A genetic analysis of epidemic typhus, which killed millions in the aftermath of the two world wars, shows that it is descended from an ancient microbe that invented aerobic respiration, when energy is produced by "burning" oxygen.
The research on the typhus microbe, Rickettsia prowazekii, also confirms a theory that the internal powerhouses of human cells - the mitochondria - were once free-living bacteria that became trapped inside the cells of other organisms.
Mitochondria look like primitive rod-shaped bacteria and possess their own genetic material, which people inherit only from their mothers because male mitochondria are lost in the fusion of egg and sperm.
Charles Kurland, professor of biochemistry at Sweden's Uppsala University, and colleagues at the University of South Alabama publish the complete DNA sequence of the typhus microbe - which unequivocally relates it to human mitochondria - in this week's issue of the journal Nature.
Epidemic typhus is carried by blood-sucking body lice and is particularly prevalent after wars and natural disasters. It is estimated to have infected up to 30 million people after the First World War. Deciphering its genes will lead to better vaccines to combat outbreaks of the disease.
The symptoms of epidemic typhus include headaches, chills, fever, delirium and skin rashes. The first recorded outbreak may have been during the siege of Athens in 430BC when a plague-like illness decimated the local population. Tens of thousands of cases of typhus occur each year, mostly in Africa, Asia and South America.
Unravelling the genetic structure of typhus provides hard evidence for the evolution of oxygen-breathing cells using mitochondria, said Michael Gray, professor of biochemistry at Dalhousie University in Halifax, Nova Scotia.
Typhus microbes invade human cells, unlike most other bacteria which live in watery gaps between cells, and so appear to have adopted a similar lifestyle to the ancestors of mitochondria which may also have been intracellular parasites. "True to expectation, these results show that R. prowazekii is ... closely related to mitochondria," Professor Gray said.