Scientists in China have placed human genes into the brains of monkeys, and some are calling the experiment an "ethical nightmare".

The research was led by the Kunming Institute of Zoology in southwest China and published in the National Science Review, a Beijing-based journal.

Researchers used MCPH1, a gene which they think may play an important part in human brain development.  Of the eleven rhesus macaque monkeys used in the experiment, six died.  The remaining five - referred to as "transgenic" - underwent a series of examinations that included MRI brain scans and memory tests. 

"You just go to the Planet of the Apes immediately in the popular imagination," said Dr. Jacqueline Glover, a bioethicist at the University of Colorado.  "To humanise 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."

The five transgenic monkeys performed better on a short-term memory test and had shorter reaction times when compared to normal monkeys; their brains also took longer to develop and had a similar development pattern to that of human brains.  And while the Chinese scientists claim they were trying to learn more about human brain development, many others don't buy that.

"The costs are terribly high and the benefits to humanity approach zero; there's growing recognition that animal models simply don't work well to study complex human processes," wrote Barbara J. King, author of How Animals Grieve and an emerita professor of anthropology at the College of William and Mary in an email to Vox media.