Requires a 'Simian Flu' scenario.
I earned this achievement in a different way that may be bugged.
Make all apes intelligent and evolve the ability to move them across oceans.
Move them all to a single colony.
Move the colony across an ocean and this achievement should unlock while they are 'in the water'.
I believe it happens because while crossing the ocean all apes were considered to not occupy any spaces so they were temporarily considered to be dead.