Mouse embryos have developed massive, fast-growing brains after receiving a dose of human DNA in the womb. Scientists suspect that the gene encoded by this DNA is what separates us from all other animals. 

Humans would be nothing without our remarkable brains. Despite sharing 98 percent of our DNA with chimpanzees, we've got much bigger brains, and they undergo a massive white matter growth explosion in the first two years of life, which means a super-fast proliferation of neural connections. Researchers have pinned this to why we're so much more intelligent than our primate brethren, but it's not actually clear where in our genetic make-up the instructions to make all this occur.  

To investigate, researchers at Duke University in the US picked over and compared areas of human and chimp DNA, focussing on 'enhancer' sequences, which are short bits of DNA that regulate the activity and expression of neighbouring genes. Several thousand enhancers are found in every genome, but in previous research, no one had found any that were specific to humans and linked directly to brain growth. 

In their initial screening, the team identified 106 enhancers that could be related to the difference in chimp and human brain growth. A closer analysis revealed that six of these sequences were located near genes thought to be involved in brain development. The group called them 'human-accelerated regulatory enhancers,' or HARE, and numbered them one through six. 

HARE5, in particular, was looking particularly special. When they inserted the sequence into mouse embryos, the mice grew brains that were 12 percent bigger than those of mouse embryos that received HARE5 sequences from chimpanzees. Which is pretty surprising, seeing as these enhancers only differ between chimps and humans by 16 letters in their genetic code.

While the difference in brain growth between the two groups of mouse embryos was initially very subtle, near the end of gestation, the burgeoning size of the brains enhanced by human DNA was noticeable to the naked eye. 

Debra Silver, the lead study author and an assistant professor of molecular genetics and microbiology at Duke, explained the results to Rachel Feltman at The Washington Post: 

"We discovered that the human DNA sequence, which only had 16 changes in it compared to the chimp sequence, was being expressed differently in mice. The human DNA was really able to accelerate the way the stem cells divide, and as a result, the mice were able to produce more neurons. HARE5 seems to promote the ability to create more neurons and increase brain size, which allows human brain development to take advantage of that." 

Interestingly, the team reports in Current Biology that the region that was most affected by this hyper-growth in the mouse brains was the cerebral cortex, which is thought to be involved in language and reasoning skills.

"We're seeing differences in brain development, particularly in the structure of the brain that becomes the cerebral cortex," Silver told The Washington Post. "The cerebral cortex has an important function in decision making and thought in humans. We're definitely interested in testing whether it might affect memory or socialisation in mice."

The question of how we got to be as intelligent as we are remains open, but we now have a fascinating little piece of DNA with which to investigate. And a potential army of malevolent rodent geniuses:

Source: The Washington Post