sc/ "Jurassic Mother" Rat is Man's Oldest Mammalian Ancestor

The 160-million year old mammal marked the split between placental mammals and marsupials

The skeleton was painstakingly reconstructed to reveal its secrets. 

 (Source: Nature/Beijing Museum of Natural History)

"You and me baby ain't nothing but mammals..." - Bloodhound Gang

Scientists may have just found mankind's oldest relative -- the "Jurassic mother from China", Juramaia sinensis.  The fascinating beast fills in crucial missing details in the fossil record of history's evolutionary tree.

I. A Tiny Rat, A Giant Discovery

The skeleton of the rat-like creature was discovered in Liaoning Province, a northern province of China that borders North Korea.  The skeleton was then analyzed by Zhe-Xi Luo, a Carnegie Museum of Natural History paleontologist; Chong-Xi Yuan and Qing-Jin Meng, Chinese Academy of Geological Sciences researchers; and Qiang Ji, a paleontologist at the Beijing Museum of Natural History.

The fossil including impressions of soft residual tissues like hair, an incomplete skull, and a partial skeleton.  The skeleton contained the forepaw bones (the "hand" bones in a mammal paw) and the skull contained a full set of teeth -- both valuable in identifying the creature's identity.

Examining those bones allowed the researchers to identify the mammal as a eutherian, a group of extinct mammals.  Furthermore, they establish that the creature was a placental-eutherian.

Placental-eutherians are distinguished for having a placenta when pregnant -- similar to man and most other modern mammals.  The placenta is a special organ in the uterine wall that handles resource exchange with the developing fetus.  Marsupials ("metatherians") -- such as kangaroos and opossums -- do not have a placenta when pregnant.

An artist depicts "Jurassic mother", the earliest known 
ancestor of man, merrily scampering through trees
in the hunt for juicy bugs. (Source: Mark A. Klinger
/ Carnegie Museum of Natural History)

II. Discovery Resolves Evolutionary Discrepancy

The discovery solves a puzzling mystery.  Advanced modern genetic analysis had indicated that the placental-eutherian clade had diverged from marsupial-eutherians approximately 160.7 million years ago.  But until now the oldest placental-eutherian was the "dawn mother" Eomaia -- a 125 million year old rat found in China.  Now the genetic evidence and fossil record are in agreement.

The fossil was dated using the 40Ar/39Ar method on sanidines -- high temperature feldspars -- in the local sediment.  That dating method was accurate to place the age to within a +/- 400,000-year range.

Man's ancestor was more fur than fight, though.  It weighed around one pound soaking wet and its specially adapted paws allowed it to scurry around in tree avoiding the numerous dinosaur predators that stalked the ground.  Its sharp teeth indicate that it feasted primarily on a diet of bugs.

Professor Luo comments, "The divergence of eutherian mammals from marsupials eventually led to placental birth and reproduction that are so crucial for the evolutionary success of placentals. But it is their early adaptation to exploit niches on the tree that paved their way toward this success."

In an interview with MSNBC, he adds, "I imagine this animal to be a shrew-sized insectivorous mammal that hunted insects and was capable of being active in the trees."

The work is published [abstract] in one of science's top peer-review research journals, Nature.

III. Work is Lauded as Groundbreaking Discovery

Often critical studies such as this one draw a great deal of debate and criticism as not all experts are convinced. In this case, however, the community of paleontology experts seems virtually unanimous in their praise of the work, which they say is a vital leap for the field.

States Gregory Wilson, an assistant professor in the University of Washington’s Department of Biology, "This new specimen is a real jewel among the spectacular treasure chest of the Chinese fossil record," Wilson said. "The exquisitely preserved anatomical details leave little doubt that we're looking at the earliest eutherian yet known not quite a placental yet but on the line to placentals."

"This significantly pulls several major branches of the mammalian phylogenetic tree even farther back into the Age of Dinosaurs, providing better understanding of the context of mammalian evolution. This work will grip the attention of paleontologists, molecular systematists, and anyone interested in their ancient forebears."

The brave little rat would thrive and flourish in the Chinese woodland eventually giving way to newer eutherians like Eomaia.  After the mass extinction of the dinosaurs -- which occurred about 95 million years after the Jurassic mother rat -- they would descend from the trees, conquering the empty domain that dinosaurs left behind.  Over the next 65 million years a host of new mammal species evolved including -- eventually -- mankind.