Scientists discover earliest common ancestor for humans and mammals

A six year study analyzed genomic information and fossil evidence to construct a mammalian family tree, and discovered our earliest common mammal ancestor lived soon after the extinction of the dinosaurs 66 million years ago. The furry insect-eater's descendants spread around the world and evolved into all the major placental groups we have today.

Maureen O'Leary of Stony Bridge University in New York state and her team combined genetic information with data on more than 4500 anatomical traits from 46 living species and 40 fossil species to construct their family tree.

None of the fossils dating from before the asteroid impact that killed the dinosaurs could be classified as modern placental mammals. The oldest fossil they identified as being a modern placental lived 200,000 to 400,000 years after the impact. Their analysis showed that rapid evolution followed, producing the first members of the major placental mammal lineages, such as primates and rodents, 2 to 3 million years later.

"This is a landmark piece of work," says Stephen Brusatte of the University of Edinburgh, UK. "The fossil record and the molecular clock are starting to converge. That's just what we want."