Monkeys, rats and dogs were found to have already enjoyed a good "laugh". What a rat or dog could think when "laughing", it's hard to say. But it seems that we, humans, have been able to laugh for many millions of years, and not since we turned into humans.

A new research published in the Biology Letters shows that our laugh originated in our ancient primate ancestor related with the apes. The study has discovered that orangutans possess a sense of empathy and mimicry essential for a laughter and pushed the age of human laughter to at least 12-16 million years ago, when orangutans split from the line that led to humans, chimps and gorillas.

Facial expressions, like the open mouth resembling laughter, were imitated by orangutans. The speed of the imitation (0.4 seconds) revealed that those expressions were involuntary, thus orangutan laughing was contagious.

The team made of Dr Marina Davila Ross, from the University of Portsmouth, and Professor Elke Zimmermann, from the University of Veterinary Medicine in Hanover, Germany, investigated the play behavior of 25 orangutans aged 2 to 12 at four primate centers: Sepilok Rehabilitation Center, Apenheul Primate Park, Tierpark Carl Hagenbeck and Leipzig Zoo.

When one orangutan showed the open, gaping mouth, its playmate responded with the same expression in most of the occasions, a behavior called "emotional contagion" in humans, a form of empathy that permits us to experience the emotions of those around us. But not every ape would smile back, this behavior being conditioned by social factors similar to humans: orangutans were more likely to return a smile to friends than to strangers.

"In humans, mimicking behavior can be voluntary and involuntary. Until our discovery there had been no evidence that animals had similar responses. What is clear now is the building blocks of positive emotional contagion and empathy that refer to rapid involuntary facial mimicry in humans evolved prior to humankind. The findings shed a new light on empathy and its importance for animals which live in groups such as orangutans", said Ross.

"Results clearly indicated that orangutans mimic open-mouth faces of their playmates within 0.4 second, which confirms rapid involuntary facial mimicry in nonhuman mammals", wrote the researchers.

Previous researches showed that yawns are as contagious to apes and monkeys as they are to us.