Bacteria live in complex multispecies communities. Intimately interacting bacterial cells are ubiquitous on biological and mineral surfaces in all habitats. Molecular and cellular biologists have unraveled some key mechanisms that modulate bacterial interactions, but the ecology and evolution of these associations remain poorly understood. One debate has focused on the relative importance of cooperation among cells in bacterial communities. Some researchers suggest that communication and cooperation, both within and among bacterial species, have produced emergent properties that give such groups a selective advantage. Evolutionary biologists have countered that the appearance of group-level traits should be viewed with caution, as natural selection almost invariably favors selfishness. A recent theory by Morris, Lenski, and Zinser, called the Black Queen Hypothesis, gives a new perspective on this debate (J. J. Morris, R. E. Lenski, and E. R. Zinser, mBio 3(2):e00036-12, 2012). These authors present a model that reshapes a decades-old idea: cooperation among species can be automatic and based upon purely selfish traits. Moreover, this hypothesis stands in contrast to the Red Queen Hypothesis, which states that species are in constant evolutionary conflict. Two assumptions serve as the core of the Black Queen model. First, bacterial functions are often leaky, such that cells unavoidably produce resources that benefit others. Second, the receivers of such by-products will tend to delete their own costly pathways for those products, thus building dependency into the interactions. Although not explicitly required in their model, an emergent prediction is that the initiation of such dependency can favor the spread of more obligate coevolved partnerships. This new paradigm suggests that bacteria might often form interdependent cooperative interactions in communities and moreover that bacterial cooperation should leave a clear genomic signature via complementary loss of shared diffusible functions.