We can thank hairy ancestors for our grin: study

Chimps can smile without laughing, just like people. Photo / Getty Images

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.

British researchers from the University of Portsmouth say 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.

Dr Marina Davila-Ross, from the university's Centre for Comparative and Evolutionary Psychology, said the research suggested this shows the evolution of this type of expression from ape to human.

"Humans have the flexibility to show their smile with and without talking or laughing," she said.

"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.

The study investigated specific types of smiles that accompany laugh sounds.

It found these smile types had the same evolutionary origin as human smiles have when they are laughing.

It suggests these human smile types 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 the evolution of humans.

Davila-Ross said there were still key differences between humans and our ape ancestors.

"Chimps only rarely display crow's feet when laughing, but this trait is often shown by laughing humans," she said.

"Then, it is called Duchenne laughter, which has a particularly positive impact on human listeners."

-PA