Evolutionary biologists have long been interested in the dimensions of the human noggin, and more specifically the size of the brain tucked inside. The human cranium is so big, they point out, that it poses risks during childbirth, and it takes a lot of energy to keep that big brain humming along. Most researchers assumed that the large size must deliver a major evolutionary advantage, like the capacity for increased intelligence, to make up for these disadvantages.
Now a study published in Nature Neuroscience [subscription required] suggests that it wasn’t an increased number of brain cells that gave humans such an evolutionary boost, but rather the increased complexity in the synapses between brain cells.
[Synapses are] 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 [AFP].
The new study puts a little more distance between the creepy crawly creatures and mankind. The researchers looked at the synapses in single-celled yeast, fruit flies, and mice, three organisms that represent very different evolutionary stages. The neuroscience team focused on the roughly 600 proteins that are found in the synapses of mice, and looked to see whether those proteins were also present in the synapses of the other two organisms. They found that only half were present in fruit flies, and about a quarter were present in the yeast synapses.
This suggests that each big evolutionary step of life on earth was occasioned by an increase in synaptic sophistication, the researchers say. 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” [BBC News].
Each increase in synapse proteins presumably allowed for greater specialization in the brains of developing species, and therefore more complex systems of learning and behavior. Researchers said that 25 percent of synapse proteins that were found in the single-celled yeast point to the evolutionary root of animal intelligence.
Remarkably, the study shows that the origins of thinking lie in feeling: 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.
“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 [Daily Telegraph].
It certainly is amazing. Browse through some past DISCOVER articles for equally fascinating glimpses of how synapses encode memories and fear, and just how complicated the complete “neural code” that governs our learning and behavior really is.