Why Humans are Smarter Than Chickens: Brain Evolution

How did humans become so smart? Scientists have discovered that a single molecular event in our cells could hold the key to how we evolved to become the smartest animal on the planet.

Brain size and complexity vary enormously across vertebrates, but it is not clear how these differences came about. Humans and frogs, for example, have been evolving separately for 350 million years and have very different brain abilities. Yet scientists have shown that they use a remarkably similar repertoire of genes to build organs in the body.

How is it that a similar number of genes that can generate a vast range of organ size and complexity? That's a good question. The key to it lies in a process known as alternative splicing (AS). This is when gene products are assembled into proteins, which are the building blocks of life. During AS, gene fragments, called exons, are shuffled to make different protein shapes. AS also enables cell to make more than one protein from a single gene, so that the total number of different proteins in a cell greatly surpasses the number of available genes. Interestingly, nowhere is AS more widespread than in the brain.

"We wanted to see if AS could drive morphological differences in the brains of different vertebrate species," said Serge Gueroussov, lead author of the new study, in a news release.

In this case, the researchers found that PTBP1 is a protein that takes on another form of different animals. More specifically, they found that the presence of a second, shorter version of PTBP1 unleashes a cascade of AS events, tipping the scales of protein balance so that a cell becomes a neuron.

So what does this mean? It's possible that a 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.

The findings are published in the journal Science.

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