Lateral gene transfer is the process by which one evolving lineage acquires genes from other lineages. This mechanism isn't thought to be especially
important for eukaryotes such as humans, but it's quite common among non-eukaryotes, the Eubacteria and Archea.
A good example of lateral gene transfer is the vastly different number of genes between the beneficial E. coli strain that all humans have in their guts and the disease-causing E. coli 0157:H7. The human-gut E. coli has about 4,300 genes, whereas the pathogenic strain has 5,400 — a huge difference. It's not clear where all these extra genes came from, but some of them clearly are related to genes from the bacteria species Shigella dysenteriae, whose name probably makes clear why these genes can turn a good E. coli bad.
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