This is a story about your butt. It’s a story about how you got your butt, why you have your butt, and how your butt might be one of the most important and essential things for you being you, for being human.  Today, reporters Heather Radke and Matt Kielty talk to two researchers who followed the butt from our ancient beginnings, through millions of years of evolution, and all the way to today, out to a valley in Arizona, where our butts are put to the ultimate test.   This episode was reported by Heather Radke and Matt Kielty and was produced by Matt Kielty, Rachael Cusick and Simon Adler. Sound design and mixing by Jeremy Bloom. Fact-checking by Dorie Chevlen. Special thanks to Michelle Legro. Support Radiolab today at Radiolab.org/donate.