TRADITIONAL MODEL. “Under this ‘phylogenetic’ model, complexity of mimetic muscle morphology increased in a simple linear, step-wise fashion up the phylogenetic scale until humans, where the ultimate in complexity was achieved. This view has traditionally also been applied to facial display repertoire, with the most simple, undifferentiated displays being rooted in the prosimians, ever increasing in a step-wise, linear fashion up to humans, where the most complex, subtle, and graded displays are found.”
http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full