The next major transition was the evolution of eukaryotic cells – the complex cell type that all plants, animals and fungi are built from. This likely happened through endosymbiosis, in which one cell engulfed another but instead of digesting it, formed a cooperative relationship. Mitochondria and chloroplasts are the descendants of these ancient symbiotic partners.
Eukaryotes and Complex Cells: 2.1–1.2 Billion Years Ago
HTML5 mp4 https://david.hitchhikers.earth/assets/history-of-the-earth/History%20of%20the%20Earth%20%5BQ1OreyX0-fw%5D.mp4#t=5:31 Eukaryotes and Complex Cells: 2.1–1.2 Billion Years Ago
Eukaryotic cells allowed for larger size, internal organisation and specialised functions. This paved the way for multicellular life. The first multicellular organisms appeared between 1.2 and 1.0 billion years ago - wikipedia 