Animal-Human hybrid in the making 

After the development of human - monkey in China, the Japanese government has approved stem cell experiments to create animal-human hybrid disregarding the species barrier and other ethical issues.


A Chimera is an organism that combines cells of two or more individuals or from different species. Chimeras can potentially be used to address many issues in the field of organ transplantation and rejection. Without using human subjects, scientists are creating animals with human cells to monitor and track cell differentiation, tissue development and organ formation. 

But the research with chimeras poses scientific, religious, and ethical problems. There are scientific concerns with regards to public health consequences of the spreading of disease between species.


A Japanese stem – cell scientist, Hiromitsu Nakauchi has received the government’s support to create human-animal chimera embryos, which are created by implanting human pluripotent stem cells (iPSCs) into animals in early development. In March 2019, Japan had lifted the ban on allowing such embryos to develop beyond 14 days or the transplant of such embryos into a surrogate uterus.  

Nakauchi leads a team at the University of Tokyo and Standford University in California and has previously grown human cells in mouse and rat embryos. His ultimate goal is to produce animals with organs made of human cells that can be transplanted into people. Nakauchi also plans to apply for government approval to grow hybrid embryos in pigs for up to 70 days.

Nakauchi moved to the US in 2013, where no federal law restricts the creation of chimeras. He received grants from the California Institute of Regenerative Medicine, a state agency set up to by-pass political interference from Washington. 

Likewise,  a team of researchers led by Prof. Juan Carlos from Salk Insititute in the US have produced monkey – human chimeras. To avoid any legal issues, the research was conducted in China. Previously, Prof.Carlos and scientists injected human stem cells into 2000 pig embryos, but the effort was not successful. 

The chimera research has been opposed by many religious groups. Judaism, Islam and sects of Christianity have opposed the chimeric research involving many lower animals, especially pigs. Hindus disapprove the chimeric research involving cows, the sacred animals that are to be treated with the same respect “as one’s mother.” In 2005, the Pontifical Council for Health stated the Catholic Church’s stance on the subject, saying that human genes “embody the characteristic uniqueness of the person, which medicine is bound to protect,” an opinion echoed by the council of the bishops of England and Wales in 2007

Incubating organs into animals can be ethically charged because adding human cells to animal embryos, could blur the line between species in many ways. The ethical groups are worried about the prospect of human cells contributing to animal brains, or in germ cells that lead to progeny—much more so than if they were only present in heart, blood, or liver tissue.


Prof Robin Lovell-Badge, a developmental biologist from London’s Francis Crick Institute, said, “I don’t think it particularly concerns in terms of the ethics because you are not taking them far enough to have a nervous system or develop in any way – it’s just really a ball of cells,” he said. He noted that if chimeras were allowed to develop further, it could raise concerns. “How do you restrict the contribution of the human cells just to the organ that you want to make?” he said. “If that is a pancreas or a heart or something, or kidney, then that is fine if you manage to do that. [But] if you allow these animals to go all the way through and be born if you have a big contribution to the central nervous system from the human cells, then that obviously becomes a concern.”

To counter these concerns, Prof. Nakauchi has proposed the use of “committed progenitor cells” which, unlike pluripotent stem cells, have already started developing and therefore lost some versatility. He has also suggested scrubbing the human cells of their immune properties before using them to produce “universal organs” compatible with any immune system.


  • For years, scientists have been trying to convert stem cells growing in Petri dishes to fully functional adult cells. Also, laboratory animals are routinely used to model human biology and disease. But they are not human and therefore cannot fully replicate human physiology. Chimeras with human tissues offer a reasonably accurate substitute for a real human body and are likely to help in the study of disease, provide a safer and more ethical method for testing of new drugs. For example, pieces of human tumours are grafted under the skin of mice providing a bio incubator for tumour growth. The mouse essentially becomes a cancer patient whose tumour can then be manipulated in various ways to understand cancer mechanisms and to test therapeutic protocols for human cancer.
  • The creation of chimeras constitutes an unnatural breach of the species barrier. It is more likely a backward evolution and could be breaching the evolutionary lineage.
  • There is no societal framework yet for part humans and part animals. Also, the restructuring could cause humans to lose their coveted position at the top of all life.
  • A humanised animal, if it attains the attributes of personhood i.e. the capacity to develop into self-conscious rational being, then it is entitled to the same dispensations as humans. This means it cannot be used for medical experiments.

Image Courtesy: Public Domain