A pair of identical long-tailed macaques were recently born in China—two weeks apart. Named Zhong Zhong and Hua Hua (after the Chinese word “Zhōnghuá,” which means Chinese Nation), these little monkeys are certainly not your average twins. As Ben Hirschler of Reuters reports, they are the product of a ground-breaking experiment, which has, for the first time ever, successfully cloned primates using non-embryonic cells.

Researchers at the Chinese Academy of Sciences Institute of Neuroscience in Shanghai relied on a process known as somatic cell nuclear transfer (SCNT), which has been used to clone several mammals, including Dolly the sheep. SCNT involves removing the nucleus from the egg cell of one individual, and replacing it with the nucleus of a differentiated body cell from another individual. The reconstructed egg, which is implanted into a third individual, develops into a clone of the individual that donated the replacement nucleus.

Back in 1999, scientists cloned a rhesus monkey named Tetra using a method called embryo-splitting, which is easier to execute than SCNT but cannot generate as many offspring. Until the most recent experiment, however, all attempts to clone primates with SCNT had failed. The Chinese team also ran into many stumbling blocks, but eventually figured out a way to introduce modulators that would switch on or off genes that were inhibiting embryo development.

As Nicola Davis of the Guardian explains, researchers tried the technique on two different sources of nuclei: cells from an aborted macaque fetus, and cells surrounding the eggs of an adult macaque. They implanted 181 embryos derived from the adult macaque cells into 42 surrogates, and two live babies were born—but they died almost immediately. The team had more success with the 79 embryos produced from fetal cells, which were implanted into 21 surrogates. Again, only two babies were born. But this time, they survived.

The results of the remarkable experiment were published Wednesday in the journal Cell. “We tried several different methods but only one worked,” Qiang Sun, senior author of the study and Director of the Nonhuman Primate Research Facility at the Chinese Academy of Sciences Institute of Neuroscience, says in a statement. “There was much failure before we found a way to successfully clone a monkey.”

The primary goal of the new study was to advance medical research. Having access to genetically identical animals can help scientists better understand the mechanisms of certain diseases. It also eliminates questions about genetic variability that arise when testing new drugs or therapies on animals.

“You can produce cloned monkeys with the same genetic background except the gene you manipulated,” Sun says in the statement. “This will generate real models not just for genetically based brain diseases, but also cancer, immune or metabolic disorders, and allow us to test the efficacy of the drugs for these conditions before clinical use.”

But some experts have cast doubt on the value of the new research. Speaking to Hirschler of Reuters, Robin Lovell-Badge, a cloning expert at the Francis Crick Institute in London, notes that the experiment boasted a very low success rate; only two live babies resulted from the implantation of more than 100 embryos.

“It remains a very inefficient and hazardous procedure,” he says.

Unsurprisingly, the experiment has also drudged up questions about the ethics of animal cloning and animal testing, both highly contentious fields of research. “It gives this sense that animals are disposable and commodities for us to use,” Kathleen Conlee, vice president of animal research issues at the Humane Society of the United States, tells Michael Greshko of National Geographic. “Is this appropriate, to have an animal you can do whatever you want to?”

Another vital and equally thorny question remains: What does this new experiment mean for the cloning of humans?

“There is now no barrier for cloning primate species, thus cloning humans is closer to reality,” Mu-ming Poo, co-author of the research and director of the Chinese Academy of Sciences’ Institute of Neuroscience, explains in an interview with the Guardian’s Davis. “However, our research purpose is entirely for producing non-human primate models for human diseases; we absolutely have no intention, and society will not permit, this work to be extended to humans.”