Earliest Placental Mammal Described in Breathtaking Detail

A new study seems to resolve the mystery of whether placental mammals evolved before or after a massive asteroid plowed into the Earth 65 million years ago, wiping about 70 percent of all species. Emerging after the dinosaurs died out, a tiny, scrambling, chipmunk-like creature is the common ancestor to all placental mammals.

Some 65 million years ago, an asteroid possibly as big as Mount Everest plowed into the Earth, wiping about 70 percent of all species, including dinosaurs, off the planet. Why this event killed off the dinosaurs but spared other species, including many mammals, birds, turtles, starfish and frogs, is a great mystery.

Also disputed has been whether placental mammals emerged before or after this event. Placental mammals are those that give birth to live young after nourishing them via a placenta rather than laying eggs. That includes 5,100 living species -- among them, us. But a team of scientists claim to have traced these mammals back to a common ancestor, according to a new study, published in Thursday's issue of the journal, Science.

A tiny, scrambling, chipmunk-like creature, they found, is the common ancestor to all placental mammals. It ate insects, climbed trees, weighed less than half a pound, and gave birth to hairless, closed-eyed babies one at a time. And it emerged after the dinosaurs died out, not before. (It has a very unimaginative name: the "hypothetical placental ancestor.")

From this strange creature sprang all the placental mammals we know today: monkeys and mice, dolphins and deer, bats and dogs and humans.

It's a fascinating finding, but most astonishing is the amount of detail the scientists have managed to gin up about this hypothetical super ancestor.

It ranged from the size of a chipmunk to that of a shrew. It had a long tail, sharp teeth and well-developed sense of smell and hearing. Its brain had a feature that linked both hemispheres, called the corpus callosum. (We have that too.) Like marsupials, it bore its young live. But unlike marsupials, the young probably stayed in the mother for a long time, said Michael Novacek, provost and curator at the American Museum of Natural History and one of the study's authors.

Novacek and other scientists explain more in this video by the American Museum of Natural History:

Researchers pieced together a picture of this animal after recording some 4,500 physical traits for 83 mammalian species. They did this using a matrix-like computer program called the Morpho Bank. Over six years, members of the 23-member team entered DNA and morphological data -- that's physical characteristics -- on these species.

"The way this worked, you could be working in the cloud, in this massive matrix, entering a character there, and you suddenly see other images pop up on the matrix, because a guy from Argentina was entering data there," Novacek said.

By the end, they had 390,000 cells, or windows, describing a particular feature for a particular species. A feature could include the presence or absence of teeth or the shape of those teeth, for example.

"We were looking at really minute evolutionary features on a very small scale, said Michelle Spaulding, also a study author, from Carnegie Museum of Natural History. "Things like the formation or condition of bones in the skull, the number of toes, the number of vertebrae, or stripes on a tail."

For years, they decided what features to include, assembled the data and entered it into a high-powered computer. The computer mapped that data, producing a tree that showed the relationships of these mammals. And using all the data, they were able to trace the species to a common ancestor, and create a fantastically detailed description of that animal.

The next step, Spaulding said: to add more and more mammals.

"The beauty of it being on the web platform MorphoBank is anyone can work on it," she said. "If you have a fossil and you want to know what it is, you can access this matrix. You don't have to travel. It's great for graduate students or full-blown scientists that just don't have the resources to travel."


Four computer scientists will receive Oscars tomorrow for "Technical Achievement in Special Effects." They'll be recognized for a software algorithm that generates swirling smoke and fiery explosions in great detail. But, according to the National Science Foundation, the technology may have broader uses that extends to understanding blood flow turbulence and how intergalactic gases move through space.

Another asteroid, this one the size of a building, will buzz by Earth next week. Here's NPR's story. In asteroid flyby terms, 17,000 miles is actually quite close. (Check out our latest post on asteroid flybys.) But it poses no danger to us or our satellites.

This small, two-wheeled robot has been driven by a male silkmoth to track down sex pheremones given off by a female mate. Here's a video.

From RadioLab and Planet Money, check out these gorgeous visualizations of high-frequency trading.


From Wired: 3-D Cybertaxonomy: Fascinating Virtual Dissections That Will Freak You Out

Tom Kennedy and Patti Parson contributed to this report.

Photo credit: An artist's rendering of the hypothetical placental ancestor, a small, insect-eating animal. Image courtesy of Carl Buell.

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