Parts of Albert Einstein's brain were unusually big
Size may not be everything when it comes to brain evolution, say experts.
Instead, UK research reveals that the rising complexity of connections between brain cells may have been the biggest driving force.
The Nature Neuroscience study found clear differences between brain junctions in mammals, insects and single cell creatures.
Changes around half a billion years ago, they said, are likely to have been more important than brain size.
Prehistoric ancestors, such as homo erectus, had significantly smaller brains than modern man.
Evidence suggests that human brain size must have conferred an evolutionary advantage sufficient to make up for the obvious disadvantages - particularly the risks during childbirth, and the increased amount of energy needed to keep a larger brain running.
However, the research from Cambridge's Sanger Institute, and the universities of Keele and Edinburgh, says that this may not be the whole story, and that some of the key evolutionary "breakthroughs" may have relied on something more than an increase in the number of cells.
It looked closely at the synapse, the junction between nervous system cells, in three different types of creature, single-celled yeast, the fruit fly, and the mouse, all of which represent three distinct stages in the evolution of life on Earth.
Of particular interest were 600 proteins found in the synapses of mammals.
To their surprise, only about half of these were found in the synapses of the fruit flies, and, in the yeast cells, which don't have a brain, approximately 25% were present.
They suggest that the big advances in evolution, from single-celled to multi-celled, and at the change between invertebrates and vertebrates, may well have been sparked by a rapid change in the complexity of the synapse.
Professor Seth Grant, one of those leading the study, said: "Our simple view that 'more nerves' is sufficient to explain more brain power is simply not supported by our study.
"The number and complexity of proteins in the synapse first exploded when multicellular animals emerged, some billion years ago.
"A second wave occurred with the appearance of vertebrates, perhaps 500 million years ago."
He said the finding offered clues not only to these massive changes, but to the ongoing evolution of humans.
"We are one step closer to understanding the logic behind the complexity of human brains," he said.
Dr Richard Emes, a lecturer in bioinformatics at Keele University, and another of the researchers, said: "It is amazing how a process of Darwinian evolution by tinkering and improvement has generated, from a collection of sensory proteins in yeast, the complex synapse of mammals associated with learning and cognition."
Dr Hugo Spiers, a neuroscientist from University College London, said that while the size of the brain could not explain all the differences in the abilities of the organ, it still had a major role to play.
He said: "We know that size isn't everything - for example, whales and elephants have much larger brains than we do.
"This new research is right in saying that there is a lot more we can learn about how synapses work to improve our understanding of the brain's complexities.
"However, it's also true that, if you are dealing with intelligence, there are certain parts of the brain which are disproportionately bigger in humans, and which do appear to make a difference."