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Originally shared by Carl ZimmerWe humans use a virus protein to build placentas. So do dogs. So do rabbits. So do mice. Each mammal uses a different virus. On the Loom, I blog about one of the weirder twists in our evolution.![](http://blogs.discovermagazine.com/loom/files/2012/02/davincifetus.png)
Mammals Made By Viruses | The Loom | Discover Magazine
![](http://blogs.discovermagazine.com/loom/files/2012/02/davincifetus.png)
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