Of course humans are smarter than chickens. But now scientists know why.

Anyone who’s lost in a game of tic-tac-toe to a chicken at a county fair can take solace in new research that definitively proves, not only that humans are in fact smarter than our feathered friends, but why.

Researchers from the University of Toronto published a paper in the August 20 issue of Science that may have traced back our species’ intellectual development back to a singular molecular event within our cells.

Benjamin Blencowe, a professor in the University of Toronto's Donnelly Centre and Banbury Chair in Medical Research and his team have discovered how a small change in a protein called PTBP1 can inspire the creation of neurons, also known as brain cells, in mammals.

The process has its foundation in something called alternative splicing (AS), which is a step in the way that cells assemble proteins. During AS, gene fragments are assembled to make different protein forms.

Previous research has shown the incidence of AS increases along with the complexity of vertebrate forms. So human beings have a higher prevalence of AS – and more complex proteins – than something like a frog, even though at a basic level the genes which are expressed are similar.

The researchers in Toronto looked into whether AS is a factor in brain development in vertebrates.

"We wanted to see if AS could drive morphological differences in the brains of different vertebrate species," says Serge Gueroussov, a graduate student in Professor Blencowe's lab and lead author of the study.

The PTBP1 protein's job in a cell is to stop it from becoming a neuron. In mammals, a new shorter version of PTBP1 is active in cells leading to a series of events that lead to the generation of neurons in an increasing number of cells. The second form of mammalian PTBP1 is shorter because a small piece is absent during AS and is not included in the completed protein shape.

The little change could have sparked the evolution of mammalian brains to be among the most developed in the animal kingdom.

"One interesting implication of our work is that this particular switch between the two versions of PTBP1 could have affected the timing of when neurons are made in the embryo in a way that creates differences in morphological complexity and brain size," said Blencowe.

Additionally, when chicken cells were genetically engineered to produce the mammalian form of PTBP1, similar events were triggered, leading to neuron development. Scientists say this discovery has led to new ideas about the role of AS has had in species development.

"This is the tip of an iceberg in terms of the full repertoire of AS changes that likely have contributed major roles in driving evolutionary differences," Blencowe said.