Having a child with a serious birth defect, particularly one affecting the brain, is a devastating experience. In such situations, parents often look to their physicians for moral guidance. Yet there is considerable debate among medical professionals and ethicists regarding the status of anencephalic infants, or infants with severely malformed or absent brains. The conflicted reaction of many to a recent case of an Oklahoma couple, who elected to bring their anencephalic child to birth so that the child’s organs could be donated, highlights the difficulty we have in thinking about such situations.

Part of the difficulty is that we naturally link our sense of self to our memories, thoughts, and choices. The capacity for rational thought is a defining feature of human beings, and mental functions such as reason and language require sophisticated neural circuitry. Consequently, the moral status of an embryo that fails to produce a brain capable of rational thought is not clear to many. Such an individual can be viewed in at least three different ways; as a defective human embryo, as a non-human embryo, or as something that is not an embryo at all.

The last possibility is the most easily dismissed. Clearly an anencephalic embryo is not a mere aggregate of living human cells; it manifests holistic integration of parts, well beyond simple cellular organization. Despite the absence of a functioning brain, the embryo will, in most cases, build a wide range of interactive systems (circulatory, respiratory, digestive, urinary, etc.) that collectively serve to promote the health and viability of the embryo as a whole. It is an organism, not a tumor.

Although an anencephalic embryo is clearly an organism, it is also clearly not undergoing normal human development. While much of the nervous system develops as it should, the regions of the brain responsible for language, memory, and decision-making are severely disorganized or absent. Thus, the embryo possesses the capability to form brain structures suitable for rationality, yet does so imperfectly. Consequently, the most reasonable view is that such an individual is a human embryo with a disordered part: a defective human embryo, rather than a non-human embryo.

Yet in severe cases of anencephaly, the brain is entirely absent, and thus the embryo would seem to lack the capacity for rational thought, a defining feature of human beings. If the conclusion that anencephalic embryos are human rests on the fact that some neural tissue forms in such cases, what if it were possible to engineer a human embryo that didn’t just produce a dysfunctional brain, but rather was utterly incapable of producing a brain at all? Put slightly differently, would it be theoretically possible to produce a novel, non-human organism that is nonetheless of human origin and that possesses an intact human genome, but is intrinsically not a “rational animal” because it entirely lacks the capacity to produce a brain capable of rational thought?

Given how human development works, this is a surprisingly difficult question to answer. Embryos do not have discrete “brain genes” (much less “rational thought genes”) that can simply be removed, while allowing the rest of development to proceed normally. Multiple embryonic structures interact to generate a normal brain. Indeed, the interdependence of diverse parts in the production of the body as a whole strongly reflects the organismal character of development. Moreover, many genes that are required for brain formation are also required for the formation of other systems, and therefore removing them has widespread and often fatal effects. In animal models, some gene deletions (Dkk1, Noggin, Wnt3) result in severe truncation of the cranial-caudal axis—in other words, deletion of the head—yet because the rest of the nervous system forms normally, this seems entirely analogous to naturally occurring anencephaly; that is, a defect in a part of an otherwise normal embryo.

Thus, while selective gene removal could produce an embryo that forms a disorganized or incomplete nervous system, it is likely to be impossible to produce an otherwise functional organism that entirely lacks the capacity to produce a brain. While one can certainly imagine an embryo that is incapable of brain formation, this is not an informative thought experiment. Purely imaginary organisms that do not respect the laws of biology can have any properties you wish and provide no insight into actual human embryos.


A more informative (albeit more fantastical) thought experiment is to sidestep the requirements of human development, and instead posit a hypothetical “zombie” transformation, in which the body of a once-living human is reanimated in a non-rational state. The zombie literary genre includes many scenarios that do not address the possibility of non-rational human animals, simply because they are based on biologically ridiculous premises; e.g., walking skeletons; zombies that move and eat but don’t require oxygen to survive, etc. Equally uninformative are apocalyptic scenarios that involve a devastating (yet potentially curable) human disease; for example, infection of living humans with a pathologic agent that induces an irrational, bloodthirsty state. Yet, one can posit a biologically reasonable, and therefore potentially informative, reanimation of a human corpse to generate a non-rational human organism, as follows.

Suppose a living human being becomes infected by a pathologic agent (a virus, bacteria or parasite) that causes the zombie transformation only after death (perhaps due to a temperature-sensitive activation of the pathogen). The human individual dies and remains dead for a long period of time (hours). Death would be clearly diagnosed by cessation of either cardio/pulmonary function or brain function. Thus, it would appear that the original human being has ceased to be and is not merely experiencing a catatonic disease state.

It is well established that some cells within a human corpse remain alive for a long period of time (hours to days) after cessation of brain activity or heartbeat. And the hypothetical zombie transformation would occur after death of the human being, while cellular life still persists in the corpse. During this interval, the pathogen would transform cells that remain alive, giving them new properties that are characteristic of a zombie (lower oxygen requirements, slower metabolic state, rapid wound healing, etc.).

It is also well established that different parts of the brain have different sensitivities to loss of oxygen, with structures responsible for thinking, memory, and coordinated behavior (the cortex, hippocampus, and cerebellum) likely to be the first to die once blood circulation ceases. Consequently, it is reasonable to posit that our hypothetical zombie pathogen would be able to maintain sub-cortical brain networks responsible for some elements of vision, hearing, olfaction and aggression, while higher brain networks responsible for reason, memory, language, and decision-making would be lost.

For the corpse to be reanimated, the altered cellular activity promoted by the pathogen would have to be sufficient to somehow “revive” minimal vital functions like heartbeat, respiration, and basic sensory/motor activity. This is, of course, not a property of any known pathogen, yet if such a reanimation took place, it would generate an aggressive, uncoordinated, non-rational organism, human in its origins and very resistant to being killed: a zombie.

What Have We Made?

This scenario raises a number of interesting philosophical questions. Is a zombie thus described a severely impaired human being, or a non-human? Brain and cardiac function were clearly not “irreversibly” lost, which raises the question whether the original human was actually dead in the first place. Is the dead human “reborn” but in a severely compromised state, or has a substantial change occurred, generating a new entity? And if the zombie is a new entity, is it a new human being or an entirely new kind of creature?

The zombie thus described is clearly a living being (an organism) of some sort; it consists of parts that function together for the support of the entity as a whole. Moreover, it would be an organism that is, in some sense, “human.” It would be genetically similar or identical (depending on the nature of the transforming pathogen) to a once living human. It would retain structural and functional components of that once living human. The key question is: what kind of soul does the zombie possess, a human, rational soul or a merely animal soul?

The most reasonable interpretation of this scenario is that the original human has died and is not resurrected. Although modern technology can, in some cases, “reanimate” individuals who in the past would have been considered dead (by shocking the heart, for example), this always involves reactivation of natural functions intrinsic to the body that have temporarily ceased. In contrast, the vital activity of the zombie depends on novel cellular functions that arise only as a consequence of the zombie transformation. If you could “cure” the condition, ridding the zombie of the pathogen and restoring natural human cellular function, the body would revert to a corpse, not to a living human being. Therefore, the zombie exhibits a qualitatively new kind of life that is distinct from a human life.

I would argue that a zombie thus described is a novel organism that arises via a biological “hijacking” of the cells and structures remaining in a corpse. Just as an oncogenic transformation can “repurpose” cells within a living human being, diverting them from their natural function as contributing parts of the body and directing them to behave as independent cancer cells, the zombie pathogen has “repurposed” the living cells of a human corpse to a novel, non-human organismal function.

But it is not by virtue of lacking rational thought that the zombie is a non-human organism. Living human persons with serious brain injuries are also unable to reason, but they remain human beings so long as they continue to function as living human organisms. In contrast, due to the death of the cortex and the inability of the pathogen to revive or reconstruct cortical circuitry, the zombie lacks the capacity for reason from the beginning. Since the capacity for reason is a defining human trait, zombies, as I have envisioned them, would be non-human animals of human origin.

Zombie Embryos?

Yet considering our original question whether it would be possible to engineer an embryo that utter lacks the capacity to produce a brain capable of rational thought, it is reasonable to ask: if a zombie transformation gives rise to a non-human organism comprising human cells, shouldn’t it be possible to produce a zombie embryo? If an embryo were engineered to produce only the brain structures present in a zombie, wouldn’t it also lack the capacity for reason from the beginning, and therefore be a non-human animal of human origin? And isn’t this what happens in cases of anencephaly?

It is not. The difference between a once-living organism repurposed by a pathogen and an anencephalic embryo is that embryos autonomously undergo development—an ordered process that sequentially produces the mature human body. Importantly, all of the structures produced by the embryo are predicated on the existence of an intact nervous system, even before the onset of neural function. For example, muscles are activated by nerves; spinal circuitry communicates with and is regulated by the brain; heart rate, breathing, and a host of other biologic functions are naturally controlled by the nervous system. The embryo is structured around a “whole” that includes a functioning brain. Any defect that disrupts brain formation in a human embryo is, by definition, a defect in a part, not a deficiency that changes the embryo into a distinct sort of being.

In contrast, the hypothetical zombie does not construct itself, and therefore cannot be seen as having an intrinsic orientation toward any form other than the one it inherits. The integrated function of the zombie entirely reflects the organization built into the tissues and structures by the (now dead) human being during embryonic life; the pathogen merely repurposes these structures. Like the mythical golem constructed of wood and clay and then brought to life by magic, the ordering of the parts within a zombie reflects an extrinsic cause, not an order arising from the nature of the entity itself. Even if a zombie were to grow in size or undergo bodily changes such as puberty or senescence, such alterations would not call into question the nature of the zombie as a non-rational animal of human origin.

Importantly, embryonic development is not merely “growth” (i.e., increase in size) or “maturation” (i.e., age-dependent alterations in physiology or function) but construction of the body. Indeed, the capacity to construct the body is the defining feature of a human embryo; that is, a human being at the earliest stages of life. And all forms of construction are ordered processes, with early stages necessarily anticipating the final outcome. Just as the size and form of a building’s foundation anticipates a completed structure of specific size and dimensions, so too all of the components built by the embryo are ordered toward a specific endpoint. Development requires autonomous self-organization in a manner that is intrinsically predicated on a mature human state, including a functional human brain.

Stated in more philosophical terms, “rational animal” is the essential definition of a human being, and “developing human” is the essential definition of a human being at embryonic stages. Therefore, if an entity is developing and of human origin it is a human being by definition, regardless of any defect it may exhibit in brain formation. Thus, if a human embryo infected by the zombie pathogen subsequently died and was resurrected, the nature of the resulting entity would depend entirely on what happened next. If the entity proceeded through embryonic development (an unlikely scenario), it would be, in virtue of this fact alone, a human being. And just as is the case for anencephaly, any impact the pathogen had on subsequent formation of the brain would reflect a defect in a part, not an alteration in the nature of the entity. Given that human beings at embryonic stages of life are defined by their capacity to undergo development, it is not possible, by definition, to engineer a developing human zombie; i.e., an embryo of human origin that utterly lacks the capacity to produce a brain capable of rational thought.

The author is indebted to Dr. Patrick Lee, a most persistent purveyor of the question, “Would a human embryo that was utterly incapable of forming a brain suitable for rational thought be a human being?” for prompting this analysis and to Drs. H.J. Yost and S.B. Condic for helpful discussions.