Darryl Granger, left, and Guanjun Shen.

The excavation of numerous 'Peking Man' fossils in the 1920s and 30s at Zhoukoudian, the site of a collapsed cave near Bejing, laid to rest disputes about whether previously discovered Homo erectus fossils were primitive humans or deformed apes. But the question of Peking Man's precise age has remained a puzzle.

Efforts to date the fossils have been hindered by a lack of suitable methods, but Guanjun Shen of Nanjing Normal University in China and his colleagues have found a solution to the problem. In collaboration with Darryl Granger at Purdue University in West Lafayette, Indiana, they have determined that H. erectus lived in northern China as far back as 770,000 years ago — much earlier than had previously been assumed.

The Zhoukoudian site comprises 17 distinct sedimentary rock layers that have been deposited over time. The age of the top — and so most recent — 11 layers, in which hominin fossils and tools have been found, provides an estimate of the time period in which H. erectus lived. The most reliable methods for dating rocks rely on the decay of radioactive isotopes such as uranium. But, Shen explains, uranium-series dating is limited to the past 600,000 years or so, which means it can be applied only to layers one to five. “We therefore looked for ways to extend that limit,” says Shen, who has been analysing Zhoukoudian rock samples since 1989.

The solution came in the form of a new isotopic dating method, published by Granger and co-workers, that measures the ratio of aluminium-26 and beryllium-10 isotopes. “The idea,” says Granger, “is that while rocks are at Earth's surface, they are exposed to cosmic rays and obtain a particular inventory of isotopes. When new sediment collects on top, the original rocks are buried and that inventory decays over time.” Determining a rock's isotopic content thus reveals the time at which it was buried.

In 2003, Shen approached Granger to see whether the burial dating method could be used to determine the age of quartz-containing rocks and tools at Zhoukoudian. The first batch of samples yielded no useful data. Further study revealed that the original samples contained a mix of pure white quartz and smoky quartz. The smoky quartz is high in the stable isotope aluminium-27, and the researchers realized that this had masked the small amount of aluminium-26 present.

Shen came up with a painstaking solution — his group literally hand-picked the grains of quartz that appeared purest. “It took about eight hours to select just a few grams, and we needed 40 to 60 grams for analysis,” says Shen. “We had to mobilize all of the students at the lab here in Nanjing.”

In the end, their analyses revealed that H. erectus lived in northern China during a mild glacial period (see page 198). “Hominins living in glacial conditions would have been adapted to the cold, either biologically or culturally,” says Granger. “Would they have used fire? The evidence for that is shaky.” What is clear, however, is that burial dating is an effective method for determining the age of older rocks and artefacts. Shen and Granger have already started work on sites in the Nihewan basin in northern China, home to the oldest known assemblage of hominin tools in East Asia.