An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.Or as one of the Nature authors, Joseph Thornton, puts it in the Science Times piece, mutations "burn the bridge that evolution just crossed." I think most people today don't see the irreversibility as a law, but a very high probability, and Thornton takes that view.
The scientists tried to undo a set of mutations in a molecule (a glucocorticoid receptor) and found that you couldn't get it back to its earlier evolutionary state. They argue that the complexity of the evolutionary path and interactions among changes shut down the road to reversal.
Dr. Thornton believes it says something important about the course of evolutionary history. Natural selection can achieve many things, but it is hemmed in. Even harmless, random mutations can block its path. “The biology we ended up with was not inevitable,” he said. “It was just one roll of the evolutionary dice.”This caught my eye because Dollo's Law is a notion that gets talked about, mostly informally, here and there in linguistics. It's part of the ongoing discussion about how linguistic change and biological evolution run parallel, or don't. It's a little coincidence then to find that This Modern World was covering "Language is a virus" at the same time (click to enlarge):
Sigh, some evolution I wish could be reversed.
*That's a Wikipedia entry that needs some editing.