Scientists say Israeli site dates back 780,000 years
NEW YORK - Harnessing a quirky aspect of the Earth's magnetic field, American and Israeli scientists have dated an archaeological site along the Jordan River in northern Israel to about 780,000 years ago, 250,000 years older than previously thought.
The new evidence helps fill an important gap in the early archaeological record of Homo erectus, considered the most immediate ancestor of modern humans. The age revision also raises new questions about a wave of early migration from Africa to Europe and Asia through Israel and the accompanying spread of a surprisingly sophisticated stone tool technology.
Exposed sediment layers at the Gesher Benot Ya'aqov site, which means "Bridge of the Daughters of Jacob" in Hebrew, have revealed preserved wood, seeds, fossils and a collection of stone tools, including axes and cleavers fashioned from volcanic basalt.
Scientists have studied the site since the 1930s. But Craig Feibel, an assistant professor of anthropology and geological sciences at Rutgers University and a co-author of the study, which was to be released Friday in the journal Science, said the "real leap" was his group's ability to analyze a sequence of sediment layers and construct a reliable time scale by using geological evidence.
That evidence is based on a well-known geological phenomenon in which the Earth's magnetic field reverses every 500,000 years, on average. The last of those shifts took place about 780,000 years ago.
"What that means is that for compasses today that point north, they would point south following a magnetic reversal, or if you go back in time prior to 780,000 years ago, they would be pointing south," Feibel said. "It's a very useful tool in studying the Earth's history. All the particles in mud, for example, as they're settling out of suspension in water, they're getting oriented just like little compass needles relative to the prevailing Earth magnetic field."
By measuring the orientation of mineral fragments in each successive layer of sediment, Feibel and his colleagues defined the layer containing the magnetic shift as a landmark with which to date adjoining sedimentary layers.
Directly above and below this landmark, scientists have found bones, tools and other evidence of Homo erectus life spanning 100,000 years along the shoreline of a former lake bed known as Lake Hula. Most of the artifacts lie directly above the magnetic shift and date back nearly 780,000 years.
The site was previously thought to be about half a million years old, the same age as other sites in Europe containing stone tools with similar features, such as the famous Boxgrove archaeological site in England. Scientists had interpreted the concurrence of dates to mean the technology had spread rapidly from its origin in Africa, through Israel and into Europe.
With the new findings, Feibel said he wonders if the wave of migration carrying our prehistoric ancestors lagged somewhere between Israel and Europe or if older evidence of their presence in Europe has merely eluded researchers.
"Like any good discovery, it poses as many questions as it answers," he said.