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Peking Man fossils show their age

New ages for the fossils feed suspicions that the ancient species migrated to Asia in two separate waves

1:33pm, March 11, 2009
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Peking Man has suddenly gotten much older, and with age comes re-evaluation. The Homo erectus fossils from China’s Zhoukoudian cave system that are referred to collectively as Peking Man date to 780,000 years ago, roughly 200,000 years earlier than usually thought, scientists say.

Along with recently revised dates for a handful of other Chinese H. erectus sites, the new evidence fits with the idea that H. erectus traveled to eastern Asia in two separate migrations.

It also suggests that this ancient member of the human evolutionary family reached northeastern China at a time of relatively cool temperatures, say geologist Guanjun Shen of Nanjing Normal University in China and his colleagues. Such climate conditions fostered the spread of open grasslands, or savannas, that H. erectus had favored in Africa and central Asia.

In it for the long haul, H. erectus inhabited Zhoukoudian and nearby sites throughout a series of shifts from cool, dry climates to warm, wet conditions, Shen’s team reports in the March 12 Nature. Evidence indicates that H. erectus lived at Zhoukoudian until about 400,000 years ago, the researchers note.

“If Homo erectus groups lived there for that long, they must have passed behavioral traditions across many generations for adapting to climate cycles,” remarks anthropologist Russell Ciochon of the University of Iowa in Iowa City.

In several papers published from 2005 to 2008, other Chinese scientists reported that H. erectus — a species that originated about 2 million years ago in Africa — reached four sites northwest of Zhoukoudian and one site in southern China almost 1.3 million years ago, not 1.7 million years ago as had been suggested by earlier studies.

Combined with the new age for Peking Man, these dates suggest two H. erectus populations:One occupied northeastern China for almost 1 million years, and one that reached the Indonesia island of Java 1.6 to 1.5 million years ago. Part of this Java population may have survived until 50,000 years ago.

A huge forested region in southern China would have deterred H. erectus from traveling between northeastern China and Indonesia, according to Ciochon.

In a comment accompanying the new study, Ciochon and Iowa geologist Arthur Bettis III hypothesize that H. erectus populations in or just outside of Africa took two separate routes eastward into Asia. Ciochon and Bettis propose that an initial migration followed Asia’s southern coast to Java, which was at the time connected to the mainland. Later, H. erectus passed through central Asia and southern Mongolia to reach the Zhoukoudian vicinity.

Most recent models of H. erectus population movements posit that this species reached Java first and later dispersed northward to China. “It may be time to rethink that scenario,” Ciochon says.

Researchers first unearthed Peking Man fossils at Zhoukoudian in the 1920s. The Chinese site, located on the outskirts of Beijing, has since yielded 17,000 stone artifacts and fossils from more than 50 H. erectus individuals.

Peking Man’s formerly younger age had been estimated using several dating techniques that are vulnerable to large inaccuracies. But Shen’s team examined Zhoukoudian soil layers for signs of reversals in Earth’s magnetic field that occurred at known times. The researchers then employed a recently developed technique for estimating how long fossils have been covered by soil, based on different decay rates of radioactive forms of aluminum and beryllium in quartz grains.

After learning of Peking Man’s readjusted age, Bettis and Ciochon examined geological evidence for the presence of cold and dry or warm and wet climates at Zhoukoudian and the five other Chinese H. erectus sites with revised ages. H. erectus adapted to both climates in eastern Asia, the two scientists say.

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