Life forms multiplied and grew more complex by co-opting others, not just by killing them. Their ancient biotechnology led to fermentation, photosynthesis, oxygen breathing, and the fixation of atmospheric nitrogen into proteins. It also led to worldwide crises of bacterial population expansion, starvation, and pollution - long before the dawn of larger forms of life. Bacteria survived these crises because of special abilities that other life forms lack and that add whole new dimensions to the dynamics of evolution. First, bacteria routinely transfer their genes to bacteria very different from themselves.
Bacteria can exchange genes quickly and reversibly.
Unlike other life forms, all the world's bacteria have access to a single gene pool and hence to the chemical prowess of the entire bacterial kingdom. This extreme genetic fluidity makes the very concept of species of bacteria meaningless. The result is a planet made fertile and inhabitable for larger life forms by a worldwide system of communicating, gene-exchanging bacteria. Bacteria also have a remarkable capacity to combine their bodies with other organisms, forming alliances that may become permanent. Mitochondria live inside our cells but reproduce at different times with different methods from the rest of our bodies' cells.