The addition does seem to have made the monkeys smarter. The transgenic animals' brains took longer to develop - more like those of human children - and they also exhibited better memory skills, and faster reaction times, compared to their unmodified peers.
"This was the first attempt to understand the evolution of human cognition using a transgenic monkey model," geneticist Bing Su of the Kunming Institute of Zoology told Technology Review.
Transgenic organisms are nothing new. The first was published in 1974, when Staphylococcus aureus genes were spliced into Escherichia coli. The first transgenic monkey, inserted with jellyfish genes, was created in 2001.
Human genes have been added to monkeys to study diseases and conditions such as autism, and mice have been modified with human cognition genes, including altered microcephalin. But the researchers believe that this is the first time researchers have used transgenic monkeys to look into the genetic origins of the human brain.
It is, scientists say, an experiment with concerning ethical implications.
The team exposed the monkey embryos to a virus carrying human microcephalin. This generated 11 transgenic rhesus monkeys carrying the human gene, only five of whom actually survived.
"Our findings demonstrated that transgenic nonhuman primates (excluding ape species) have the potential to provide important - and potentially unique - insights into basic questions of what actually makes humans unique, as well as into disorders and clinically relevant phenotypes," the researchers wrote in their paper.
But not everyone agrees. In fact, a 2010 paper expressly condemns the entire concept of editing apes with human brain genes (although not necessarily monkeys), calling such potential studies "ethically unacceptable" due to the elevated risk of harm to the animals.
But using monkeys could be a step down that path.
"The use of transgenic monkeys to study human genes linked to brain evolution is a very risky road to take," geneticist James Sikela of the University of Colorado, who co-authored that 2010 paper, told Technology Review.
"It is a classic slippery slope issue and one that we can expect to recur as this type of research is pursued."
In addition, one of the researchers of this latest study, computer scientist Martin Styner of the University of North Carolina, noted that there were aspects of the study that would not be allowed in a country with stricter regulations, such as the US. In fact, the research was unable to find a publisher in the West.
Chinese genetic research is already being side-eyed after the work of geneticist He Jiankui, who claimed to have edited the germline of human twins. His American collaborator, Michael Deem of Rice University, has also come under fire.
It's difficult to know whether Su's new research would receive the same reception were it not under Jiankui's shadow, but the geneticist is not letting it slow him down. He is already at work making new transgenic monkeys.
But Styner said he considered taking his name off the paper.
"Now we have created this animal which is different than it is supposed to be. When we do experiments, we have to have a good understanding of what we are trying to learn, to help society, and that is not the case here," he said.
"They are trying to understand brain development. And I don't think they are getting there."
The research has been published in National Science Review.
