Chimpanzees have the same types of smiles as humans when laughing, according to a new study which suggests that the common smile types evolved from positive expressions of ancestral apes.
Researchers from the University of Portsmouth in UK also found that chimpanzees are able to produce these smile types silently, without being constrained by the accompanying laughing sound.
The findings suggest that chimpanzees' communication is more similar to humans than was previously known, researchers said.
"Humans have the flexibility to show their smile with and without talking or laughing," said lead researcher Dr Marina Davila-Ross from the University's Centre for Comparative and Evolutionary Psychology.
"This ability to flexibly use our facial expressions allows us to communicate in more explicit and versatile ways, but until now we didn't know chimps could also flexibly produce facial expressions free from their vocalisations," Davila-Ross said.
The researchers filmed 46 chimpanzees at the Chimfunshi Wildlife Orphanage and used ChimpFACS - a facial action coding system designed for chimpanzees - to measure their facial movements.
Co-author on the paper, Professor Kim Bard, who designed ChimpFACS, said: "The coding system allows us to examine very subtle facial movements and compare human and chimpanzee facial expressions, based on their shared musculature."
The study investigated specific types of smiles that accompany laugh sounds and found that these smile types have the same evolutionary origin as human smiles when they are laughing.
It suggests that these smile types of humans must have evolved from positive expressions of ancestral apes.
The study further suggested that flexibility in facial expressions was already present in ancestral apes and emerged long before humans evolved.
Other researchers have found that chimpanzees, just like humans, are capable of metacognition, or thinking about one's own thinking, and can adjust their behaviour accordingly.
A recent study found that chimpanzees have the cognitive abilities required for cooking and prefer the taste of cooked food.
