The existence of humans in the Spore Galaxy is debated. No game or expansion explicitly mentions humans or their civilization, but many hints suggest their existence and fate, the most famous of them being the Solar System with a uninhabited Earth existing as an easter egg in the galaxy, and Steve, a probe telling the player to look after Earth.
Many Spore users have tried to create humans using the Creature Editor, but since the editor is not designed for that, their creations don't fit much of the appearance and behavior of a real-life human.
Some players report finding these user-made "Humans" with Empires spawning in the vicinity of the aforementioned Solar System. It is unknown whether this is a coincidence or observation bias, or whether there is a feature that causes creatures named (or tagged) "Human" to appear in that region of the galaxy, in a similar manner which causes simpler creatures to appear in the early Creature Stage.
Some hints in the game strongly suggest that, at the time the game takes place, humans had been extinct by an enviromental disaster. One of those hints is the lack of apparent vegetation in the Amazon Rainforest area, implying a large deforestation.
The description of some artifacts also purportedly mention humans.