The Japan Times
February 26, 2001

New uranium dating resets clock with dates of 580,000-620,000 years ago

Nanjing Man tests 'Out of Africa' theory

SYDNEY (Reuters) Australian and Chinese scientists have dated China's Nanjing Man as 580,000 to 620,000 years old, further supporting a multiregional theory of human evolution that argues Asians evolved locally and not out of Africa.

The scientists said Nanjing Man (Homo erectus), a male and female skull discovered in 1993 in Tangshan Cave near Shanghai, shows that humans evolved in isolation in China and much earlier than previously thought.

"We can confidently say the Nanjing Man fossils are older than 580,000 years and probably at least 620,000 years old," said Zhao Jianxin from the University of Queensland.

Scientists had previously estimated Nanjing Man to be about 400,000 years old. The "Out of Africa" theory of evolution argues that Homo erectus in Asia was replaced by Homo sapiens newly arrived out of Africa about 100,000 to 200,000 years ago.

"The Nanjing Man finding supports the multiregional evolution model that argues modern Asian populations evolved directly from Asian Homo erectus, rather than evolving from populations out of Africa," Zhao said.

The scientists from Queensland and Nanjing universities said the limited dating range of radiocarbon had previously hindered accurate dating of human fossils and human evolutionary history.

But by using a new thermal ionization mass spectrometer, which measures the decay in radioactive uranium, a more accurate and older dating range can now be achieved, they said.

The scientists dated Nanjing Man measuring the rate of decay of uranium by counting the number of thorium atoms, but instead of dating the fossils, which are porous, they dated rocks, which better retain uranium, above and below the fossils.

"The problem with dating has been that the fossils themselves were actually dated, and we know that they don't retain the radioactive product, so any age you get from the fossil is a minimum age," said Ken Collerson of Queensland University.

The Nanjing Man finding was recently published by the respected U.S. journal Science, which said the Nanjing Man dating was consistent and would now allow a more accurate assessment of early migration out of Africa and Asian evolution.

"These ages, along with those from other sites in China, imply that most of the Homo erectus specimens there are older than previously thought and perhaps do not overlap significantly with younger Homo sapiens," Science said.

Collerson said scientists now believe Nanjing Man and the more famous Peking Man family, which had been estimated at 230,000 years old, evolved in small communities in eastern Asia.

"Under the multiregional evolutionary model, these small communities of Homo erectus probably developed in widely separate locations but had some communications, which explains the DNA similarities which developed," Collerson said.

The basic view is that a common ancestor of chimpanzees and humans emerged in Africa 8 million to 10 million years ago. The species evolved in many branches, eventually giving rise to Homo erectus and then Homo sapiens.