Say hello to your great-great-great-great-great-grandparents (times a few million or so)—Protungulatum donnae, a rat-sized insect eater believed to have lived 66 million years ago. A new six-year study of the mammalian family tree, looking at DNA and anatomical evidence in greater detail than ever before, has identified Protungulatum donnae as the most likely common ancestor for all 5,400 placental mammal species, reports the New York Times. The critter, however, is only hypothetical, notes the LA Times, explaining that scientists used data to "reverse-engineer" it.
The study examined 83 mammals and fossils for more than 4,500 traits, creating a database 10 times larger than any previous database. Because Protungulatum donnae would have emerged 200,000 to 400,000 years after the great extinction that ended the dinosaurs—about 36 million years later than previous estimates—scientists say this is a clear sign the rise of the mammals was tied to that mass extinction. The abstract to the original article is at Science magazine.