When Android Studio consistently says "default activity not found"

Today I worked on updating my Android app to the newest versions of various components. When I went to run the app, it failed with a "default activity not found" bubble. I tried File | Invalidate Caches / Restart and clearing the .idea folder - both good things to do when Android Studio gets stuck - but they did not change the error. After some stumbling around I happened to look at the Merged Manifest tab of the AndroidManifest.xml editor. The right pane showed an error about one of my dependencies having a minimum API level above that of my app. Switching to a slightly older version of that dependency fixed the problem.