Stone Age people somehow managed to live in the forbidding environs of Siberia as early as 300,000 years ago -- hundreds of thousands of years before experts had thought possible -- according to a new analysis of geological evidence.

Even now, in a relatively warm period of Earth's recent climate history, the region 75 miles south of Yakutsk, where signs of archaic habitation have been found, gets as cold as minus 70 degrees Fahrenheit.

So scientists have long assumed that human settlement was not possible there until about 40,000 years ago, coincident with the evolutionary advent of anatomically modern humans who had the capacity to develop sophisticated shelters, make suitable clothing and control fire.

That determination has been reinforced by the fact that, for five decades, the earliest convincingly dated sites of human occupation in upper Siberia have been shown to be no older than 35,000 years.

But three researchers report in today's issue of the journal Science that an ancient quarry site called Diring Yuriakh, apparently used by people to make simple stone tools, is at least 260,000 years old.

"It kind of breaks our mind-set," said coauthor Michael R. Waters, a Texas A&M geoarchaeologist, by indicating "that people were able to push into this rigorous environment and make a living there."

Diring -- located on the Lena River at the same latitude as central Alaska -- has been something of an enigma since it was discovered in 1982 by Russian archaeologist Yuri Mochanov.

Eventually, thousands of artifacts were removed from the excavation, and Mochanov decided the site was at least 2 million years old.

That claim, which would put Siberia on a chronological par with east Africa for early evidence of human-like habitation, met with profound skepticism from most quarters. Estimates of the tools' true age, the authors write, varied widely: from Mochanov's maximum of 3.2 million years to as little as 15,000 years.

Waters, along with geologists Steven L. Forman and James M. Pierson of the University of Illinois, traveled to Siberia to investigate the site.

The once-open quarry had been covered by blowing sand and other sedimentary material; and because there was no other material to use for dating, the scientists were obliged to employ a generally reliable method called thermoluminescence (TL).

The technique relies on the fact that buried mineral crystals (of the sort common in certain rock types and the wind-blown sand at Diring Yuriakh) are gradually exposed to low-level radiation, whether from radioactive material in the rock or in the surrounding earth.

The energy from that radiation dislodges electrons that get trapped in defects within the crystal. When the material is later heated in the lab, those trapped electrons are released, shedding their energy as light: The more light emitted, the longer the sample has been buried and subjected to radiation.

Exposure to sunlight, however, generally removes all the trapped electrons in a sample, "resetting" its internal clock to zero. So the sand that blew into the Diring quarry would have had no trapped electrons at the time it was first deposited; trapping would have begun only when that geological layer was buried.

TL studies of specimens from the site, the Texas-Illinois team concluded, indicated the sand around the stone tools was 250,000 to 350,000 years old.

If the tools are actually tools (and some scientists believe the Diring stones may have cracked naturally) and are genuinely 300,000 years old (which some researchers would prefer to see confirmed using another dating method), it still does not necessarily mean archaic humans were able to live there continuously.

"What happened," said Rick Potts, director of the Human Origins program at the Smithsonian Institution, "when the weather got bad that year or the following year? We just don't know." CAPTION: This cutting tool apparently used in Stone Age was found at Diring Yuriakh, an ancient quarry site on Lena River.