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, reports a study released Sunday.
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.
Looking at the density and molecular makeup of synapses, the study, published in Nature Neuroscience, found dramatic differences between species.
Grant and his team examined more 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 single-cell yeasts and fruit flies.
"We were surprised to find that only 50 percent of these are also found in invertebrate synapses, and about 25 percent 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 genetic transition from single-celled to multi-celled organisms, and then again from invertebrates to vertebrates, both involved huge increases in the architectural complexity and signalling capacity of synapses, the study concludes.
"It is amazing how a process of Darwinian evolution, by tinkering and improvement, has generated ... complex synapse of mammals associated with learning and cognition," said co-author Richar Emes, a professor at Keele University in Britain.
Behavioural studies in animals in which certain synapse genes have been blocked through mutation bolster the idea that the emergence of more complex synapse proteins in vertebrates allowed for higher mental functions.
One of these "vertebrate innovation" genes called SAP102, for example, is necessary for a mouse to learn how to navigate a maze. When this gene is defective in humans, it results in a form of mental disability.
"The molecular evolution of the synapse is like the evolution of computer chips -- the increasing complexity has given them more power and those animals with the most powerful chips can do the most," said Grant.
The new findings will help scientists understand normal functioning of the human brain, and could lead to treatments for disease, he added.