When life evolves on a planet, the planet gets a shield and 4 indicator arcs (each 90 degree wide) inside - initially "dark". Then, as time progresses, these start filling in. Also, defensive laser cannons appear on the surface. Furthermore, the planet starts producing spaceships.
Which of the things is a representation of how much life is there on the planet? I'm asking in regards to the "Maximum life in your system" achievement.