"Peking Man" is older than he looks. That's the conclusion of pioneering geochronologist Richard Teh-Lung Ku, whose new analysis of one of the most important finds in human evolutionary history indicates that the fossils date from at least 100,000 years earlier than scientists previously believed.
Ku dated the limestone caverns in which Peking Man dwelled at Zhoukoudian, not far from Beijing. In excavations that began in 1921, archeologists and paleontologists have recovered the remains of at least 40 individuals belonging to a hominid species that used fire and crudely fashioned stone tools but was anatomically distinct from Homo sapiens.
Working with professor Guanjun Shen of the University of Guizhou in China, Ku gathered cave limestone samples from the Peking Man site. Back in the lab, the researcher used state-of-the-art methods to show that the limestone from strata just above the layers containing Peking Man fossilized bones is close to 400,000 years old. Previous studies, using less advanced methods, dated the remains in the 200,000- to 300,000-year-old range.
Ku - a professor in the Department of Earth Sciences who was recently elected to fellowship in the American Geophysical Union for his pioneering work in the field of geochronology - helped develop the sophisticated methods used in the new analysis. The two scientists' preliminary findings will appear as a lead article in the August 1996 issue of the journal Acta Anthropologica Sinca, published in the People's Republic of China.
The dating of cave limestones ("speleothems") like those of Zhoukoudian uses principles of chemistry and nuclear physics. Such limestone forms where groundwater contains a high concentration of dissolved calcium and carbonate ions. When this water emerges from the ground, part of the dissolved carbonate "degasses" as carbon dioxide. At the same time, calcium carbonate precipitates, forming the spectacular deposits seen in limestone caves, such as stalactites, stalagmites and flowstones.
Fortunately for geochronologists, such limestone-forming groundwater often also contains minute quantities of dissolved uranium, which gets incorporated into the limestone crystals as they form.
The most common form of uranium, U-238, is unstable. At a slow but steady rate, it sends off subatomic particles. The process transmutes U-238 first into another uranium isotope, U-234, and then into an isotope of the element thorium, Th-230.
Thorium, unlike uranium, does not dissolve in water. Therefore, scientists reason, all Th-230 found in limestone must be the product of radioactive decay of U-238. By making precise determinations of the amounts of all three isotopes (U-238, U-234 and Th-230), and knowing the rate at which the transmutations occur, researchers can estimate the age of a rock. Basically, the higher the ratio of Th-230 to U-238, the older the limestone.
To get these ratios, scientists previously measured the minute amounts of radioactivity emanating from limestone. With proper instruments, they can distinguish between the radiation coming from decaying U-238, U-234 and Th-230 atoms.
The theory is good, but implementing it is difficult. The amounts of radiation produced are minimal, and even small measurement errors can lead to big errors in estimating age.
To significantly reduce the measurement errors requires the use of a device known as a mass spectrometer. This device can actually measure quantities of U-238, U-234 and Th-230 atom by atom, instead of waiting for them to decay and measuring their radiation.
Previously, even a mass spectrometer was not capable of counting the extremely small amount of U-234 and Th-230 found in samples like the Zhoukoudian material. But by upgrading and calibration of instruments pioneered by scientists at Caltech, Ku and his Caltech colleagues demonstrated in 1987 the feasibility of using mass spectrometry to make the kind of measurements demanded by the Zhoukoudian dating project. They achieved a 10- to 100-fold improvement in sensitivity and precision over the radiation-measuring methods.
The increased sensitivity also means that less material is required for analysis.
"We are able to select pure limestone crystals," said Ku, thus reducing the risk of a sample's being accidentally contaminated by older or younger non-carbonate material.
The mass spectrometric method also extends the time horizon for the method, allowing scientists to date material as far back as 600,000 years.
In addition to using a more sensitive dating method, Ku believes he has reduced error by analyzing undisturbed limestone from the strata in which the bones were found instead of studying the fossils themselves, as previous investigators had done.
According to Ku, there is evidence that radioisotopes can migrate in and out of bone during the fossilization process, affecting the accuracy of dating.
To double-check his data, Ku compared his mass spectrographic age determinations with results obtained from an alternate method - one using the isotopes Proactinium-231/Uranium 235. Though less precise than the U-238 isotopic ratio analysis, this method, too, confirmed the greater age of the finds.
The collaboration between Shen and Ku goes back more than a decade, to Shen's research on European caves for his dissertation at the University of Paris. Ku, then taking sabbatical leave from USC as a Fulbright Senior Scholar and Guggenheim Fellow visiting France, was brought in as an outside expert on geochronology. The two scientists have stayed in touch ever since.
In addition to his work at Zhoukoudian, Ku is currently engaged in numerous studies using subtle clues in ocean or lake sediments to study the past climate on Earth. One such study, for example, uses ratios of oxygen isotopes found in sediments from California's Mono Lake to reconstruct detailed climate fluctuations in the area since 10,000 years ago, shortly after the last Ice Age had ended.
Ku's research on the Zhoukoudian finds is supported by a grant from the National Science Foundation. Further field sampling and lab analyses are continuing.