OXFORD UNIVERSITY PRESS, JULY 2014The past 20 years has seen the tree of life take quite a beating. In the 1990s, the time-honored notion that the genealogy of all living beings could be represented as a single branching tree began to look shaky. By the turn of the century it had collapsed under a mound of molecular data too chaotic to ignore. Researchers found prokaryotic organisms—bacteria and archaea—to engage in rampant genetic exchange, both within and among their ranks. The extent of this horizontal gene transfer between distantly related prokaryotes is such that many biologists now see the tree of life as more of a tangled web, through which genes have flowed both vertically and horizontally since the dawn of cellular life on our planet.
Long before genomes could be sequenced and the implications of interspecies gene swapping debated, an even more perplexing issue was brought to bear on the sanctity of life’s tree: endosymbiosis. In the 1960s and ’70s, the American biologist Lynn Margulis (1938–2011) championed the idea that certain compartments within eukaryotic cells had evolved from once free-living bacteria. Fueled by the mixing and matching of genes from two evolutionarily distinct cells, endosymbiosis led to the formation of a single new organism with emergent and transformative biochemical properties.
The idea that our mitochondria and the light-gathering chloroplasts of algae and plants originated from outside the eukaryotic cell was heretical. It was also surprisingly old, having been published in various guises more than 100 years ago by scientists in ...
