But as researchers reported Thursday in the journal Science, the animal had several anatomical characteristics for live births that anticipated all placental mammals and led to some 5,400 living species, from shrews to elephants, bats to whales, cats to dogs and, not least, humans. The scientists theorized that this hypothetical critter would have begun to diversify into new species of mammals after the demise of the dinosaurs roughly 66 million years ago — a concept at odds with some current thinking. In a comprehensive six-year study of the mammalian family tree, scientists have identified and reconstructed what they say is the most likely common ancestor of the many species on the most abundant and diverse branch of that tree — the branch of creatures that nourish their young in utero through a placenta. Given some belated stature by an artist’s brush, the animal hardly looks the part of a progenitor of so many mammals (which do not include marsupials, like kangaroos and opossums, or monotremes, egg-laying mammals like the duck-billed platypus). Estimates based on DNA, or so-called molecular evidence, suggest that placental mammals appeared in the mid-Cretaceous period when dinosaurs like Tyrannosaurus rex and Triceratops still trod the Earth. Using a vast online database called Morphobank, the team of 23 scientists built a composite of the hypothetical ancestor based on the morphology, or appearance, of existing and extinct animals, along with the DNA data.  Humankind’s common ancestor with other mammals may have been a roughly rat-size animal that weighed no more than a half a pound, had a long furry tail and lived on insects. Based on their findings, the scientists said placental mammals began their spread about 36 million years later than estimates based on the DNA-derived molecular clock model.
– 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.