Relentless evolution towards more intelligent species may have been driven not just by progressively larger brains but by the increasingly complex way in which they were wired.
Scientists in Britain probing the origins of the human brain focused on the role of synapses, the junctions between nerves which transfer electrical signals - and information from one brain cell to the next via a series of biochemical switches.
Most research to date has assumed that synapses, made of proteins, are essentially the same in all animals, ranging from the lowly earthworm all the way up the evolutionary ladder to humans.
What makes some species more intelligent than others, according to this prevailing wisdom, is sheer mass - more neurons equals greater data-processing capacity.
"The view that 'more nerves' is sufficient to explain 'more brain power' is simply not supported by our study," said lead researcher Seth Grant, who heads the Genes to Cognition Programme at the Wellcome Trust Sanger Institute in Britain.
Most research to date has assumed that synapses, made of proteins, are essentially the same in all animals, ranging from the lowly earthworm all the way up the evolutionary ladder to humans than 600 types of synaptic proteins found in mammals, and then looked to see how many would turn up in less evolved life forms, notably singlecell yeasts and fruit flies.
"We were surprised to find that only 50 per cent of these are also found in invertebrate synapses, and about 25 per cent are in single-cell animals, which obviously don't have a brain," said Grant.
These differences correspond to the evolutionary path of synapses over time, and point to two major leaps in sophistication that may have made later brain specialisation possible.
The new findings will help scientists understand normal functioning of the human brain, and could lead to treatments for disease, he added.