The creature that gave rise to all the placental mammals - a huge group that includes whales, elephants, dogs, bats and us - has at last been pinpointed.
An international effort mapped out thousands of physical traits and genetic clues to trace the lineage.
Their results indicate that all placental mammals arose from a small, furry, insect-eating animal.
A report in Science resolves the debate as to when the creature lived; it came about after the demise of dinosaurs.
That had been a hotly debated question over years of research.
Placental mammals - as opposed to the kind that lay eggs, such as the platypus, or carry young in pouches, such as the kangaroo - are an extraordinarily diverse group of animals with more than 5,000 species today. They include examples that fly, swim and run, and range in weight from a couple of grams to hundreds of tonnes.
A wealth of fossil evidence had pointed to the notion that the group, or clade, grew in an "explosion" of species shortly after the dinosaurs' end about 65 million years ago.