Why can humans talk and chimps cannot?
The question has long since puzzled researchers, considering we share up to 98.5 per cent of the same DNA.
But scientists say they have found a clue - and it lies not simply in the genetic code, but in how the genes function.
A team at the University of California looked specifically at a gene called FOXP2, which looks very similar in many animals - up until the time the human lineage split off from the chimpanzee line four million to seven million years ago.
Slight hang up: Chimpanzees have a different version of the 'talking' gene
FOXP2 is a transcription factor, meaning it affects the activity of other genes. It plays an important role in human speech and affects coordination in other animals such as mice.
Dr Dan Geschwind's team used human and chimpanzee brain tissue to analyse what FOXP2 actually does.
Although the DNA code itself is similar, they found subtle differences in two amino acids produced by the gene.
These differences were enough to account for variations in brain structure that may be involved in language, Geschwind's team reported in the journal Nature.
They could also account for some of the physical characteristics of the jaw and throat that help humans speak - while our hairy cousins can only manage a few grunts.
While many animals, such as bees and dolphins, have complex communication systems, humans are the only ones to develop spoken language, at some time in the last 100,000 years.
'Earlier research suggests that the amino-acid composition of human FOXP2 changed rapidly around the same time that language emerged in modern humans,' Geschwind said.
'We showed that the human and chimp versions of FOXP2 not only look different but function differently too,' added Geschwind, who is currently a visiting professor at King's College London.
'Our findings may shed light on why human brains are born with the circuitry for speech and language and chimp brains are not.'
