Scientists say they have grown part-human, part monkey embryos in lab Petri dishes. What could possibly go wrong?

We may not be looking at the Planet of the Apes (yet), but this experiment is uncomfortably bizarre and certainly raises some ethical questions.

Scientists injected dozens of human stem cells into developing monkey embryos, and the resulting hybrids survived for up to 20 days in lab dishes.

While it sounds like something straight out of a Sci-Fi movie, this is not the first time scientists have created such “chimeras.” A chimera is a single organism that contains the DNA of at least two different distinct species. In the past, scientists have attempted to incorporate human stem cells into pig and sheep embryos, with the eventual goal of growing human organs in the livestock for use in transplant surgeries. But very few human cells survived the experiments.

None have lived as long as the “monkey-human” chimeras in this experiment. The researchers involved say that these human-monkey embryos could someday serve as helpful models for human disease, embryonic development, and aging. By zooming in on the interaction of human and animal cells in the embryos, scientists could also learn how to help human cells survive amongst animal cells, potentially advancing the effort to grow human organs in living animal models.

The potential for what this kind of research can yield aside, it raises some astronomical ethical concerns, especially in regard to how long such embryos should be allowed to develop. “One of the main concerns with human-animal chimeras is whether ‘humanization’ of the chimeras will occur, for example, whether such chimeras acquire human-like cognition,” said Alejandro De Los Angeles, a stem cell biologist at the Yale University School of Medicine, who was not involved in the study.

However, these concerns don’t necessarily apply to the new experiment, given that the embryos were only allowed to develop for a limited time and weren’t implanted in a uterus, he said to the journal Live Science. But for future studies, “it will be important to discuss how long experiments should be allowed to go for,” he said.

When thinking about the future of chimeric embryos, scientific ethics must enter the discussion, Henry Greely, director of Stanford’s Center for Law and the Biosciences, and Nita Farahany, director of Duke’s Initiative for Science & Society, wrote in a commentary of the new study. These embryos, by their nature, raise different concerns than the direct transplantation of human cells and tissues into animals, they wrote.

With direct transplantation, researchers can predict how transplanted tissues will integrate into the larger animal and can determine whether a transplant might enter an area of concern, such as the brain and gonads, they wrote. But in a chimeric embryo, human cells can potentially become incorporated throughout the entire organism, assuming the embryo is allowed to develop for some time.

In the new experiments, “the embryos here were not transferred into a uterus, and thus could not lead to living chimeric animals or even fetuses,” and growing full-fledged chimeras in a womb is not the long-term goal of this research, they noted.

“Implantation of human-monkey embryos would be ethically contentious and will need to be discussed by scientists, ethicists, and the public before moving forward with such experiments,” De Los Angeles said.

There are no current plans to do so – implant a human-monkey chimera embryo into a human or primate uterus and allow it to develop into… what?

But there is a lot of history of scientists doing something because it could be done, without questioning if it SHOULD be done, and it is only a matter of time before someone, sanctioned or not, takes this next bold step.

We can only hope that the first human-monkey hybrid born is not named “Caesar.”