A Harvard archeologist has discovered floors of some caves in northern Israel are composed largely of ash — residue of fires that burned 200,000 years ago.

Neanderthals ignited fires that left the ash in the Hayonim and Kebara caves, according to tests developed at the Weizmann Institute of Science in Rehovot and conducted by professor Ofer Bar-Yosef and Weizmann professor Stephen Weiner.

Fires in the caves signal important clues about life, and the environment, in the ancient Middle East, the researchers say, and could test Bar-Yosef's theory that dramatic changes in weather altered the course of history.

The fires suggest the caves may have hosted frequent human visitors or even permanent residents — in contrast to European caves that seem to have provided ancient people with only temporary shelter, the researchers say.

The caves may have provided a refuge to waves of people whom climatic changes drove out of southeastern Europe some 200,000 and 70,000 years ago, Bar-Yosef speculates. Recent Harvard studies have suggested that the caves on Mount Carmel were probably inhabited in winter, while those in Galilee may have served as a summer retreat.

After spending a year at the institute in the late '80s, Bar-Yosef has returned there for a six-month sabbatical during which he will pursue joint projects with researchers working in environmental sciences.

Throwing light on another aspect of life in the caves, Bar-Yosef — along with Weiner and Texas geologist Paul Goldberg — recently revealed that Homo sapiens apparently cannot take credit for a seemingly modern practice, the division of space into areas based on use.

A portable laboratory that Weiner established at the Kebara Cave to analyze the minerals in its sediments showed that the cave's Neanderthal dwellers separated their hearths from their garbage dumps.

Together with Weizmann Institute scientists and the Israel Antiquities Authority, Bar-Yosef also recently launched a new project dealing with one of the hottest topics in archaeology today; the response of ancient humans to natural disasters such as droughts and famines.

These calamities are now believed to have produced significant societal changes throughout history, Bar-Yosef says.

This view has emerged over the past decade, during which large-scale disasters such as floods, volcanic eruptions, droughts and tornadoes affected unprecedented numbers of people, even causing political and social turmoil.

Many archaeologists and historians, however, still embrace the once-popular opinion that societal changes mainly stem not from natural disasters but from political and socioeconomic circumstances.

"My personal view is that changes in climate accompanied by natural disasters serve as triggers of societal change that pass through what I call a cultural `filter' — namely, they affect each culture differently, depending on its way of dealing with disasters," Bar-Yosef said.

"In particular, [disasters] produce the greatest upheavals in countries that are the worst equipped for dealing with them economically."

He intends to test this hypothesis over several years by correlating Israel's archaeological history over the past 6,000 years with the history of its climate.

While the region's archaeology is well-documented, its climatic record is less well understood. Weizmann's Dan Yakir will address certain aspects of the weather question using chemical and isotopic analyses of ancient and modern wood samples from the area.

Because wood contains different carbon and oxygen isotopes depending on the prevailing precipitation, measurements ofthese isotopes elicit conclusions about the past climate.

Wood collected at various archaeological sites will be provided for the project by Uri Baruch of the Israel Antiquities Authority, who has also studied Israel's vegetational history via his analyses of ancient pollen.

The wood samples will be dated at the Weizmann Institute's Radiocarbon and Tritium Laboratory, the only facility in Israel to perform such analysis. The laboratory is headed by Weizmann's Israel Carmi and is operated jointly with the Antiquities Authority.

Bar-Yosef intends to combine Yakir's findings with Baruch's pollen studies in order to distinguish the effects of human activity on vegetation from climatic effects, and to obtain a better picture of past climatic change.

The findings may shed light on some of the most controversial issues in Middle Eastern history and archaeology:

*Did climatic changes spark the collapse of the Early Bronze urban societies in the third millennium B.C.E.?

*Did such changes weaken the region's society, thereby aiding the conquests of the Israelites led by Joshua in 1200 B.C.E.?

*Did climatic or political factors spur the decline of Byzantine settlement in Palestine in the eighth century C.E.?

"No matter what we come up with, our findings will be controversial because the impact of climate on history is such a hotly debated topic, and you cannot go back in time to verify things," Bar-Yosef says.

"But it will be interesting to get `proxy' data on this subject backed by solid scientific finds," he adds.