She's spent more than two years tracing its earliest use among the native peoples of California and the Pacific Coast.
"We think tobacco in the western U.S. was present 8,000 or 10,000 years ago, and hunter gatherers probably figured out the special properties of the plant fairly early," said Tushingham.
She says the clues were burned into ancient pipes, some found in nearly pristine condition during digs at the Smith River basin near the Oregon border and another undisclosed site here in the Bay Area.
First, to document exactly what was being smoked and when, Tushingham and her colleagues at UC Davis used test pipes to burn dozens of native plant species. They were then able to compare the residues to what still remained inside the stone pipes.
"The residues that are in the open spaces will endure thousands of years," says researcher Jelmar Eerkens. "So when we started obtaining results, it was a very exciting thing."
Still, for more accuracy, the microscopic samples were analyzed using advanced technology, including gas chromatography, and mass spectrometers.
Oliver Fiehn, Ph.D. runs the lab at the UC Davis Genome Center.
"You have to fragments the molecule, and look at the fragments to see what the compound was," he said.
The verdict: tobacco.
And they say the evidence suggests that Native Americans in Northern California smoked it more than a thousand years ago, and perhaps cultivated it for centuries, far earlier than first imagined. Although it's likely they consumed it much less frequently, and for a different purpose than the smoking we know today according to Tushingham.
"There was recreational smoking," she believes. "But more in ceremonial context that has roots deep in time," she said.
Tushingham says early tobacco would have had less than half the nicotine content of what's used in modern cigarettes. Still, the data could be used in future studies to learn if early ritual smoking could have produced any health effects.
Written and produced by Tim Didion