Reconstruction of the skull and a life restoration of Hadrocodium wui, with a paper clip (32mm or 1.25 inches) for scale.

Luo says these Jurassic-era mammals - including the tiny Hadrocodium, whose skull was smaller than a paper clip - had brains that were two or three times larger than any mammal relatives before them.

“Hadrocodium has this big head, small mammal, relative to the body size, it has already achieved about the same range of brain size as modern mammals.”

Luo was thrilled to see this impressive brain development in our 190-million-year-old mammalian relatives. It's an important evolutionary milestone that helped make mammals - and humans among them - one of the Earth’s dominant animal classes.