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However, in a new book on the science and ethics of chimeras, Yale University researchers say it’s time to explore, cautiously, the creation of human-monkey chimeras.

“The search for a better animal model to stimulate human disease has been a ‘holy grail’ of biomedical research for decades,” they wrote in Chimera Research: Ethics and Protocols.

“Realizing the promise of human-monkey chimera research in an ethically and scientifically appropriate manner will require a co-ordinated approach.”

For me, crossing into the central nervous system, with any species, is probably crossing the line

To some, that may seem all fine and good. However, some scientists aren’t waiting for the ethics to be worked out.

In April, Chinese researchers announced they had inserted a human brain gene into monkey embryos, a gene critical for human brain development.The experiment provoked revulsion, and fascination. Ethicists and philosophers condemned it as hugely morally risky, while the scientists behind the work are said to be keen on implanting even more human genes into monkey embryos, including one presumed to play a role in human intelligence.

Chimeras were once only monsters out of Greek mythology. Today, stem-cell technologies and gene-editing tools like CRISPR may make the generation of these once fantastical creatures possible.

In fact, scientists are already using “xenotransplantation” techniques to create human-pig chimeras, in the hope of one day generating human organs for transplant.

The approach involves using pluripotent human stem cells, cells that have the ability to morph into virtually any type of cell in the body. If the technical hurdles can be overcome, the idea is to block or delete genes critical for the development of a given organ (a pancreas, say) in a fertilized pig egg, and then inject human stem cells into the embryo, to fill the empty gap or niche, and grow the missing pancreas.