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Research Increases Peking Man's Age by 100,000 Years

May 01, 1996|ROBERT LEE HOTZ | TIMES SCIENCE WRITER

Underscoring the antiquity of human evolution in Asia, scientists have discovered that one of China's most illustrious ancestors--a famous collection of fossils from pre-humans known as Peking Man--is at least 100,000 years older than previously believed.

The finding by scientists at USC and the University of Guizhou in China is the latest in a series of revised ages for pre-human fossils that has scientists reconsidering humanity's place in the cradle of time.

Richard Ku, a dating expert at USC, took advantage of a more sophisticated technique for determining the age of the limestone cave deposits in which the fossilized bones were found. His preliminary work shows that the bones are at least 400,000 years old and perhaps considerably older, he said Tuesday. The research, funded by the National Science Foundation, is to be published this summer in the Chinese scientific journal Acta Anthropologica Sinica.

The remains of the tool-using Peking Man, which today is considered an example of Homo erectus, were discovered in 1921 in a cavern at Zhoukoudian near Beijing. They were among the first pre-human fossils found in Asia and have played a prominent role in theories of human development.

In the years since, archeologists working at the site have unearthed the remains of at least 40 hominids, as the pre-human ancestors of modern mankind are called, who used stone axes and tamed fire.

F. Clark Howell, a noted UC Berkeley authority on human origins, called the new research "terrific."

No one knows just how long ago the earliest ancestors of humanity settled Asia. Some anthropologists argue that primitive hominids migrated out of Africa across the Asian continent in two great waves separated by almost a million years, settling some parts of Asia as long as 1.8 million years ago. Other experts use the evidence of early pre-human fossils on the mainland of Asia and on Java to suggest that modern humanity, with all its racial variations, may have evolved contemporaneously in different parts of the world.

Until now, some experts on human evolution have argued that the Peking Man deposits at Zhoukoudian were so recent that they could be taken as evidence that the primitive hominids co-existed with the direct ancestors of anatomically modern humans.

The newest date for the fossils, however, suggests that the primitive hominids may have died out before the rise of modern humans or evolved into our direct ancestors.

"The Zhoukoudian deposits have really stood as evidence of late survival of the species in the Far East," said Philip Rightmire, an anthropologist at the State University of New York in Binghamton. "If you push even the latest levels back to 400,000 years ago, then a claim for overlap begins to fall away. It begins to look more like a gradual evolution from one form to the next, rather than an overlap."

To date the deposits from the cave, Ku and his colleagues measured minute amounts of radioactivity in the limestone with an especially sensitive form of mass spectrometry developed at Caltech. Atom by atom, they measured three isotopes--uranium-238, uranium-234 and thorium-230--to gauge the rate of radioactive decay. The higher the ratio of thorium to uranium, the older the limestone.

"There is a revolution in the techniques of these measurements," Ku said. "It is a couple of orders of magnitude more sensitive than the older ways."

Previous studies have indicated that the newest limestone deposits in the cave were no more than 200,000 to 300,000 years old. Ku's work shows that they are at least 400,000 years old. He confirmed his results by a second, less precise dating technique using traces of other radioactive isotopes.

Ku said he plans to return to China this summer to collect more material from the cave to further refine the age of the bones themselves.

"We are dating the limestone itself associated with the bones," Ku said. "The Peking Man fossil was found in a layer of limestone older than what we have so far dated. We dated the strata just above the occurrence of the fossils. The limestone seems to be older than people thought.

"It implies that Peking Man could be considerably older than people thought," he said.

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

Revising Prehistory

Scientists have discovered that remains of the so-called Peking Man, the youngest known specimen of Homo erectus in Asia, are at least 400,000 years old--about 100,000 years older than believed. The bones were found at Zhoukoudian near Beijing in 1921.

What the Date Means

When experts believed the Peking Man deposits to be recent, they argued that the bones could be evidence that primitive hominids coexisted with modern humans. The older date suggests that the hominids either died out before modern humans or gradually evolved into them.

DATING CAVE LIMESTONE

1. Limestone often contains minute amounts of dissolved uranium, which decays at a slow but steady rate.

2. The most common form of uranium, U-238, gradually decays into an isotope called U-234 and then into an isotope of the element thorium, Th-230.

3. By measuring all three isotopes, scientists can estimate the rock's age.

4. The higher the ratio of thorium to uranium, the older the limestone.

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