The author's implied premise that a smile is a universal form of emotional expression vs a cultural adaptation was presented in the Darwin paper he cited: "The Expression of the Emotions in Man and Animals". Interestingly, a 2012 paper tested this hypothesis:

"In sum, our data directly show that across cultures, emotions are expressed using culture-specific facial signals. Although some basic facial expressions such as fear and disgust (2) originally served as an adaptive function when humans “existed in a much lower and animal-like condition” (ref. 1, p. 19), facial expression signals have since evolved and diversified to serve the primary role of emotion communication during social interaction. As a result, these once biologically hardwired and universal signals have been molded by the diverse social ideologies and practices of the cultural groups who use them for social communication."

"Facial expressions of emotion are not culturally universal", http://www.pnas.org/content/109/19/7241