According to a recent study published in Nature Neuroscience (“Evolutionary expansion and anatomical specialization of synapse proteome complexity,” Nature Neuroscience 11, 799 - 806 (2008), Published online: 8 June 2008), “understanding the origins and evolution of synapses may provide insight into species diversity and the organization of the brain.”
The study notes that scientists currently know little about “synaptic molecular evolution,” despite the fact that synapses play a key role in information processing. The authors hypothesized that “the evolution of synapse complexity… has contributed to invertebrate-vertebrate differences and to brain specialization.”
With this in mind, they compared human brains with the brains of 19 other species, ranging from single-celled organisms that do not have nervous systems (e.g., brewer’s yeast), to invertebrates (e.g., insects and worms), non-mammalian vertebrates (e.g., fish), and mammalian vertebrates (e.g., rats and chimpanzees).
In particular, researchers examined proteins located in the postsynaptic region of the synapse in each of the different species. They then compared which proteins the various species shared and what functions they served.
Results indicate that, as organisms become more complex, they possess a greater variety of postsynaptic proteins, as well as a higher number. In particular, “mammals have a higher percentage of proteins” in their synapses (about 600) than invertebrates (which show only half as many) or single-celled organisms (which contain only a quarter of the amount). These proteins evolved over time to become more complex, which ultimately contributed to differences in cognitive abilities between species and to the adaptation of different regions of the brain for different functions.
Professor Seth Grant, Head of the Genes to Cognition Programme at the Wellcome Trust Sanger Institute, explains that "[o]ur 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".
ScientificBlogging.com reports that, “[s]ince the evolution of molecularly complex, 'big' synapses occurred before the emergence of large brains, it may be that these molecular evolutionary events were necessary to allow evolution of big brains found in humans, primates and other vertebrates.”
Professor Grant believes that “[t]his work leads to a new and simple model for understanding the origins and diversity of brains and behavior in all species. We are one step closer to understanding the logic behind the complexity of human brains.”
He equates "[t]he molecular evolution of the synapse" to "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".
Here are links to the research study and the article that sparked my curiosity in the subject:
http://www.nature.com/neuro/journal/v11/n7/full/nn.2135.html
http://www.scientificblogging.com/news_releases/the_complex_synapses_that_drove_brain_evolution
Roxanne Enman
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