The first 2 billion years was single celled. Since then, life on earth has been a combination of single celled prokaryotic cells, and much larger (100x) eukaryotic cells.
The evolution of eukaryotes, which comprise all the plants and animals you see on David Attenborough's Life on Earth show, didn't occur until the Earth had developed an oxygen rich atmosphere.
What's amazing about eukaryotic cells is that they formed from colonies of prokaryotic cells, in a process called endosymbiosis.
There's a reason why the mitochondria in eukaryotic cells look like little bacteria like organisms. They actually were once separate prokaryotic organisms. Same for chloroplasts in plant cells.
How Two Microbes Changed History