Once upon a time, there were no anuses. These ingenious organs allowed our primordial ancestors to grow bigger and more complex, but scientists still don’t understand how they evolved. And they’re still grasping at a mystery that literally surrounds it: Why is the human butt so big? For more go to http://vox.com/unexplainable It's a great place to sign up for our newsletter, view show transcripts, and read more about the topics on our show. Also email us! unexplainable@vox.com We read every email. Support Unexplainable by making a financial contribution to Vox! bit.ly/givepodcasts Learn more about your ad choices. Visit podcastchoices.com/adchoices