Researchers today announced the discovery of the earliest known ancestor of the group of mammals that give birth to live young. The finding is based on a well-preserved fossil of a tiny, hairy 125-million-year-old shrewlike species that scurried about in bushes and the low branches of trees.

“We found the earliest ancestor, perhaps a great uncle or aunt, or perhaps a great grandparent–albeit 125 million years removed–to all placental mammals,” said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. “It is significant because a vast majority of mammals alive today are placentals.”