It's huge. For the first 2 billion years of the history of Earth, life was unicellular. The only type of cell in existence was prokaryote. The atmosphere contained no oxygen.
Then there was 1 billion years of history in which Earth's atmosphere changed. At the end of that billion years, two main differences were apparent. Firstly, the atmosphere was comprised of 21% oxygen. Secondly, life on Earth was comprised of two different types of cells - prokaryotes, and eukaryotes.
Eukaryotic cells are 100 times larger than prokaryotes. Unlike prokaryotes, they form multi-cellular organisms. Everything you see around you with your eyes, and every living thing you ever watched David Attenborough talk about on TV are comprised of Eukaryotic cells.
Endosymbiosis is the explanation of how Eukaryotic life came to exist.