Ever since their beginnings, primates have been splendidly adapted for life in the trees—at least until our hominid ancestors climbed down from the canopy a few million years ago.

Now scientists have discovered that this lifestyle likely existed much deeper in the family tree, back in the time when eutherians, the group that represents about 90 percent of living mammals (including placental mammals from rodents to humans), had begun to differentiate themselves from marsupials (mammals with pouches).

A team led by Ji Qiang of the Chinese Academy of Geological Sciences has unearthed a 125-million-year-old fossil of a seven-inch-long (18-centimeter-long) eutherian called Eomaia scansoria, or “dawn mother who climbs,” in Liaoning Province. It’s so well preserved that its fur, teeth, and tiny foot and hand bones can still be distinguished. Eomaia’s fingers and toes are long and tipped with curved claws, adaptations for life in the branches. (To view a picture, click here).

The fossil is about 50 million years older than the earliest eutherians previously known. Because those creatures weren’t adapted to life in trees, most paleontologists assumed that eutherians evolved from ground dwellers. The Eomaia find could mean that today’s eutherians come from a lineage that’s been up in the trees since deep in the age of dinosaurs.

— Christopher P. Sloan