2-1 billion years ago
The first organisms were simple cells like modern bacteria, but some of them became much more internally complex. These ‘eukaryotes’ developed lots of specialised equipment within their cells. They also had a new source of energy: sausage-shaped objects called mitochondria that were once free-living bacteria, but which were absorbed in a process called endosymbiosis. Every animal and plant you have ever seen is a eukaryote.
See other: History of Life