  We investigate the selective forces that promote the emergence of modularity
in nature. We demonstrate the spontaneous emergence of modularity in a
population of individuals that evolve in a changing environment. We show that
the level of modularity correlates with the rapidity and severity of
environmental change. The modularity arises as a synergistic response to the
noise in the environment in the presence of horizontal gene transfer. We
suggest that the hierarchical structure observed in the natural world may be a
broken symmetry state, which generically results from evolution in a changing
environment. To support our results, we analyze experimental protein
interaction data and show that protein interaction networks became increasingly
modular as evolution proceeded over the last four billion years. We also
discuss a method to determine the divergence time of a protein.
