Do All Life Forms Fall into a Nested Hierarchy?

A biology graduate student and alumnus of our Summer Seminar on Intelligent Design recently contacted me to ask where ID comes down on common descent. I explained to him that in the ID camp, some folks accept common descent while others are skeptical. ID proponents can respectfully disagree on that question (and others too), while agreeing on the powerful evidence for design in nature. But my correspondent asked about an argument in favor of common ancestry he had heard that basically went l...