Chimpanzees have the same types of smiles as humans when laughing and do not even need to make a sound to be understood, according to a new study.
Researchers from the University of Portsmouth say that chimpanzees' communication is more similar to humans than was previously known as they are able to produce these smile types silently without being constrained by the accompanying laughing sound.
Marina Davila-Ross, from the university's Centre for Comparative and Evolutionary Psychology, said the research suggests that this showed the evolution of this type of expression from ape to human.
She said: "Humans have the flexibility to show their smile with and without talking or laughing. 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."
The researchers filmed 46 chimpanzees at the Chimfunshi Wildlife Orphanage in Zambia and used ChimpFACS - a facial action coding system designed for chimpanzees - to measure their facial movements.
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, published in the journal PLOS ONE, further suggests that flexibility in facial expressions was already present in ancestral apes and emerged long before humans evolved.
Dr Davila-Ross said there were still key differences between humans and our ape ancestors.
She said: "Chimps only rarely display crow's feet when laughing, but this trait is often shown by laughing humans. Then, it is called Duchenne laughter, which has a particularly positive impact on human listeners."