Washington, June 9 : Scientists at the Wellcome Trust Sanger Institute say that they have gained fresh insights into the evolutionary origins of the brain, and how it evolved into the remarkably complex structure found in humans.
The researchers say that a study conducted by them suggests that it is not the size alone that gives more brainpower, but the increasingly sophisticated molecular processing of nerve impulses allowed development of animals with more complex behaviours during evolution.
They say that the evolution of complex brains might be attributed to two waves of increased sophistication in the structure of nerve junctions.
Scientists have always thought that most animals, from humble worms to humans, have similar protein components of nerve connections called synapses, and that it is the increase in the number of such protein components in larger animals that allows more sophisticated thought.
"Our simple view that 'more nerves' is sufficient to explain 'more brain power' is simply not supported by our study. Although many studies have looked at the number of neurons, none has looked at the molecular composition of neuron connections. We found dramatic differences in the numbers of proteins in the neuron connections between different species," said Professor Seth Grant, Head of the Genes to Cognition Programme at the Wellcome Trust Sanger Institute and leader of the project.
"We studied around 600 proteins that are found in mammalian synapses and 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," he added.
Defining synapses as the junctions between nerves where electrical signals from one cell are transferred through a series of biochemical switches to the next, the researchers said that they act as mini-processors that give the nervous systems the property of learning and memory.
The research group said that their study suggested that some of the proteins involved in synapse signalling and learning and memory are found in yeast, where they act to respond to signals from their environment, such as stress due to limited food or temperature change.
"The set of proteins found in single-cell animals represents the ancient or 'protosynapse' involved with simple behaviours. This set of proteins was embellished by addition of new proteins with the evolution of invertebrates and vertebrates and this has contributed to the more complex behaviours of these animals," said Professor Grant.
"The number and complexity of proteins in the synapse first exploded when muticellular animals emerged, some billion years ago. A second wave occurred with the appearance of vertebrates, perhaps 500 million years ago," he added.
He also claimed that his team was the first to isolate the synapse proteins from brains of flies, confirming that invertebrates have a simpler set of proteins than vertebrates.
In what might be essential to the understanding of human thought, the researchers had found that expansion in proteins that occurred in vertebrates provided a pool of proteins that were used for making different parts of the brain into the specialised regions like cortex, cerebellum and spinal cord.
The researchers say that molecular evolutionary events, such as the occurrence of the evolution of molecularly complex and 'big' synapses before the emergence of large brains, might have been necessary to allow evolution of big brains found in humans, primates, and other vertebrates.
According to them, behavioural studies on animals, wherein mutations were found to disrupt synapse genes, went on to suggest that the synapse proteins that evolved in vertebrates gave rise to a wider range of behaviours, including those involved with the highest mental functions.
The team said that a study had shown that irregularities in a gene called SAP102, necessary for a mouse to use the correct learning strategy when solving mazes, could give rise to a form of mental disability in humans too.
"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 Professor Grant.
Simple invertebrate species have a set of simple forms of learning powered by molecularly simple synapses, and the complex mammalian species show a wider range of types of learning powered by molecularly very complex synapses.
"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," said Dr Richard Emes, Lecturer in Bioinformatics at Keele University, and joint first author on the paper.
The researchers are of the opinion that their findings might helpfully enable scientists to gain a better understanding of the human brain, and be directly relevant to disease studies.
Professor Grant claimed that he and his colleagues had identified recently evolved genes involved in impaired human cognition, and modelled those deficits in the mouse.
"This work leads to a new and simple model for understanding the origins and diversity of brains and behaviour in all species" said Professor Grant, adding that "we are one step closer to understanding the logic behind the complexity of human brains"