Scientists have grown larger monkey brains by giving marmoset fetuses a gene that's unique to humans. It might sound like the start of a Planet Of The Apes rip off, but the research could shed light on how this profoundly important gene helped to make us human. 

Reporting their work in the journal Science, scientists at the Max Planck Institute of Molecular Cell Biology and Genetics expressed the gene in 101-day-old fetuses (still 50 days pre-birth date) of common marmosets and found it produces an enlarged neocortex, a part of the cerebral cortex that’s exceptionally larger in humans compared to other primates and is responsible for advanced cognitive abilities, such as reasoning and language.

The gene in question is ARHGAP11B, which helps to create an enlarged neocortex by triggering brain stem cells to form more stem cells. Given the importance of a large neocortex in the development of humanity's cognitive skills, it’s thought the gene played a fundamental role in the evolutionary history of humans and other closely related hominins. ARHGAP11B is a human-specific gene that occurred on our lineage after the divergence from chimpanzees but before our split with Neanderthals and Denisovans, meaning these extinct relatives also had the "big-brain gene."

The gene has previously been expressed in mice and ferrets, but this is the first time it’s been shown to have the same brain-enlarging effect in a non-human primate.

“We found indeed that the neocortex of the common marmoset brain was enlarged and the brain surface folded. Its cortical plate was also thicker than normal,” Michael Heide, lead study author, said in a statement. “Furthermore, we could see increased numbers of basal radial glia progenitors in the outer subventricular zone and increased numbers of upper-layer neurons. This type of cortical neuron increases in primate evolution.”

Scientists were long mystified as to when and how this crucial gene arrived in our story, but in 2016, researchers from the Max Planck Institute of Molecular Cell Biology and Genetics found that the gene’s brain-growing ability likely evolved from a mutation that arose a mere 1.5 million to 500,000 years ago. 

“The mutation of a single genetic letter, namely the change from a C to a G, in the ARHGAP11B gene leads to the loss of 55 nucleotides in the formation of the corresponding messenger RNA,” explained Wieland Huttner, whose lab led all the studies on the gene mentioned in this article. “This results in a shift in the reading frame, which in turn leads to the human-specific, functionally essential sequence of 47 amino acids in the protein.” 

“Such point mutations occur relatively frequently, but in the case of ARHGAP11B its advantages of forming a bigger brain seem to have immediately influenced human evolution,” Huttner added

The researchers were also keen to point out that the marmoset fetuses featured in this experiment were dealt with using high ethical standards and were not brought to term. 

"To let them come to be born, in my opinion, would have been irresponsible as a first step," Huttner told Inverse. "because you don't know what kind of behavioral change you'll get."