He talks about the hypothesis that a two and a half billion years ago one cell got inside another cell, and they lived better together than they were living separately, and managed to reproduce.
This became the origin of the eukaryotic cell, the cell from which we and all complex living things are made. Without that single act 2.5 billion years ago there would likely be no complex life on Earth.
https://www.facebook.com/reel/880972647240647?mibextid=9drbnH