REHOVOT, Israel -- September 24, 1996 -- Prehistoric ashes can now be recognized and analyzed with precision for the first time, thanks to a discovery made by a Weizmann Institute of Science researcher.
The finding, reported in the September issue of the Journal of Archaeological Science, is expected to shed new light on the ways primitive humans used fire, and on their lifestyles and environment. While humans are believed to have first harnessed fire at least 500,000 years ago, ash -- the most direct evidence of fire -- is hard to find and even harder to recognize, because most of the minerals in it are highly reactive and unstable, and begin changing within days of a fire's going out.
As a result, until now evidence of the use of fire has been mostly indirect, coming from blackened bones or fractured flint tools. Now Weizmann Institute's Prof. Steve Weiner -- working with post-doctoral fellow mineralogist Dr. Solveig Schiegl, archaeologist Prof. Ofer Bar-Yosef of Harvard University, and geologist Prof. Paul Goldberg of Boston University -- has traced the chemical transformations that ashes undergo over time, and has discovered that a small group of minerals remains relatively stable and can serve as a telltale sign of ash even after thousands of years.
"We know how to recognize this component now and can look for it, and we have a much better chance of proving something was ash even though it doesn't look like ash at all," says Weiner, a structural biologist who heads the Institute's Environmental Sciences and Energy Research Department.
By analyzing the ashes, scientists can learn about the circumstances in which early humans used fire, which can help explain how they lived, cooked, kept warm, and protected themselves. The ashes and surrounding sediments can also throw light on changes in climate, geology and ecology over the millennia.
"Fire is such an important part of the archaeological record that it may come as a surprise to learn that until now very little analysis has been done on something as basic as ashes," Weiner said.
Archaeologists have assumed that certain sediments are ash if they are found in the form of a hearth, but there has been little scientific analysis of the composition and microscopic structure of ash minerals.
Weiner made his discovery while studying sediments in two northern Israeli caves that had been inhabited from at least 250,000 years ago up to 40,000 years ago.
At the caves, he set up a portable laboratory that included a sophisticated infrared spectrometer, enabling him to analyze mineral samples on-site -- the first time such equipment was used at any archaeological dig. Weiner found that sediments across the floor of one cave, Hayonim, kept giving enigmatic readings on the spectrometer.
The breakthrough came after he realized he had seen the same spectrum in some hearths preserved in the other cave, Kebara.
"That gave me the connection to fire, and I realized that this reading indicated ashes," Weiner said.
To check the finding, Weiner and colleagues conducted experiments in which they burned wood from local trees. They found that the ashes contained a small, relatively stable group of minerals, known as siliceous aggregates, which make up only about 2 percent of the ash's original volume.
It was this fraction that was preserved in the caves. In fact, Weiner found that these siliceous minerals made up a large proportion of the cave sediments, which were up to 3 meters (9 ft.) thick in places -- a finding which suggested the caves had been intensively occupied over millennia.
Weiner and colleagues are now seeking to uncover the record of human habitation in the caves by analyzing the different sediment layers: They hope to distinguish between periods of occupation and non-occupation on the basis that ash would be present only in sediments from periods of occupation.
In addition, by analyzing the other minerals in the ash remnants, they are trying to obtain clues about the types of vegetation present in prehistoric times, which will yield information about the prevailing climate and ecology.
As a result of his discovery, Weiner has recently become the first Western scientist in 60 years to work at China s famous Zhoukoudian cave, home to the bones of Peking Man, believed to be among the first humans to use fire. Weiner will spend the next year analyzing samples he brought back from China in May, and trying to determine precisely what they comprise, whether there were indeed fires at the site, and whether they were lit by humans.
Prof. Stephen Weiner holds the Weizmann Institute's I.W. Abel Chair of Structural Biology and heads the Sussman Family Center for the Study of Environmental Sciences.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.