A small, shrew-like creature from the Jurassic era may be the earliest-known ancestor of the present placental mammals, which includes humans, a new study reveals.
Documented in the latest issue of the journal Nature, the new-found fossil dubbed as the Jurassic Mother from China (Juramaia sinensis) provides important details regarding the evolutionary split between marsupial and placental mammals.
A critical event in the history of the evolution of mammals is the divergence of eutherians from the metatherian marsupials. The present placentals including humans, whales and mice are the only living variants of the eutherian group of mammals.
Eutherians, on the other hand, arose from metatherians, the mammal lineage that led to marsupials such as kangaroos.
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, the journal Science mentions.
The new discovery establishes that the divergence took place 160 million years ago. According to the National Geographic report, lead author Zhe-Xi Luo stated that the new discovery brings the fossil record in line with DNA evidence, which had indicated the split between ancestral marsupials and placentals.
At 160 million years old, Juramaia is now the oldest known eutherian, preceding the previous record-holder, Eomaia, by about 35 million years, Science quoted paleontologist John Hunter of the Ohio State University, Newark.
As the earliest eutherian known to date, Juramaia sinensis represents what some of our own precursors were like during the Jurassic period, says Science. What characterizes this animal as a eutherian, and not another kind of archaic mammal, is the number of molars and premolars in its jaws, the arrangement of cusps on its teeth, and minute characteristics of its arms and wrists...
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.
The new finding also provided vital information about the diversification of other mammals apart from placentals and marsupials.