Illustration of our placental ancestor. Image: Carl Buell

It’s us!

Don’t worry, we’re not alone. Our entire clade is made up of rat-hamsters, rat-porpoises, rat-antelope and many, many more. That’s because we, the placentals, owe our ancestry to a tiny, half-pound rat-like creature that lived a little less than 65 million years ago.

Researchers from all over the world have been tirelessly whittling down the list for the last six years by creating an even bigger one: The MorphoBank, a giant database of  4,500 traits and 86 mammals on our evolutionary tree. The project, called “Assembling the Tree of Life,” was spearheaded and funded by the National Science Foundation to flesh out the fossil record and find the exact moment our branch arose.

But you might be asking who are “we”? Placentalia is the most diverse branch of mammals in existence. Basically, we’re all the mammals that don’t lay eggs or have pouches. We have live births, some form of hair, and warm blood. If you’re a kangaroo, opossum or duck-billed platypus, you’re in the other orders, Marsupialia and Monotremata, and don’t spring from the loins of this little rat.

Finding placentals’ earliest common ancestor wasn’t the result of some new fossil discovery, rather it was taking a closer look at the fossils we already have. As researchers entered more and more animals into MorphoBank, shared traits began to couple up and branches began to emerge. Study co-author and paleontologist at the American Museum of Natural History Michael Novacek says the work was “hugely laborious,” but in the end they did find their man, or rather, rat.

The creature is known as Protungulatum donnae. It has no common name because up until now, it’s just been an obscure little mammal with its distinguising great-great-great-grand-paternalism unknown to all. There’s also something a bit surprising about the creature that scientists discovered in their life tree assembly; Protungulatum only arose about 200,000 to 400,000 years after the extinction of the non-avian dinosaurs, about 65 million years ago. That pushes the theorized origin of Placentalia 35 million years into the future. Judging by how long certain traits have taken to evolve throughout the fossil record, scientists had pegged the rise of our ancestor about 90 million years ago. Detectives everywhere must be nodding their heads; all you need to find the correct answer is a closer look at the evidence.