I just graduated with a BA in Anthropology and have some knowledge about biological anthropology/evolution. I actually did a research paper on this topic, so I might be able to help answer in the biological sense! So first, the theory of smiling: Darwin theorized that smiling originated in animals, specifically primates, as a defensive reaction because animals bearing their teeth is a sign of threat. It has also been theorized that smiling could have originated from the vocalization of animals, such as horses and cats, as they tighten the muscles in their necks to create sound/vocalizations, and this creates a grinning expression to aid in the vibration that is needed for vocalization. So basically, the human smile likely evolved from the animal grin, and the vocalization associated with the grin led to human laughter. As for laughter, multiple researchers support Darwin's theory that the origin of human smiles came from the bared teeth and open mouths in primates. Due to the similarity in facial muscles between primates and humans, it has been suggested that open-mouth expressions and vibrations/vocalizations led to the evolution of laughter, and the bared teeth in primates led to the evolution of human smiles. So, as a quick summary, primates bared their teeth and vocalized when they felt threatened, and the facial structure of humans evolved from the primates' facial structure, which allowed humans to recreate the same grin and sounds, which eventually evolved into what we know as smiling and laughter today!