Scientists have traced the evolution of kissing from primates to humans
Kissing may be much older than thought.
A new study by scientists at the University of Oxford shows that this behaviour most likely emerged in the common ancestor of humans and great apes around 21-17 million years ago. In addition, Neanderthals, the authors estimate, may well have kissed too. About it reports Phys.org with reference to the article in the journal Evolution and Human Behaviour.
Kissing occurs in a variety of animals, but from an evolutionary perspective it seems puzzling: it increases the risk of transmission of infections while offering no obvious survival or reproductive advantage. Although kissing often has a strong cultural and emotional meaning in human societies, its evolutionary roots have long received little attention.
A team led by evolutionary biologist Matilda Brindle from Oxford's Department of Biology has for the first time attempted to reconstruct the evolutionary history of kissing using an interspecies approach and a primate 'family tree'.
The results showed that kissing as a form of behaviour is an ancient trait of great apes and originated in the common ancestor of this group around 21.5-16.9 million years ago. At the same time, kissing behaviour persists today in most great apes.
Neanderthals and kissing
A separate conclusion concerns our extinct relatives, the Neanderthals. The authors note that there is already evidence that humans and Neanderthals:
exchanged oral microbes (i.e., saliva transfer occurred),
interbreeding and exchanging genetic material.
Taken together, this makes it highly likely that there was kissing between humans and Neanderthals, not just formal "contact for the sake of reproduction."
"This is the first time anyone has looked at kissing through a broad evolutionary prism," notes Matilda Brindle. - 'Our results add to the growing evidence for a striking diversity of sexual behaviour in our primate relatives'."
How scientists defined "kissing" in animals
One key step was formulating a working definition. Many forms of mouth-to-mouth contact in animals look similar but are not kissing. Because the study spans different species, a universal formulation was needed.
The researchers defined kissing as unprotected (non-aggressive) mouth-to-mouth contact without food transfer.
They then collected data from the literature on which modern primate species have experienced such contact. The analysis included primates from Africa, Europe and Asia, including chimpanzees, bonobos and orangutans - in all of these species kissing has been described by observers.
Kissing was then treated as a behavioural 'trait' that was mapped onto a phylogenetic tree of primates. Using Bayesian modelling, the scientists repeatedly "played" different scenarios of the evolution of this trait along the branches of the tree to estimate which ancestral species were highly likely to have kissed too. The model was run 10 million times to make the statistical estimates reliable.
"By combining evolutionary biology and behavioural data, we can make valid inferences about traits that do not leave fossil traces - such as kissing," explains the paper's co-author, Professor of Evolutionary Biology Stuart West. - 'This allows us to study the social behaviour of both modern and extinct species.'
Kissing: innate behaviour or cultural invention?
The authors emphasise that existing data is still limited, especially for species outside of great apes. However, their work sets a framework for future research and offers primatologists a unified approach to fixing kissing in different animals.
Interestingly, even among humans, kissing is not at all universal.
"Although kissing seems like something common and ubiquitous, it is documented in only 46% of human cultures," notes study co-author Katherine Talbot, an assistant professor in the College of Psychology at the Florida Institute of Technology. - "Social norms and context vary widely, and this raises the question: is kissing an evolutionarily fixed behaviour or a cultural invention? Our work is a first step toward answering that question."