What’s Unique about Human Brain Evolution?

Even though we’ve been separated by about 450 million years of evolution, humans and zebrafish share homology in roughly 70% of their genes. Yet in spite of this commonality, few would deny that humans and zebrafish look little alike.

We humans as a species are very proud of our brains. Not only do we have complex societies, we’ve designed numerous tools, and also have the spoken and written language to pass down that culture and those tools. Compared to our closest living cousin, the chimpanzee, our skills are indeed impressive. But is our unique behavioral skill set reflected in the brain? What makes the human brain so special?

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First off, we’re probably all familiar with the fact that, compared to our size, humans have the largest relative brain volume. This numeric value is referred to as an “encephalization quotient.” However, we’re not alone in the Big-brained Mammal Club. A number of different cetaceans (whales, dolphins) and elephants also have particularly large encephalization quotients, as do other primates like the chimpanzee.

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What’s interesting is that, given it’s involvement in things like planning, socialization, cooperation, and working memory, the frontal cortex appears to be different in humans compared to our other primate cousins. When scientists studied patterns of gene transcription across the brain in humans versus other primates, they found that in humans the frontal cortex was particularly special: although chimpanzee and macaque were quite similar in their gene expression profiles, about 1/3rd of the gene transcripts studied were expressed differently in humans [1].

The frontal pole, which is the anterior-most portion of the frontal cortex, is also larger in humans than it is in other primates. And what’s more, it contains a unique microanatomy that indicates it shares more connectivity with other distant areas of the brain [2]. This suggests that not only do humans have more processing units by way of a larger brain, but for its size the human brain may be more interconnected compared to other primates than one might expect. This may help us not only to see important details but to make connections between disparate ideas– the very essence of human creativity and ingenuity.

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A lot of the evolution across species has occurred, not in the protein-coding sequences of genes, but in the regulatory sequences in and around them. In this way in particular, transposable elements, from which many regulatory sequences are derived, are an important source of material for what is ultimately macroevolution, which refers to the processes of speciation.

Scientists have discovered that both humans and chimpanzees have experienced accelerated evolution in and around many of these regulatory sequences in subsets of neural genes. Interestingly, although both species display evolutionary acceleration, the acceleration doesn’t occur in the same genes in human and chimp. So since the 6-7 million years since our two species shared our last common ancestor, we have both dramatically evolved but in different ways relating to the brain [3]. Specifically what those genetic changes mean to brain development and function are yet to be studied but will continue to elucidate precisely what makes us most human.

Human brain evolution has been a complex process, undoubtedly involving varying effects sizes from many different genes. Not only has the human ancestral line progressively developed larger and larger brains, but it’s quite probable that connectivity and neuronal and glial functions have been targets for selective pressures. So when it comes to human intelligence, size is not all that matters. Just consider the extinct Neanderthals: they had even bigger brains than we do!

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