Researchers Zero In On Date Of Early Hominids

The findings by researchers from Oregon State University and the Geophysical Institute of Israel add important evidence about the timing and migration path of hominids out of Africa. It suggests that hominids migrated northward from Africa into Europe and Asia through the Levantine Corridor.

Results of the research have been published in the new (October) issue of the journal Geology. Shaul Levi, a professor emeritus of oceanic and atmospheric sciences at OSU, and Hagai Ron, an Israeli geophysicist, were able to narrow down the range of ages for the formation by studying the reversals of the Earth's magnetic field recorded in the sedimentary formation.

"The Earth's magnetic field has reversed its polarity numerous times in the past 50-60 million years," Levi said. "Some of these reversals, especially in the last 5 million years, have very good age control, so we were able to narrow down the range of ages to within 300,000 years. In terms of human history, that is a long, long time. But in geologic terms, it isn't much at all." Most scientists believe that central Africa was the springboard for hominid evolution in the late Miocene or early Pliocene - about 4-5 million years ago. However, there is much less agreement on when hominids migrated out of Africa.

One of the oldest, scientifically accepted hominid sites is the 'Ubeidiya Formation, about three kilometers south of the Sea of Galilee on the west bank of the Jordan River. Excavations there have yielded skull fragments, teeth, and evidence of early tools. Biostratigraphy and cultural evidence indicate an age of about 1.4 million years.

Other out-of-Africa sites with ancient hominid remains have been found in Java, Indonesia, and Dmanisi in the Republic of Georgia. Scientists have been able to date the lava flows beneath the Dmanisi site at 1.6 to 1.8 million years, though there is no indication of how much later the artifacts were deposited atop the lava.

The Erk-el-Ahmar site is the closest to Africa and there anthropologists from The Hebrew University of Jerusalem have over time discovered core choppers, flakes of flint and other artifacts associated with Oldowan tools.

Located about 10 kilometers south of the Sea of Galilee, Erk-el-Ahmar rises up some 50 meters above the Jordan River. The entire region once was covered by the ancient Dead Sea, and when it receded, the rock formation was exposed to erosion by water and desert winds. What hasn't eroded are ancient lake deposits - layers of clay, silt and sand - that sometimes contain fish remains and other fossils.

That composition, Levi said, is what made Erk-el-Ahmar difficult to date.

"Carbon dating is good only back to about 50,000 years, and it depends on the availability of charcoal or other organic material," he pointed out. "Potassium-Argon dating can be used from about 10,000 years to more than a billion years, depending on the rock, but it requires volcanic material, such as lava flows or volcanic ash. Because the Erk-el-Ahmar Formation contains no volcanic material, and its age far exceeds the range for carbon dating, neither of these methods could be used. "Paleomagnetism, however, provides an additional avenue for dating," Levi added.

The first attempt to date the formation with paleomagnetism was made in 1990 by Doron Braun, a graduate student of Ron's, who was able to identify several magnetic reversals. When Levi and Ron returned to Erk-el-Ahmar, they conducted more comprehensive tests. These tests looked not only at the existence of magnetic reversals, but also at the degree of fluctuations of the horizontal and vertical components of the magnetic field.

In a "normal" period, such as now, the magnetic field is approximated by a dipole at the center of the Earth, and the compass needle points generally north, Levi explained. During "reverse" periods, the dipole configuration is opposite and the compass needle points south.

The magnetic field originates deep in the Earth's fluid core. The field arises from organized flow in the electrically conducting fluid, composed primarily of iron and nickel alloy, with additions of other lighter elements. What triggers magnetic reversals isn't understood, Levi said, and reversals from one polarity to another may take thousands of years.

Scientists have shown that during these transitions, the intensity of the magnetic field decreases dramatically from average values, he added.

All of these differences do, however, leave magnetic "fingerprints" on the rocks. And when Levi and Ron took 120 samples from Erk-el-Ahmar into the laboratory, where they demagnetized them to recover their original magnetic state, they had a series of markers from different depths of the formation that allowed them to hone in on a more precise date.

What they also discovered was that the Erk-el-Ahmar magnetic profile matched almost exactly a magnetic profile taken from the ocean floor in the North Atlantic - research conducted under the auspices of the Ocean Drilling Program.

"That validated our findings quite nicely and is worthy of further study, globally," Levi said. "The fact that the patterns found at Erk-el-Ahmar matched those from the North Atlantic core supports our interpretation that the age of the formation - and the hominid tools it contains - is between 1.7 and 2.0 million years old."

EDITOR'S NOTE:

A map and photo image of Erk-el-Ahmar are available on the following web site: http://osu.orst.edu/dept/ncs/photos/index.html