A minute particle within a protein allowed humans to become the most intelligent creatures on the planet, say scientists.
It holds the key to understanding why our brains are so much bigger and more complex than any other animal, according to new research.
It may also explain how its unequalled mental capacity evolved so rapidly and dramatically, a mystery that has baffled researchers for decades.
Human brains are surprisingly large compared to the rest of the body.
More importantly, its ratio to body size is significantly larger and it has a much greater cerebral cortex, the area that controls higher thought processes, with a higher concentration of neurons.
Professor James Sikela, of the University of Colorado, said: 'We wanted to know why.
'The size and cognitive capacity of the human brain sets us apart. But how did that happen?
'This research indicates that what drove the evolutionary expansion of the human brain may well be a specific unit within a protein - called a protein domain - that is far more numerous in humans than other species.'
The protein domain issue is known as DUF1220.
Humans have more than 270 copies of DUF1220 encoded in their DNA, far more than other species.
© AlamyDophins are regarded as one of earth's most intelligent species.
Chimpanzees have the next highest number, 125. Gorillas have 99, marmosets 30 and mice just one.
Prof Sikela said: 'The one over-riding theme that we saw repeatedly was the more copies of DUF1220 in the genome, the bigger the brain.
'And this held true whether we looked at different species or within the human population.'
His researchers also linked DUF1220 to brain disorders.
They associated lower numbers of DUF1220 with microcephaly, when the brain is too small, and larger amounts with macrocephaly, when the brain is too large.
They compared genome sequences from humans and other animals and also looked at the DNA of individuals with microcephaly and macrocephaly and of healthy people.
Professor Sikela, whose findings were reported online in The American Journal of Human Genetics, said 'The take home message was brain size may be to a large degree a matter of protein domain dosage.
'This discovery opens many new doors. It provides new tools to diagnose diseases related to brain size.
'And more broadly, it points to a new way to study the human brain and its dramatic increase in size and ability over what, in evolutionary terms, is a short amount of time.'
Human brains are up to three times larger than those found in an ape.
did I miss the statistics about the dolphins?