3/4 of the time period of life on earth was single-celled organisms.... but what people might miss when they look at this was that it was always an interactive system. What we call 'life' arose on top of chemical and physical processes that were only enabled by the flows of energy from (1) our young and energetic Sun and (2) our highly radioactive earth core. From the very point where the system started cooling down enough to allow carbon-carbon bonds to form the context of an aqueous environment, life was already there, the potentiality at least.

Of course, by this measure of our own history, we should expect ~3/4 of the Earth-like planets we discover to have nothing more advanced than single-celled life.

Now, the question of how single-celled life could have given rise to multicellular life and the origination of chromosomes, etc. is a whole other question, see slime molds.