The same thing might have just happened to our understanding of human prehistory with the radioactive dating of Nanjing Man.

Nanjing Man is the name given to a Homo erectus fossil found in China. He was thought to be around 400,000 years old. But, last month, a quartet of scientists from China and Australia re-dated Nanjing Man using uranium-series dating and found he was actually at least 620,000 years old.

This made the scientists question the dates given to other major archaeological finds - finds that have come to shape evolutionary theory as the world knows it.

  Suddenly, Nanjing Man became the catalyst for an evolutionary coup d'etat that could end Mitochondrial Eve's reign as the most accepted theory of human evolution.

The Mitochondrial Eve theory claims Homo sapien populations migrated from Africa between 100,000 and 200,000 years ago and replaced Homo erectus populations wherever they went.

But, based on his new age, Nanjing Man could very well be chalked up as another victory for the less-popular multi-regional model of evolution.

Multi-regionalism argues that modern Asian populations were not replaced by African sapiens. Instead, they evolved directly from Asian Homo erectus and Nanjing Man's new age would provide more time for this evolutionary process to have occurred.

  Nanjing Man's age and his implications for evolutionary theory have prompted the Chinese-Australian team of scientists who dated him to re-date other major archaeological finds using the uranium-series dating technique that derived Nanjing Man's age.

"Many Homo erectus sites are not well-dated," says Dr. Ken Collerson, a member of the team who dated Nanjing Man.

This is because of the limited capabilities or questionable results from other dating techniques such as electron-spin resonance, which Collerson refers to as "in the realms of alchemy."

By re-dating major archaeological finds, Collerson says he thinks the multi-regional theory of evolution will displace the Mitochondrial Eve theory as the most accepted evolutionary model.

"If I had to bet, I think the multi-regional model will probably be the one to emerge as a more realistic interpretation."

  Zhao Jian-xin, a member of the team that dated Nanjing Man, agrees. An archaeologist since 1997, he started his career as a geologist who knew little about evolutionary theory.

"I think the multi-regional theory, within the next 10 years, will take the upper hand," Zhao says. "The most important thing we want to push is a chronological framework to show that (Mitochondrial Eve) is wrong,"

Zhao says the Eve model's timeline of human evolution is too compressed to be accurate. Modern hominids had to have evolved and replaced ancient hominids around the world in too short a time period for the Eve theory to be plausible.

Nanjing Man's age suggests that Asian erectus had a longer time to evolve into modern hominids than previously thought. This makes it less likely for modern hominids from Africa to have replaced Asian erectus. The two populations would not have overlapped as the Eve theory speculates.

The four scientists credited with dating Nanjing Man measured the decay of radioactive uranium (U) by counting the number of thorium (th) atoms present in the stratigraphic layers above and below where Nanjing Man's fossil was found. They used thermal ionization mass spectrometry (TIMS) to do this.

Over time and at a steady rate, uranium transforms at the atomic level into thorium. This means that as objects age, the amount of thorium they contain increases as the uranium in them decrease. Because this happens at a steady rate, thorium provides a good estimate for the age of rocks.

This provides a bracket of dates between which Nanjing Man lived with the bottom layer providing the earliest possible date and the top layer the latest date.

TIMS U-Th dating is particularly helpful in dating fossils found in limestone caves where a method like potassium argon (KAr) dating would be inadequate.

Potassium argon (KAr) dating uses crystallized volcanic ash found on or around a fossil in an open area to derive the fossil's age. KAr dating is not suited for dating cave fossils because there is often little volcanic ash to be found inside caves for this method to work.

But, TIMS U-Th dating is suited for dating cave fossils because the limestone retains radioactive elements.

TIMS U-Th dating also provides a larger, more accurate window of time for fossil dates, possibly up to one million years, unlike other techniques such as carbon dating.

Carbon dating actually dates the fossil itself, which does not retain any radioactive elements, and only provides a maximum age of up to 50,000 years.

However, TIMS U-Th dating is nothing new. Henry Schwarcz, a geologist at McMaster University in Hamilton, developed it in 1975.

"We've looked for many different clocks to trace human evolution," Schwarcz says. "The decay (of uranium) is like a ticking clock and this will give us an answer. It is the most precise and accurate."

As for the impact that his dating method will have on evolutionary theory, Schwarcz says that while everyone thinks Africa was where all the action was, this idea won't last.

"It appears China was an important area for evolution. This opens up a whole new window into the time scale of human evolution in that part of the world."

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