BAP complex is to oppose Polycomb, whereas in yeast, genetic studies clearly demonstrated that histones and presumably nucleosomes are the major targets for SWI/SNF action. Several other strategic changes to chromatin occur in multicellular organisms, including the appearance of histone H1, which may also create chromatin structures that require BAF complexes for conversion to more accessible DNA. With the evolutionary emergence of vertebrates and DNA methylation, nine additional essential subunits are added, which do not have homologs in yeast or flies. In vertebrates, the complexes become polymorphic and combinatorially assembled, allowing them to take on highly specialized functions (14–18). Finally, very late in vertebrate evolution, four additional neuron-specific subunits emerge that are essential for the formation and wiring of our complex nervous systems (19, 20). Thus, the evolution of these complexes might be illustrative of the way that functionalities of the complex are gained and lost with each major change in gene expression and epigenetic strategies over the past half-billion years.
