Earliest placental mammal ancestor pinpointed

he 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.

But a range of genetic studies that look for fairly regular changes in genetic makeup suggested that the group arose as long as 100 million years ago, with mammals such as early rodents sharing the Earth with the dinosaurs.