago. But a series of new dates for fossils from one site in Java, Indonesia, in particular, have recently shown that Homo erectus was there 1.66 million years ago and possibly earlier. This changed the old textbook view that human ancestors spread around the globe only after they had big brains and more advanced stone hand axes, which appear in Africa about 1.6 million years ago.

Now, a team of Chinese and American researchers has redated the Yuanmou Basin site using a paleomagnetic technique that relies on rock samples to determine how Earth's magnetic poles were oriented when the rocks were formed. Although the original hillside where the fossils were found has been excavated, the discoverers took photos and recorded the layer of sediment where they uncovered the teeth and tools. The new team traced that sediment layer--or time horizon--throughout the basin, collecting 318 rock samples from it. In an article in press in the Journal of Human Evolution, the researchers report that the fossils came from a layer of rock just above a magnetic landmark known as the Olduvai-Matuyama reversal boundary, which is at least 1.77 million years old. This makes the fossil site slightly younger, about 1.71 million years old.

This age estimate represents "the oldest definite fossil and archaeological evidence of early hominins in China and mainland East Asia," says co-author Rick Potts, a paleoanthropologist at the Smithsonian Institution in Washington, D.C. The finds are a bit younger than the oldest Homo erectus fossils from western Asia, which are 1.77 million years old and come from Georgia, and a bit older than the most conservative dates for the Java remains and 1.66-million-year-old stone tools from northeast China. Taken together, these dates from at least three fossil sites are convincing many researchers that early humans were moving rapidly across Asia 1.77 million to 1.66 million years ago. "What's so important about this paper is that we finally have good, solid paleomagnetic dates," says paleoanthropologist Susan Antón of New York University in New York City. "I think the body of data for early Homo in China is getting much stronger."