The discovery of partial skeleton of a small, shrew-like mammal, in the fossil beds of Liaoning, China, has helped explain the ancestry of living mammals on earth, including humans.

The tiny mammals that lived during the heyday of the Jurassic dinosaurs were the ancestors of the placental mammals, which includes everything from humans to bats to whales, the Science journal has reported, quoting research published by Zhe-Xi Luo, Curator and Associate Director for Research and Collections at Carnegie Museum of Natural History.

The newly discovered little mammal, which weighed only about 15 grams, has been named Juramaia sinensis, or Jurassic mother from China. Luo, the lead author of the study, says though it is not certain if Juramaia sinensis is a direct ancestor of modern placentals, it's either a great grand-aunt or a great grandmother.

New findings prove that Juramaia sinensis is the earliest known ancestor of placental mammals. Placental mammals are animals that give birth to relatively mature, live young.

The great evolutionary lineage that includes us had a very humble beginning, in terms of body mass, said Luo.

Living mammals are split into subgroups like the egg-laying monotremes, the pouched marsupials and the placental mammals. Each group diverged at different times, and determining when marsupials and placentals split from each other has been problematic. Fossil discoveries point to the Cretaceous, about 125 million years ago, whereas estimates made on genetic differences among living mammals suggest that the split happened even earlier, says Science.

The new discovery establishes that the divergence took place 160 million years ago. Lou said that the new discovery brings the fossil record in line with DNA evidence, which had indicated that the split between ancestral marsupials and placentals occurred around 160 million years ago, the National Geographic reported.

At 160 million years old, Juramaia is now the oldest known eutherian, preceding the previous record-holder, Eomaia, by about 35 million years, says paleontologist John Hunter of Ohio State University, Newark, according to Science.

As the earliest eutherian known to date, Juramaia sinensis represents what some of our own precursors were like during the Jurassic, Science says. What characterizes this animal as a eutherian, and not another kind of archaic mammal, are the number of molars and premolars in its jaws, the arrangement of cusps on its teeth, and minute characteristics of its arms and wrists...

Lou's findings show that the forerunners of marsupial mammals—called metatherians—and placental mammals diverged even earlier still and have been separated by over 160 million years of evolutionary change.
This puts down a new evolutionary milestone for the origin of placental mammals, which are important because they make up more than 90% of all of the living mammals, he said.