Published online 11 March 2009 | Nature | doi:10.1038/news.2009.149

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Classic Homo erectus fossils in Zhoukoudian caves are more than 700,000 years old.

A replica Homo erectus skull reconstructed from some of the Zhoukoudian fossils. Nature

Researchers have sifted the sands of time to show that Homo erectus lived at China's most famous anthropology site at least 250,000 years earlier than was thought.

The new date means that this early human ancestor â€" the first lineage to migrate out of Africa â€" prospered in an earlier, colder climate, and its physical development in China matched that in Africa, where the species first evolved.

Discovered in 1918, the Zhoukoudian caves near Beijing have yielded surprises for nearly a century. Layers in the hillside cave system overlooking a river valley have produced some 17,000 stone artefacts and fossils of 50 H. erectus individuals, including six skulls. The species had a distinctive barrel-shaped torso and stood 145â€"180 centimetres tall, walking upright in a similar way to modern humans (Homo sapiens).

Now, work by a team of scientists based in China and the United States reveals that the Zhoukoudian cave fossils are about 770,000 years old â€" much more ancient than previous estimates of 230,000â€"500,000 years.

The new dates are based on the effects of cosmic rays on aluminium and beryllium isotopes in miniscule quartz grains â€" which Chinese researchers meticulously selected from sedimentary sand in weeks of painstaking work. The isotopic method was also applied successfully to three quartz tools. The research is published in Nature1.

"This is a catalyst for a new era of re-dating," says palaeoanthropologist Russell Ciochon of the University of Iowa at Iowa City, who was not involved with the work but has co-authored an accompanying News & Views article in Nature2.

Cosmic dating

The aluminium/beryllium technique offers new opportunities for re-dating other palaeoanthropology locations, particularly in China where the age of sites is difficult to determine because of the lack of more easily dateable volcanic ash. The method works for samples up to at least 3 million years old.

Guanjun Shen of Nanjing Normal University, one of the authors of the new study, says these studies are already underway. "We are collecting samples from early Pleistocene hominin archaeological sites in China," he says, "Notably at Xiaochangliang and Majuangou in the Nihewan basin of Hebei Province." This is northwest of Beijing, where this region's earliest H. erectus lived.

To secure the new dates, Shen collaborated with co-author Darryl Granger at Purdue University in West Lafayette, Indiana.

First, the team had to select clear, white quartz crystals that were buried with the fossils, not grey ones that might have been washed in later. This meant tedious grain-by-grain examination of sediment. One student working for eight hours could only isolate two grams of quartz, Shen says.

Because 60â€"100 grams is needed for each test, Shen says: "I had to mobilize my students to work for weeks, work not so easily conceivable by my US collaborator."

The dating method is based on the radioactive decay of 26Al and 10Be isotopes. While they're on Earth's surface, the two isotopes within the quartz are produced at a known ratio through exposure to cosmic radiation. When sediment buries the quartz, the generation of isotopes by cosmic rays nearly stops. The researchers dated the fossils by determining the isotopic content of the quartz and calculating backwards to work out the ratio of the two isotopes when the quartz was buried.

Ancient migrants

More precise dates will fill in gaps about the migrations of H. erectus into northern China and to Java in Indonesia, which occurred at least 1.6 million years ago. H. erectus evolved about 2 million years ago in equatorial Africa, possibly surviving to 50,000 years ago in Indonesia.

Some anthropologists argue that there may have been two H. erectus migrations: the first along the southern Asian coast to Java, the other through Georgia then across northern China.

But Philip Rightmire, a palaeoanthropologist at Harvard University, isn't convinced. "I don't want to speculate on migration routes until there is more evidence," he says, such as from a fossil site in Pakistan.

But he says the new dates clearly show that Zhoukoudian H. erectus developed physically in the same way as African H. erectus fossils found at the Tighenif site (previously known as Ternifine) in Algeria.