In it the authors discuss the evolution of the eukaryotic cell: a multi-part cell that evolved due to penetration or symbiosis of multiple organisms. Pretty much all complex life evolved out of eukaryotic cells. In particular the mitochondria that power your cells did not evolve from the same stuff that the rest of your cells did -- they were an addition later on.
Likewise, most of the genetic diff between us and Chimpanzees (or for that matter, blue green algae) are from external sources -- viruses and 'parasites'.
For that matter, 90% of the cells in your body are not genetically yours -- they are other microbes (though these account for only 10% of your body mass). If you are purged of these cells, you die. This lack of microbial transfer was one of the reasons suggested for why infant mortality was so high in the years when babies were isolated from their mothers and other human contact upon birth.
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