Over the past year or so I’ve gotten to know some extraordinary people. They were born with a single mutation to a single gene that caused them to grow a second skeleton. Their condition, called fibrodysplasia ossificans progressiva, affects only one person out of every two million. If you traveled across the entire the United States and gathered everyone with the condition, you could fit them all comfortably on a single Greyhound bus.

I was inspired to meet them on a visit to Philadelphia last summer. The Mutter Museum, housed at the College of Physicians and Surgeons, is a collection of medical specimens of the sort you will see nowhere else in the United States. You feel an alternation of fear