If Monkeys Could Talk

May 13, 2021 • Posted in Blog

Joyce A. Shelton, Ph.D.
Professor of Biology, Trinity International University
Guest Column

Recent news announcements proclaimed, with both excitement and alarm, that Tan and colleagues, scientists from China and the US, had successfully produced human-monkey hybrid embryos. (1, 2) The hybrids (also termed chimeras) were made by injecting human pluripotent stem cells from an induced pluripotent stem cell line into 132 early- stage monkey embryos. Human pluripotent stem cells have the capacity to develop into a range of tissues and cell types which ultimately form all the structures of the human body including the brain and reproductive cells (sperm and egg). (3) In these human-monkey hybrids, according to Juan Carlos Izpisua Belmonte, one of the US collaborators, “every embryo contained human cells that proliferate and differentiate to a different extent” (2). The embryos were allowed to develop in culture for up to 19 days, several days beyond the internationally recognized 14 day limit when neural tissue begins to form (4). The researchers were not able to control which tissue types in the human-monkey hybrids were populated by the human cells (3).

To pronounce this research controversial is a hefty understatement. Scientists, philosophers, and the general public have long voiced and are now renewing ethical concerns about the mixing of human stem cells and animal (especially primate) embryonic cells (2, 5). Tan and colleagues claim that they followed existing ethical guidelines, sought external bioethics consultations to guide their research, only conducted ex vivo experiments on early stage embryos and never intended for embryos to be developed to term (3). The reality is that, in the US and China, there are very few existing guidelines and they are nonbinding (4). The National Institutes of Health requirements for funding human stem cell research include a panel review of projects that introduce human cells into early or late stage embryos that could change an animal’s brain functions. They prohibit breeding human-animal hybrids and restrict putting human stem cells into early-stage primate embryos like monkeys (6). It seems several of these ethical boundaries were breached by this new research.

Tan and colleagues have also been quick to deem their goals praiseworthy (2). They seek to advance understanding of developmental processes and, ultimately, to utilize human-animal hybrids for the production of human-compatible organs for transplant, the overarching goal of regenerative medicine. Production of hybrids for organ transplant would certainly require crossing the barrier of bringing the hybrid embryos to full gestational term.

Many ethical concerns surround the production of human-primate hybrids, but one commonly held is that the humanized monkeys might have a sufficient number of human-derived brain cells to develop human cognitive capacities such as reasoning, self-awareness, or even moral consciousness. The question is whether they would be human enough to justify an increase in moral status (7). Such an increase in moral status should afford them enhanced consideration as research subjects.

As Françoise Baylis, a bioethicist at Dalhousie University in Canada, suggests “there are only two types of research subject, human and non-human, and there are clear distinctions on how to treat them. With chimeras, researchers risk creating a third category for which there are no research guidelines. We just tend to say we’ll treat them like non-human animals, as if nothing happened, but if they are human then, they deserve to be given the rights and privileges of a human being” (7). The treatment of research animals is governed by animal welfare regulations, which are much less stringent than requirements for human research subjects (8). The rights of the human research subjects, in the US, include informed voluntary consent and the right to withdraw from a research study. Those with diminished autonomy are entitled to protection and exclusion. Research subjects cannot be exploited just because they are available or malleable (9).

Since they can’t tell us what they think, we can only speculate on whether the human-monkeys feel pain, unhappiness, or helplessness at being used as a means to our end. Perhaps they don’t enjoy being involuntarily forced to be a research subject or, even worse, an organ factory, sacrificing their lives for the real human beings. We can conveniently ignore their unheard opinions because they are not capable of speaking to us, at least for now. Research by W.T. Fitch et al. shows that, contrary to prior assumptions, nonhuman primate vocal cord anatomy does not prevent speech. They conclude that “if a macaque monkey had a brain capable of vocal learning…its vocal tract would be able to produce clearly intelligible speech”(10). Perchance, the potential for vocal testimony from our human-monkey “creations” will finally motivate us to face the ethical issues we have thus far ignored. Meanwhile, this perilous research continues to advance apace.

The Tennessee Center for Bioethics & Culture encourages respectful discussion and debate of bioethics issues, and strongly supports freedom of speech. To that end, we invite and welcome other voices to the discussion of bioethics issues. Invited authors’ views are their own, and do not necessarily represent those of The Tennessee Center for Bioethics & Culture.


1. Briggs, H. (2021, April 15). Human cells grown in monkey embryos spark ethical debate. BBC News.

2. Subbaraman, N. (2021). First monkey–human embryos reignite debate over hybrid animals. Nature 592,497.

3. Tan, T. & 24 authors (2021). Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo. Cell, 184(8), 2020-2032.

4. Hurlbut J.B. (2017). Experiments in Democracy: Human Embryo Research and the Politics of Bioethics. Columbia University Press, NY, USA.

5. Robson, D. (2017, January). The birth of the half human half animal chimeras. BBC News.

6. National Institutes of Health (2016, August 4). Request for Public Comment on the Proposed Changes to the NIH Guidelines for Human Stem Cell Research and the Proposed Scope of an NIH Steering Committee’s Consideration of Certain Human-Animal Chimera Research.

7. Levine, S. & Grabel, L. (2017, October). The contribution of human/non-human animal chimeras to stem cell research. Science Direct; 24,128.

8. National Academy of Sciences (2004). Regulation of Animal Research (in Science, Medicine and Animals). National Academies Press.

9. Federal Policy for the Protection of Human Subjects Final Rule. (2017, January 19). Federal Register 82(12), 7149-7274.

10. Fitch, W.T., deBoer, B., Mathur, N., & Ghanzanfar, A.A., (2016). Monkey vocal tracts are speech-ready. Science Advances 2(209), e1600723.