Power, Sex, Suicide: Mitochondria and the Meaning of Life by Nick Lane Page A

Woese’s group in 1987, belonged to a tinyparasite, no larger than a bacterium, which lives inside other cells—indeed, can
only
live inside other cells. This was the microsporidium
V. necatrix
. As a group, the microsporidia are named after their infective spores, all of which come replete with a projecting coiled tube, through which spores extrude their contents into a host cell, then multiply to begin their life cycle afresh, ultimately producing more infective spores. Perhaps the best-known representative of the microsporidia is
Nosema
, which is notorious for causing epidemics in honeybees and silkworms. When feeding inside the host cell,
Nosema
behaves like a minute amoeba, moving around and engulfing food by phagocytosis. It has a nucleus, a cytoskeleton and small bacterial-style ribosomes, but has no mitochondria or any other organelles. As a group, the microsporidia infect a wide variety of cells from many branches of the eukaryote tree-of-life, including vertebrates, insects, worms, and even single-celled ciliates (cells named after their tiny hair-like ‘cilia’, used for feeding and locomotion). As all microsporidia are parasites that can survive only inside other eukaryotic cells, they can’t truly represent the first eukaryotes (because they would have had nothing to infect) but the diverse range of organisms that they do infect suggests that they have ancient origins, going back to the roots of the eukaryotic tree. This assumption seemed to be confirmed by genetic analysis, but there was a catch, as we shall see in a moment.
    Over the next few years, the ancient status of the three other groups of primitive eukaryotes was confirmed by genetic analyses—the archamoebae, the metamonads, and the parabasalia. All three groups are best known as parasites, but free-living forms do also exist, perhaps fitting them better than the microsporidia as the earliest eukaryotes. As parasites, these three groups occasion much misery, illness, and death; how ironic that these repellent and life-threatening cells should be singled out as our own early ancestors. The archamoeba are best represented by
Entamoeba histolytica
, which causes amoebic dysentery, with symptoms ranging from diarrhoea to intestinal bleeding and peritonitis. The parasites burrow through the wall of the intestine to gain access to the bloodstream, from where they infect other organs, including the liver, lungs, and brain. In the long term, they may form enormous cysts on these organs, especially the liver, causing up to 100 000 deaths worldwide each year. The other two groups are less deadly but no less smelly. The best-known metamonad is
Giardia lamblia
, another intestinal parasite.
Giardia
does not invade the intestinal walls or enter the bloodstream, but the infection is still thoroughly unpleasant, as any travellers who have incautiously drunk water from infected streams know to their cost. Watery diarrhoea and ‘eggy’ flatulence may persist for weeks or months. Turning to the third group, the parabasalia, the best known is
Trichomonas vaginalis
, which is among the most prevalent, albeit least menacing, of the microbes that cause sexually transmitted diseases (though the inflammation it produces may increase the risk ofcontracting other diseases such as AIDS).
T. vaginalis
is transmitted mainly by vaginal intercourse but can also infect the urethra in men. In women, it causes vaginal inflammation and the discharge of a malodorous yellowish-green fluid. All in all, this portfolio of foul ancestors just goes to prove that we can choose our friends but not our relatives.
The eukaryote’s progress
    For all their unpleasantness, the archezoa nonetheless fitted the bill as primitive eukaryotes, survivors from the earliest days before the acquisition of mitochondria. Genetic analysis confirmed that they did branch away from more modern eukaryotes at an early stage of evolution, some two thousand million years ago, while their uncluttered morphology