Eukaryotes

Perhaps it was during the Paleoproterozoic Era that revolutionary new forms of cells first appeared.

They probably started when the anaerobe made a big new membrane to trap the aerobe. It took some of the genes from the aerobe so it could never leave. The aerobe now became a mitochondrion, a permanent resident, unable to live outside the parent cell. The process of one organism capturing another and living together for mutual benefit is called endosymbiosis, meaning "symbiosis inside".