Sometimes a smile says more than words. We smile silently and we laugh loudly, both examples of how we adapt our faces for our social world.
When we smile, several features of our face change. We can do it independently of talking or making laughter sounds.
But how our smiles came to be has not always been clear. It was believed for a long time that human laughter had come from the "fearful grins" of our ancestors, a sort of forced grimace.
An analysis of the spontaneous laughter enjoyed by chimps as they play puts this idea to rest, showing that they too smile in a positive way, using their top lips as we do.
That chimps laugh as they rough and tumble is obvious but a detailed analysis of their facial features shows surprising similarities to our own.
It reveals that this sort of laugher must have evolved before humans or chimps, in our common ancestor.
Your smile could therefore have evolved more than five million years ago, according to a new study published in the journal PloS One.
A team analysed the facial expressions of 46 chimpanzees as they were playing. These chimps were from four different colonies in Zambia, Africa, to account for any individual differences.
They used a special coding system that reveals very subtle changes in facial movements. The chimp faces could then be compared to human smiles.
Chimps, the team discovered, can even smile without making a sound. This reveals they use their smiling faces much more flexibly than was previously believed.
Marina Davila-Ross, lead author of the study from the University of Portsmouth, UK, says the way we smile is important in order for us to communicate in explicit and versatile ways.
"We didn’t know chimps also have the ability to use their facial expressions in such ways too."
"For 40 years researchers were stating that chimpanzee smiles are submissive 'fear' grin-like expressions. The supporting evidence was that chimps do not raise their upper lips while laughing. Our detailed analysis showed they do do this."
Our smiles therefore must have evolved from these very positive "laugh faces" of our ancestors, not the submissive ones.
Laughter and smiles may have originated in play but as laughter became intertwined so deeply in our social world, it must have become "more detached from the play context and emerged into a fundamental tool of language and emotional intelligence in humans", the authors report.
This research comes after the discovery that chimpanzees dip leaves in alcoholic plant sap to "get drunk" and that they have the mental capacity to cook.
"The more we search for these commonalities between chimpanzees and people the more we are likely to find them," adds Davila-Ross.
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