Some of these respiratory and photosynthetic genes are housed outside the nucleus, in separate membrane-bound compartments called (respectively) mitochondria and chloroplasts.

Scientists have a specific scenario for how this came about. Two or three billion years ago, one of the archaea acquired the ability to eat bacteria (in the process developing a nucleus to protect its own DNA). As a result, mitochondria and chloroplasts are just bits of undigested bacterial food that evolution has turned into symbiotic energy factories for the cell. Definitive proof of this notion was first produced by Linda Bonen when she was a technician in my lab, and Scott Cunningham, a student of Mike Gray at Dalhousie University. Thanks to the work of Tom Cavalier-Smith and Patrick Keeling and Andrew Roger, we now know that the acquisition of mitochondria was one of the earliest events in eukaryote evolution.