英语时差：DNA2

The sample didn’t have any visible animal or plant remains in it, but researchers found loads of DNA anyway. They have already identified such animals as lemmings, bison, and –going way back– woolly mammoths. But if there were no carcasses, how did the DNA get in the soil? That isn’t known, though some scientists speculate that animal dung contains enough stray cells to deposit DNA. That may mean a bison pooped here thousands of years ago, unknowingly dropping its genetic signature–just waiting for future biologists to dig it up again.