At the end of ep 11, right before the credits roll, there's a set of credits that appear on the top half of the screen. I thought it was odd for that to appear before the regular credits, so I took time to read it.
It says that this 2-part story "Gravel Earth" (for eps 11 & 12) is based off an FMA novel by the same name, written by Takahashi Natsuko and adapted into a script by Inoue (erm, can't read his first name, "Shin" I guess).
So this story arc for the anime was actually based off a novel that was based off the manga... er, it's a rather pointless observation. My apologies. ^_^;