Read this book around 1975/79. Can remember the story but not the title.
Basically: group of humans in deep sleep/hibernation in underground bunkers. Awake to find 1000s years gone by and ants and bees etc have evolved into intelligent beings. Story of how they cope and alliances made. Twist at the end is that they were 'left' to see if Earth was okay before the elite (who fled in spaceships and sat in orbit) waited to return.
Appears those in space can't return as they haven't become immune to some new radiation.