Looks like some kind of insect damage, like carpenter ants.
Casey

Oh, for heaven's sake! We never considered it might be critters. Thanks, Casey!

Termites? I don't know but I'd bet its insect damage. Yikes!

Looks like termites or similar to me. You should get a pest inspector in to check for more damage, and whether they are still active.

Looks like subterranean termites, they come up through the plumbing near wet areas. Are you on a slab? If so they come up where the piping comes through the slab. Will need to treat and depending on the damage to the wood replacement may be required.

Normally the damage is localized not throughout the house.

It's definitely termites. The extent of the overall damage will depend on how far away this is from the edge of the building.
If this is in the middle of the house, then they;ve been eating since they got in at the houses edge.
Did you have an incident where flying insects were in the house. Or did you see insect wings laying all around with some bugs walking . If so, you had a termite swarm. That's when the reproductive males and females breakout to find new places to build a nest.
Ron

This is so fascinating. That wall area is just a couple of feet from the exterior of the house. Here's hoping... I have a pest inspector scheduled for tomorrow morning. Thanks, everyone! (Don't remember a swarm...)