The Uniquely Human and the Question of Chimeras

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. 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.

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.


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 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.

Experimental Insights about Chimeras

Experiments with interspecies chimeras have already yielded two critical insights. The first is that there are significant barriers to chimerizing different species. It is easy for science fiction to play up the possibility of monstrous part-human-part-animal creatures. But the science tells a different story. Chimeric experiments have emphasized that it is not so easy to incorporate cells from one species into the body of another—as the literature puts it, there are “interspecies barriers” to biological compatibility. Cells compete for survival and signaling dominance, develop on different timescales, play different roles, exist in different states of potentiality, utilize different molecular, electric, and mechanical signals, etc. For these reasons, it has been difficult to achieve significant levels of chimerism between all but the most closely related species, such as mice and rats. Human-animal chimeras produced have generally not survived past early embryonic stages.

Second, experiments suggest a graft-host paradigm of body plan dominance. Inside a mouse, scientists have generated a “rat pancreas” (i.e., a pancreas composed almost entirely of rat cells), yet the pancreas was still mouse-sized and functional in the mouse’s body. The inverse experiment has also been done, with a functional, rat-sized mouse-cell-derived pancreas developed in a rat’s body.

In a particularly striking blastocyst complementation experiment, scientists produced a mouse genetically incapable of developing its own gallbladder and injected rat stem cells into it. The chimera grew a mouse-sized gallbladder composed entirely of rat cells—even though rats do not have gallbladders. Some scientists hypothesize that the cellular microenvironment created by the mouse’s developing body influenced the rat stem cells to unlock a gallbladder developmental program that is normally suppressed in rat development.

The graft-host paradigm holds true at the microscopic level, as well. In an experiment published in 2021, human stem cells were inserted into very early-stage cynomolgus (macaque) monkey embryos (6 days post fertilization). The lab produced 132 of these interspecies chimeric embryos and allowed them to grow in specialized laboratory dishes until the last of the embryos died at 19 days after fertilization. Notably, the trajectories of gene expression in the cells of these chimeric embryos correlated more closely with the gene expression patterns expected of macaque embryos (0.673) than those expected of human embryos (0.460). Furthermore, donated human pluripotent stem cells, outnumbered and functionally dominated by native macaque embryonic stem cells within the “host” macaque embryo, tended to cluster with one another and did not easily survive. Those human cells that did survive even began to act more like macaque epiblast cells in their gene expression. Two prominent reviewers pointed out that it remains unclear whether the donor cells engrafted into host embryos are “progressing ‘autonomously’” or are cooperating with host tissues as a true whole. But in either case, these clues imply the dominance of “macaque-ness” in the chimeric embryo, to put it in metaphysical terms.

It is also worth mentioning that the dominant species in these experiments has also been the maternal species in chimeric animals brought to term. Scientific understanding is only beginning to illuminate the manifold contributions of the mother to the intricate and ongoing processes of development within the fetus itself. For example, there is new evidence that maternal microchimeric cellular contribution to the fetal brain (i.e., cells from the mother that cross the placenta and populate her child’s developing brain) influences early neuroimmune processes and the maturation of the fetus’ behavioral abilities. This burgeoning literature suggests a tight integration of purpose in maternal and fetal physiology, another highly refined system honed through the course of evolutionary history for stability of purpose.

All in all, chimeric experiments not only reveal biological barriers to integrating cells from different species but a propensity for cells (both native and donor!) to go on organizing according to specific, pre-established “blueprints” of the body, tried-and-true systems of structural and functional coherence. In all of these studies, the organization of the organism tends toward the stable bodily configurations that have been established through the long continuity of the evolutionary process. Whether because of cell number or type or role, viable chimeric animals have tended to follow a graft-host paradigm of “body plan” dominance in which the engrafted donor cells must “con-form”—emphasis on form!—to the basic typology of the host embryo species or else die.

Four Principles of Biological Ontology

A wider perspective can help to interpret these experimental results and enable an understanding of the moral status of a chimeric organism and an estimation of properties such as its mental powers. We ought to defend four ontological principles.

The first principle is that “natural kinds” of living things do exist. In particular, even at the biological level, humans can and should be distinguished from other kinds of animals. As chimeric experiments are showing, natural kinds are manifest in patterns of stable coherence of organismic structure and function. Our “species barriers” in the laboratory are the interwoven processes of development—genetic, epigenetic, mechanical, chemical, electric, etc.—that distinguish organisms from one another and unify their internal operations. At the level of mental experience, these biological constraints also consolidate the psychical possibilities of matter and constrain the mental characteristics (and thereby the moral status) of each species with its distinctive nature and body plan.

There have been many proposals to classify the moral status of chimeric organisms according to thresholds of mental capacities on a gradient of functional expression. A 2021 committee report by the National Academies of the Sciences, Engineering, and Medicine suggested that morally relevant functional capacities include the possession of pain, consciousness, and emotion and perhaps more complex abilities such as intentional thought, goal-directed behavior, problem-solving, and self-awareness. This is a reasonable starting point but needs to be extended, for things matter morally not simply because of what they do, but more fundamentally because of what they are. That is, the functional question is secondary to the ontological one.

Of course, the two are closely connected. Things behave according to the kind of things they are. In Aristotelean terms, doing follows being. We are therefore not misguided to ground our moral classifications in observations of functional powers. And, after all, when it comes to these new chimeric creatures, what else do we have to go on besides our observations of them? But we must remember that how things function is less important in and of itself than as a clue to what they are. “Beneath” function is “form.”

Biological structures and functions are not jumbled haphazardly. Structures and functions tend to collocate as interweaving features and processes of stable biological systems forged in the relation of environment, felt needs, and goal-oriented drive that is the life of each organism and its community. This is why Leon Kass can say in his foundational book The Hungry Soul, “Not philosophers but living nature is responsible for the existence of natural kinds, distinctively and recognizably formed after their kind.” Because of the complexity of biological life, natural species display nearly “omni-dimensional” evidence of species membership; everything about the “wild-type” organism—its genes, proteins, cell types, organs, morphology, behavior, etc. bespeaks its specific identity and way of life. Traits point us to real identities, including species categories. This approach allows us to reaffirm the classical notion of natural kinds without asserting an oversimplified means of discerning them.

The second important principle or insight is that the brain and body co-evolved and remain indispensably interdependent. The brain-body relationship unique to each species developed as a morphological and functional unity.

The third principle—which is closely related to the second, and is arguably a corollary of it—is that the organismic whole is the locus of mindedness. In other words, we experience with our whole body.

Though we colloquially call the brain the organ that “thinks,” it is more accurate for us to conceptualize the brain as the organ of sensation and movement (whether internal or external) while the body is the frame of reference by which the organism encounters and is encountered within the external world—and also desires and is desired within, knows and is known within that world. So the whole organism is the locus of integrated experience.

Sensation or “feeling” includes not only pain, proprioception, etc., but also emotion, which may be more difficult to appreciate at the physiological level for those not trained in the biological sciences. Neuroscientist Antonio Damasio, in his seminal book Descartes’ Error: Emotion, Reason, and the Human Brain, characterizes emotion as the brain’s participation in homeostasis. Homeostatic operations require the interplay of diverse systems: sensing pressure and chemical concentrations, producing and responding to hormonal signals, closing and opening blood vessels, triggering instincts or desires, etc. In its interface with these various physiological activities, the nervous system generates a “multidimensional mapping” of the organism’s interior. Coupled with the external senses, which contain information about objects of pain, pleasure, aversion, and appetite, the brain becomes the integrative organ of feeling or sensation.

Movement, another essential aspect of animal life, is also governed by the brain while still dependent on the musculoskeletal and other systems. Note that there is a dynamic relation between movement and feeling. Movement is guided and governed by feeling; feeling orients and urges movement. The brain’s participation in homeostasis—“feeling”—is meaningless without the ability to reach toward an object of desire or revolt from an object of dread. If feeling is the embedding of mind in the world, movement is mind’s expression in the world. Without both essential contextual aspects, which are the two basic pre-requisites for intentionality (and are lacking, for example, in “organoids” and artificial intelligence programs), mindedness does not make sense.

But brains alone are not sufficient for biological mindedness. Moving and feeling are functionally paired only in the life of an organism (or, perhaps, in its more sophisticated synthetic approximations). Phenomenologist Hans Jonas in The Phenomenon of Life: Toward a Philosophical Biology points out that moving and feeling bridge the “rift between subject and object” and thereby come to underlie the minds of animal organisms with their appetites, desires, fears, inclinations, habits, and responses. Yet moving and feeling are paired only in the organism with its complex brain-body unity.

Alongside the operation of the brain, the body frames the kinds of possibilities for sensation, movement, and mind. Consider the human form, which is peculiar among mammals for its uprightness. In his book Phenomenological Psychology, Erwin Strauss observes, “Upright posture pre-establishes a definite attitude toward the world; it is a specific mode of being-in-the-world.” Uprightness takes humanity from a nose-in-the-ground posture of sniffing or scrounging to a horizon of sight that bespeaks a comprehensive perspective, open possibility, allows for planning, and even sets a “contemplative” distance between the person and any object with which we interact. Strauss points out that in us, “Bite has become subordinated to sight . . . . To eyes looking straight forward—to the gaze of upright posture—things reveal themselves in their own nature. Sight penetrates depth; sight becomes insight . . . . Man in upright posture, his feet on the ground and his head uplifted, does not move in the line of his digestive axis; he moves in the direction of his vision.” This posture expresses a mind whose liberty is played out in the arena of more than mere appetite and aversion but also of understanding and creativity.

The human form is a mode of experience in the world that is entirely unmatched in the biological world. Jonas describes our upright posture as increasing the subject-object rift and therefore calling forth an intensification of the powers of moving and feeling to overcome it. Kass describes the result in this way:

The improved or newly developed and accompanying human powers for overcoming distance—including powers of seeing, imagining, speaking, understanding, pointing, grasping, embracing, handling, fingering, planning, making, and acting-at-a-distance — make possible, perhaps for the first time in the story of life, a new kind of world relation, one that admits of a knowing and accurate encounter with things, of genuine and articulate communion and meaningful action between living beings, and of conscious delight in the order and variety of the world’s many splendored forms—in short, a world-relation colored by a concern for the true, the good, and the beautiful.

These principles bring us to a final one: that human morphology (in all its variation, including the various types and degrees of disabilities) is normative for morally inviolable forms of embodied mindedness. Among biological systems known to exist, only the human body plan (again, in all its variations) has demonstrated the ability to support modes of interpreting and engaging the environment that allow the natures, purposes, and meanings of things to be understood. The culmination of these abilities is our capacity to recognize persons, to know the subject who is the object of our attention.

In you and me, sensation and movement are refracted through the prism of the human form, becoming understanding and creativity. But understanding and creativity fulfill the more basic animal powers of sensation and movement so perfectly as to transcend them and become part of a qualitatively new and unique mode of being in the world that involves freedom and intellectual abstraction.

Metaphor plays an important role in the transfiguration of feeling and motion to intellection and creation. Our language is grounded in bodily metaphors that allow us to leap from particular experiences, memories, imaginations to the abstractions of thought that allow us to piece together coherent stories and develop meaningful relationships with others.

This kind of mindedness has only been achieved in the human form, and perhaps only the human form (or something very similar) can sustain it. In the biological world, diverse body plans often contain analogous parts for similar functions (e.g., the compound eye for sight or the wing for flight), suggesting that “optimal solutions” to biological problems exist and that biology has a reliable tendency to achieve those solutions. That there appears to be no organism on earth whose mindedness is analogous to ours suggests that the human form may alone be the “optimal solution” for coordinated comprehension and flexibility of intention.

From Theory to Practice

This discussion gives rise to at least two guidelines for human-animal chimera research while also raising several questions for further inquiry. (Note that the argument in this paper does not take up the question of whether a fully human cell-derived brain in a chimeric animal could “represent an offense against the dignity of human beings on account of the admixture of human and animal [developmental] elements capable of disrupting the specific identity of man,” a concern about interspecies hybridization by nuclear transfer raised in the Congregation for the Doctrine of the Faith’s instruction Dignitas Personae, §33.)

First, if human-animal chimeras are produced, the “body plan” of the non-human species must be clearly dominant. “Body plan” refers to a stable, species-characteristic developmental, morphological, and physiological system. (This should include a restriction against gestating human-animal chimeras in human mothers, at least because of the growing evidence for the influence of maternal biology on developmental processes if not also for more weighty and central considerations about human dignity and distinctiveness.)

Second, there should be no production of neural chimeras between humans and non-human primates, including embryonic chimeras in which human cells may contribute to the developing non-human primate brain and its developmental progenitors. At this point, this would be a prudential restriction, not necessarily a principled one. It is not clear that human-non-human-primate chimeras would definitely cross an ontological threshold. After all, to achieve stable, “healthy” neural chimerism with human stem cells is difficult in any non-human species, including primates. Beyond the embryonic stage, the operation of the extant human neurons or glia—if any survive to that point—would be heavily influenced by the other species’ cranial microenvironment—anatomical structures, molecular signals, electrical and mechanical processes, etc.—not to mention the processes by which synaptic connections are pruned, promoted, and positioned through the developing organism’s exercise of its sensory and motor abilities.

Yet at the same time, it may be impossible at this point to “guarantee” that any one human-primate neurologic chimera would not demonstrate the higher-order development of mind that we exhibit. We do not yet understand the morally relevant biological dividing lines between humans, our modern primate relatives, and our common ancestor. We should not risk recapitulating those key genetic or structural turning points in evolution by tinkering with human-primate chimeras. (Leaving aside the fact that much of the public also finds this idea repulsive to begin with.)

Finally, there are several questions, both scientific and scholarly, that should be pursued in future research:  Can we define what constellation(s) of observable biological properties are capable of supporting an organismic existence that is specifically human? Which species-specific traits and abilities are morally relevant and why? Which bodily and neurological features of humanity support these morally relevant traits and abilities? Which neural structures and properties undergird human-specific thought processes, especially language, and what are the mechanisms by which they act? What characterizes the mental and emotional life of fully non-human interspecies neural chimeras created by blastocyst complementation (i.e., host species neural knockout and non-human graft species unopposed neural proliferation)?

The wide scope of such questions makes it clear that we need an anthropological inquiry that is interdisciplinary, involving biology, psychology, philosophy, theology, and more.

EDITORIAL NOTE: This article is part of a collaboration with the Society of Catholic Scientists (click here to read about becoming a member). A version of this article with extensive footnotes is available here

Featured Image: Image by Dean Moriarty from Pixabay


Dillon Stull

Dillon Stull is a medical student at Stanford University.

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