"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
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
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.
II. Discovery Resolves
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
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
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
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