Essence of mitochondria

摘要

All known life-forms are either prokaryotes or eukaryotes; there is nothing in between. Eukaryotic cells have a nucleus and compartments that are surrounded by two membranes, but prokaryotic cells never do. The prokaryotes came first; eukaryotes (all plants, animals, fungi and protists) evolved from them, and to this day biologists hotly debate how this transition took place, with about 20 different theories on the go. In most textbook accounts, an ancient prokaryotic lineage supposedly evolved a nucleus, giving rise to a eukafyote, which then acquired mitochondria ― the double-membrane-bounded powerhouses of eukaryotic cells. A cornerstone of this view has been the single-celled eukaryote Giardia intestinalis, which many consider to be a primitive intermediate, a 'living fossil' from the time of the prokaryote-to-eukaryote transition, because it possesses a nucleus but lacks mitochondria. Or so we thought.