Above: “Two Composite Elephants,” 18th century, unknown author.
DILLON STULL
Is a graduating medical student at Stanford University.
Human-Animal Chimeras: considered physically, mentally and morally
In the last ten years, scientists have placed various components of the human brain into animals of other species, producing mice with human brain cell grafts, rhesus macaques with a key human neurodevelopmental gene MCPH1, and most recently cynomolgus monkey embryos with human stem cells inserted into them.1 The mice with human neural grafts outperformed normal lab mice in learned maze navigation; the genetically modified macaques showed improved reaction time and short-term recall; and the human-monkey embryos demonstrated cooperation between the human and monkey cell types.
These experiments are raising important questions. Before the age of biotechnology, mind found its expression in naturally evolved organic bodies. But now, we are creating new bodies that did not evolve as coordinated systems. How can we discern the moral significance of these new creatures? As developing individuals of one species receive the cells or genes of another, what kind of being results? Could the transfer of neurologically significant human genes, cells, or tissues into a non-human animal cause an animal to be ‘humanized’? What is the moral status of such beings?
Clarity on the morality of these experiments calls us to return to the perennial questions of metaphysics, epistemology, and ethics with increasing urgency. Likewise, the biological (and therefore moral) significance of transplanted bodily components (such as genes, cells, or tissue grafts) can only be understood in light of their context within the whole organism and its environment. It is essential to seek a broader vision of organismic unity and complexity in order to estimate the mental properties that can be plausibly attributed to a biological entity, especially a novel or ambiguous one such as an interspecies neural chimera.thought and mind of man strengthening the power of vision.”
This article will define what chimeras are and note a few key themes in experimental observations of them. Then, drawing on Aristotelian metaphysics and existential phenomenology, it will delineate four principles that are important for understanding the ontological implications of these chimeric experiments. In the end, it will suggest two guidelines for human-animal chimera research and propose questions for further inquiry, both experimental and philosophical.
DEFINITIONS
By the word ‘chimera’ I will mean an organism produced by combining cells or tissues from at least two distinct organisms. An interspecies chimera is formed when the cells or tissues combined to produce the one chimeric organism had been originally derived from organisms belonging to different species. Note that chimera is a broad term, including not only mice with human neural tissue grafts but also human patients with porcine heart valves — it also includes most mothers, whose bodies and brains, even decades later, have been found to contain fetal cells that crossed the placental barrier during their pregnancies!
Important for our consideration is that chimeras can be created at embryonic stages. A common technique is to inject pluripotent stem cells (not necessarily embryonic stem cells but potentially) from one organism into the blastocyst (a very early embryonic stage in mammals, prior to implantation) of another organism.
This technique for producing embryonic chimeras can also be coupled with gene editing. Scientists can inhibit specific developmental ‘programs,’ such as the generation of a pancreas, by ‘knocking out’ the genes that control those processes. This creates a developmental ‘niche’ or functional vacancy in the embryo, which the injected donor stem cells are able to occupy. The chimeric organism would then have an organ derived almost entirely from donor cells. This technique is called ‘blastocyst complementation.’
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