Figure 3.18: Possible homologues to two types of human smiles The silent bared-teeth display (a) is believed to be homologous to the human greeting smile, and the relaxed open-mouth display (b) is believed to be homologous to the human laugh and happy smile. The animals in both photos are chimpanzees.
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Norman Owen Tomalin/Photoshot
Norman Owen Tomalin/Photoshot