What Makes Our Brain Human? The Search for Answers

“The Thinker” by Auguste Rodin (photo by Brian Hillegas)


Humans’ most unique traits, such as speaking and abstract thinking, are rooted in the outer layer of our brains called the cerebral cortex. This convoluted sheet of grey matter is found in all mammals, but it is much larger and far more complex in Homo sapiens than in any other species. The cortex comprises nearly 80 percent of our brain mass, with some 16 billion neurons packed into more than 50 distinct, meticulously organized regions.

In an effort to explore the evolution of the human cortex, many researchers have looked to changes in the portion of the genome that codes for proteins. But a new paper, published in the journal Science [1], shows that protein-coding DNA provides only part of the answer. The new findings reveal that an even more critical component may be changes in the DNA sequences that regulate the activity of these genes.

In the study, a team led by Steven Reilly and James Noonan at Yale University, New Haven, CT, compared the development of the cortex in human, rhesus macaque, and mouse. Instead of focusing on protein-coding DNA, the researchers set their sights on tracking the activity of regulatory elements in the genome. Specifically, they looked at two key types of regulatory elements: promoters, which are DNA signals that generally lie just upstream of protein-coding genes; and enhancers, which are DNA regions that can be located some distance away, but modulate a gene’s output, rather like the dimmer function on a light switch.

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