This 2012 artist’s rendering provided by the American Museum of Natural History shows a hypothetical placental mammal ancestor, a small, insect-eating animal. A study released Thursday, Feb. 7, 2013... more 
This 2012 artist’s rendering provided by the American Museum of Natural History shows a hypothetical placental mammal ancestor, a small, insect-eating animal. A study released Thursday, Feb. 7, 2013 details the family tree of mammals that have lengthy gestations before birth. It goes back to this shrewish critter slightly bigger than a mouse with a nasty set of teeth. And it first popped in the world a little more than 65 million years ago - just after the cosmic crash which is theorized to have caused the extinction of non-avian dinosaurs. (AP Photo/American Museum of Natural History, Carl Buell) less 
1 / 31
Associated Press | Photo By American Museum of Natural History, Carl Buell
Thu, Feb 7, 2013 2:07 PM EST