Lost Novella or Short Story: Either Uplifted or Evolved Animals finally break through a Barrier to discover that the Humans have left Earth

The protagonist, possibly an uplifted or naturally evolved bear is in charge of a
project to breach a barrier. This has kept all intelligent species from leaving
the area. They eventually break through to discover a final message left by Humans.
They have left the Earth to move out into space and were either responsible for the
Animal uplift in intelligence or acted responsibly when it naturally occurred.
They left Earth to allow the animals to develop naturally, uninfluenced by Human activities.
I read this sometime in the 1970’s, it may have been the first and longest story in an anthology.