OXFORD, ENGLAND -- According to a statement released by the University of Oxford, kissing may have evolved as much as 21 million years ago in the ancestors of humans and large apes. To explore the history of kissing, evolutionary biologist Matilda Brindle and her colleagues first defined the act as nonaggressive, mouth-to-mouth contact without a transfer of food. They then reviewed scientific literature for information on modern primates in Africa, Europe, and Asia to identify species that had been observed kissing. Next, they mapped the primate family tree, treating kissing as a trait, and modeled possible evolution scenarios along the tree's branches. After running the model some 10 million times, the researchers surmised that kissing is an ancient trait dating back to an ancestor who lived between 21.5 and 16.9 million years ago. This means that extinct human relatives probably kissed too. The researchers noted previous studies showing that modern humans and Neanderthals shared oral microbes, likely via the transfer of saliva. Read the original scholarly article about this research in Evolution and Human Behavior. To read more about ancient microbial DNA, go to "Worlds Within Us."