Human brains could have grown larger under the direction of specific snippets of DNA, Kent State University researchers report. When a segment of human DNA that controls gene activity was placed into laboratory mice, brains of the rodents grew significantly.
Biologists, philosophers, and other people have spent centuries arguing and debating the qualities that differentiate humans from other animals. Our species exhibits an extremely large brain for our body mass, as well as opposable thumbs, making grasping and manipulating object easy, as well as an upright posture.
It was not until a decade or so ago that genomes for other great apes were deciphered, making them available to compare to the human genetic code. Biologists identified around two dozen segments of code that could, potentially, play a role in the numerous advantages enjoyed by our species. However, none of these suspected snippets of code could be proven to bring about the purported changes.
Enhancers are sections of DNA code that direct the action of neighboring genes. In order to test segments of code that could provide our species with the qualities that make us human, researchers searched for enhancers that were both near genes involved in brain development, and were different in humans, compared to great apes.
A gene known as Hare5 was found to be highly active in the development of the cortex in brains. This segment of DNA controls the Frizzled8 gene, critical in the development of brains. Researchers found that mice provided with the human version of Hare5 developed brains 12 percent larger than those bearing the chimpanzee version of the enhancer.
"They have found a smoking gun in the human genome that connects a regulatory element with a proposed pathway for increasing brain size," Todd Preuss from the Yerkes National Primate Research Center, who was not part of the new research, said.
Although brain size could be controlled through the introduction of these enhancers, that does not mean that the rodents with more massive brains were smarter than their peers. Future research will examine if these changes correlate with increased intelligence.
Humans and chimpanzees separated from each other as species between five and seven million years ago. During that time, some segments of DNA evolved much faster than other regions, and these quickly-changing sections are known as human-accelerated regions (HARs). Some of these are enhancers, directing other genes.
Examination of the role of enhancer genes in the development of brains in mice was profiled in the journal Current Biology.
