Human intelligence is one of evolution's most momentous innovations. It is the result of a race that began millions of years ago, leading to ever bigger brains and new abilities. Eventually, humans stood upright, made tools, and created civilization, while our primate cousins remained in the trees, left behind on the evolutionary curve.

However, in a controversial move, Chinese scientists have used gene-editing techniques to insert a human brain version of a gene known as 'MCPH1' into, rhesus macaques, a breed of monkeys. The gene made the monkeys' brains develop along a more human-like timeline. The results were that the monkeys became smarter, had better memories and were more human-like compared to an unaltered specimen.

"This was the first attempt to understand the evolution of human cognition using a transgenic monkey model," says Bing Su, the geneticist at the Kunming Institute of Zoology who led the effort. Not only were the monkeys subjected to gene-editing, they were also cloned. The team successfully created 11 transgenic rhesus monkeys carrying the gene. They said that a transgenic monkey model is practical and to a large extent can imitate the human-specific status.

However, several scientists also raised concerns regarding the technique, some even calling it 'reckless'. Geneticist James Sikela told one news source, "The use of transgenic monkeys to study human genes linked to brain evolution is a very risky road to take. It is a classic slippery slope issue and one that we can expect to recur as this type of research is pursued." He is concerned that the experiment shows disregard for the animals and will soon lead to more extreme modifications.

Research using primates is fairly difficult in Europe and the US, but China has rushed to apply the latest high-tech DNA tools to the animals. The country was first to create monkeys altered with the gene-editing tool CRISPR, and this January a Chinese institute announced it had produced a half-dozen clones of a monkey with a severe mental disturbance.

Su's monkeys have raised some extraordinary questions about animal rights. In 2010, Sikela and three colleagues wrote a paper called "The ethics of using transgenic non-human primates to study what makes us human," in which they concluded that human brain genes should never be added to apes, such as chimpanzees, because they are "too similar to us,".

"You just go to the Planet of the Apes immediately in the popular imagination," says Jacqueline Glover, a University of Colorado bioethicist who was also an author of Sikela's paper. "To humanize them is to cause harm. Where would they live and what would they do? Do not create a being that can't have a meaningful life in any context."

Su agrees that the small number of animals was a limitation. He says he has a solution, though. Meanwhile, he is making more of the monkeys and is also testing new brain evolution genes.