In the spring of 2012, botanist and graduate student Benjamin Wilder was camping on Tiburón Island, a large island in the Gulf of California whose flora he has studied for most of a decade.
Wilder wanted to find out more about the evolutionary history of the plants on Tiburón, so he was looking for fossil woodrat middens—piles of vegetation and other debris collected by woodrats, glued together with woodrat pee, and cached away for tens of thousands of years. For scientists, woodrat middens are like care packages from the past.
In a shallow cave in the foothills of the Sierra Kunkaak, Wilder, a lab mate, and two friends from the local Seri community found something that resembled a midden, but didn’t look quite right: It had a lot of dung in it, and not many plants. Wilder took it back to Tucson, Arizona, and showed it to his mentor Julio Betancourt, who recognized it with a look and a sniff. “That’s bighorn sheep dung,” he said. Its identity was confirmed by DNA analysis, and when samples were radiocarbon dated, they turned out to be 1,500 years old.
Which was, as far as anyone knew, impossible.




March 10 – 14
