If you tickle a young chimp, gorilla or orang-utan, it will hoot, holler and pant in a way that would strongly remind you of human laughter. The sounds are very different. Chimp laughter, for example, is breathier than ours, faster and bereft of vowel sounds (“ha” or “hee”). Listen to a recording and you wouldn’t identify it as laughter – it’s more like a handsaw cutting wood. But in context, the resemblance to human laughter is uncanny.
Apes make these noises during play or when tickled, and they’re accompanied by distinctive open-mouthed “play faces”. Darwin himself noted the laugh-like noises of tickled chimps way back in 1872. Now, over a century later, Marina Davila Ross of the University of Portsmouth has used these noises to explore the evolutionary origins of our own laughter.
Davila Ross tickled youngsters of all of the great apes and recorded the calls they make (listen to MP3s of a tickled chimp, gorilla, bonobo and orang-utan). She used these recordings to build an acoustic family tree, showing the relationships between the calls. Scientists regularly construct such trees to illustrate the relationships between species based on the features of their bodies or the sequences of their genes. But this is the first time that anyone has applied the same technique to an emotional expression.
The tree linked the great apes in exactly the way you would expect based on genes and bodies. To Ross, this clearly shows that even though human laughter sounds uniquely different, it shares a common origin with the vocals of great apes. It didn’t arise out of nowhere, but gradually developed over 10-16 million years of evolution by exaggerating the acoustics of our ancestors. At the very least, we should now be happy to describe the noises made by tickled apes as laughter without accusations of anthropomorphism, and to consider “laughter” as a trait that applies to primates and other animals
This is the sixth of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
Physically, we are incredibly different from our ape cousins but genetically, it’s a different story. We famously share more than 98% of our DNA with chimpanzees, our closest living relatives. Our proteins are virtually identical and our chromosomes have more or less the same structure. At the level of the nucleotide (the “letters” that build strands of DNA), little has happened during ape evolution. These letters have been changing at a considerably slower rate than in our relatives than in other groups of mammals.
But at the level of the gene, things are very different. Entire parts of the genome can be duplicated or deleted and the rate at which this happens has actually accelerated in the primate lineage. Some families of genes (including many that play important roles in the brain) have expanded and contracted with remarkable speed.
Duplication provides raw fuel for rapid evolution by creating back-up copies of parts of the genome. If mutations with harmful effects crop up in one of these copies, there’s always a spare kicking around to take up the slack. So duplicated segments of the genome become relatively free to pick up new mutations and unsurprisingly, they are often very dynamic places that change with incredible speed.
Today, they make up about 5% of the human genome and have probably been a major driving force in the ape evolution. Now, Tomas Marques-Bonet from the University of Washington has reconstructed the evolutionary history of these duplications by comparing them across the genomes of four primates – humans, chimpanzees, orang-utans and macaques.
Using computer programmes, he produced a “comparative map” that revealed duplications unique to each of these four genomes, along with those that are shared between them. The map showed that about a third of the duplications in the human genome are unique to us, and most of the remaining duplications are ones we share with chimps.
The rate at which these duplications cropped up had greatly accelerated in the part of the primate family tree that includes humans and the African great apes. These rates doubled and hit their peak in the last common ancestor of ourselves and chimpanzees. As a result, both chimps and humans have far more of these doubles than either orang-utans or macaques. This burst of activity coincided with a time when other types of mutation, such as changes to single nucleotides, were slowing down. Marques-Bonet thinks that these accelerated rates of gene duplication played a pivotal role in the success and evolution of the great apes.