Today, an isotope rewrites history. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Fall, 1974: I trudge through the forest along the not-so-blue Danube, under a light drizzle. This is Jugoslavia's side of the river. I'm on a two-mile walk to a 7500-year-old archaeological site. It's the late Mesolithic village of Lepenski Vir.

We pass a rude peasant hut and an old woman. Outside, a simple wooden table and chair overlook the river. On the table are a knife and pieces of apple. I suppose it's where she sits, looking at the great river, to think about time and mortality.

Half an hour later we reach a small circle of stone huts in the muddy woods. Lepenski Vir was part of the great fuss kicked up by carbon dating. Up to now, prehistory dating had been detective work based on circumstantial evidence. Event B had to fall after A. Event C happened sometime after A, but before B, and so on.

Chronologies had few hard dates to go by. Archaeologists had once dated Lepenski Vir in 2500 B.C. Now it retreats to 5500 B.C. In 1974, scholars were at each others throats trying to stem the damage that carbon dating was doing to their textbooks.

Here's how carbon dating works: The cosmic radiation of neutrons converts atoms of nitrogen into the heavy isotope of carbon, called carbon-14. First that isotope combines to form high-altitude carbon dioxide. By the time plants and animals take up carbon dioxide, one carbon atom in a trillion is carbon-14.

But carbon-14 is radioactive. Every 5730 years, half of it decays back to regular nitrogen. Measure the fraction of carbon-14 left in any artifact that once lived -- in bone, ash, shell, or grain -- and you can tell how long ago it lived.

To make carbon dating work, you have to calibrate it with known dates. We know dates of artifacts from certain Egyptian royalty. The rings of old trees give accurate dates for the wood inside. You might think tree rings couldn't take you back very far. But some California bristlecone pines have lived five thousand years.

At first, carbon dating had tough sledding. We had to make corrections. For one thing, Earth's atmosphere has changed. The amount of carbon-14 wasn't quite the same four thousand years ago.

As the work went on, old dates moved back into time. Agriculture became two thousand years older than we'd thought. Traditional history suffered the most right here in these rainy woods along the Danube. These people lived in villages, made art, and worshipped the gods of the river long long before the Pharaohs.

On the way out we pass the table again. The woman seems older this time. She eats her apple and gazes out at the river. What are three thousand years more or less, as she honors the gods of the one thing no one can date -- the timeless water moving relentlessly below her.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Renfrew, C., Before Civilization: The Radiocarbon Revolution and Prehistoric Europe. New York: Alfred A. Knopf, 1973.

I am grateful to Professors Randolph Widmer, UH Anthropology Department, Frank Holt, UH History Department, and Henry Chafetz, UH Geoscientist, for their counsel on this episode; and to Laura Douglas, Gammage's Bookstore, for directing me to Renfrew's classic text.