From the New Yorker, here, as archaeologists have tried to assess what path the Carthaginian general Hannibal took, with soldiers, elephants, and horses, in the legendary crossing of the Alps:

“Last month, a team of scientists published a two-part study suggesting that Hannibal and his ever-dwindling company of soldiers and animals took a southerly course, travelling by way of the Col de la Traversette, a nearly ten-thousand-foot pass. The authors point to geological evidence—some already published—and to new microbiological evidence: genetic fragments from bacteria common to manure. The bacterial bits were found in a layer of possibly hoof-roiled sediment that might date to around 218 B.C., the year of Hannibal’s traverse.”

More generally, “The whole business of looking at sediments is bubbling up now—it is taking off because of advances in DNA sequencing… There is a realization that the environment is full of DNA . . . and you can detect it in sediments even in the absence of fossil remains.”