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

rigid as bacterial cells. So there is more to being ‘eukaryotic’ than just lacking a cell wall; but might it be no more complex than lifestyle? Were the ancestral eukaryotes simply wall-less archaea, which modified their existing cytoskeleton into a more dynamic scaffolding that enabled them to change shape and eat food in lumps, by phagocytosis? Might this alone account for how they came by their mitochondria—they simply ate them? And if so, might there still be a few living fossils from the age before mitochondria lurking in hidden corners, relics of those primitive eukaryotes that shared more traits with the archaea?
The archezoa—eukaryotes without mitochondria
    According to the theory put forward by Cavalier-Smith as long ago as 1983, some of the simple single-celled eukaryotes living today
do
still resemble the earliest eukaryotes. More than a thousand species of primitive eukaryotes do not possess mitochondria. While many of these probably lost their mitochondrialater, simply because they didn’t need them (evolution is always quick to jettison unnecessary traits), Cavalier-Smith argued that at least a few of these species were probably ‘primitively amitochondriate’—in other words, they never did have any mitochondria, but were instead primitive relics of the age before the eukaryotic merger. To generate their energy, most of these cells depend on fermentations in the same way as yeast. While a few of them tolerate the presence of oxygen, most grow best at very low levels or even in the complete absence of the gas, and thrive today in low-oxygen environments. Cavalier-Smith named this hypothetical group the ‘archezoa’ in deference to their ancient roots and their animal-like, scavenging mode of living, as well as their similarities to the archaea. The name ‘archezoa’ is unfortunate, in that it is confusingly similar to ‘archaea’. I can only apologize for this confusion. The archaea are prokaryotes (without a nucleus), one of the three domains of life, while the archezoa are eukaryotes (with a nucleus) that never had any mitochondria.
    Like any good hypothesis, Cavalier-Smith’s was eminently testable by the genetic sequencing technologies then reaching fruition—the capacity to work out the precise sequence of letters in the code of genes. By comparing the gene sequences of different eukaryotes, it is possible to determine how closely related different species are to each other—or conversely, how remote the archezoa are from more ‘modern’ eukaryotes. The reasoning is simple. Gene sequences consist of thousands of ‘letters’. For any gene, the sequence of these letters drifts slowly over time as a result of mutations, in which particular letters are lost or gained, or substituted one for another. Thus, if two different species have copies of the same gene, then the exact sequence of letters is likely to be slightly different in the two different species. These changes accumulate very slowly over millions of years. Other factors need to be considered, but to a point the number of changes in the sequence of letters gives an indication of the time elapsed since the two versions diverged from a common ancestor. These data can be used to build a branching tree of evolutionary relationships—the universal tree of life.
    If the archezoa really could be shown to be among the oldest of eukaryotes, then Cavalier-Smith would have found his missing link—a primitive eukaryotic cell, that had never possessed any mitochondria, but which did have a nucleus and a dynamic cytoskeleton, enabling it to change shape and feed by phagocytosis. The first answers became available within a few years of Cavalier-Smith’s hypothesis, and apparently satisfied his predictions in full. Four groups of primitive-looking eukaryotes, which not only lacked mitochondria but also most other organelles, were confirmed by genetic analysis to be amongst the oldest of the eukaryotes.
    The first genes to be sequenced, by