The Moment A Human Life Begins

by Petra W
/ Commentary

To understand the moral implications of abortion, we must first know what abortion does.

But the truth of what abortion does is often intentionally obfuscated by pro-choice proponents who use dehumanizing and scientifically incorrect language to refer to the victims of abortion, preborn humans.

Nowhere else is this more apparent than in how abortion proponents talk about the point at which human life begins. Unless and until a prenatal child is wanted, they are just a “clump of cells,” “mere tissue,” “non-living,” or “not human.”  

Or one might hear an abortion proponent concede that a prenatal human is human, “but only in the way cancer or skin cells are human.” In other words, a prenatal human is not a human being or a human organism. This is an intellectually dishonest argument. Such abortion proponents are not arguing prenatal humans are human but not valuable humans; these are abortion proponents who will not even admit a basic scientific fact. They are confusing parts with wholes to conflate (intentionally or not) mere human parts with whole human organisms.

Some believe we cannot answer the question of when human life begins. Some believe as Justice Anthony Kennedy wrote in Planned Parenthood v. Casey,

“At the heart of liberty is the right to define one's own concept of existence, of meaning, of the universe, and of the mystery of human life.”

But that is nonsense. Not only can we answer the question of when human life begins, but science already has answered that question. We have known the answer for a long time — even before Roe v. Wade — and it is at the fusion of egg and sperm (aka, fertilization or conception).

In the field of human embryology — the study of early human development — fertilization encompasses three stages, Carnegie Stage 1a, 1b, and 1c. The rest of this article will explain why a new human organism begins to exist at Carnegie Stage 1a, or first contact between a sperm and egg cell.

What is an Organism?

The definition of “organism” in biology is actually still quite debated, as is the definition of “life.” There are several definitions of “organism” which have closely overlapping ideas or criteria, but as we will see, under any definition, a human embryo is a living organism. A human embryo is not an example of something on the fence (like a virus, perhaps, or an AI) which some embryologists or biologists (or other specialized scientists) would say is a living organism, and others would argue it is not.

A general, overarching definition of an organism for biologists which leaves quite a bit of room for things normally considered organisms with no dispute (like a vertebrate animal) and those which may be harder to define (such coral polyps) is

“a functionally integrated and cohesive whole made of interdependent and interconnected parts.”

An organism has historically been seen as a “physiological individual,” that is, the definition given above. However, there is another way of thinking about organisms, which evolutionary biologists may be more likely to use, which is

“most commonly a system containing many genes and several levels of organized hardware and software used in replication. Natural selection acts at the level of the organism. It does not select particular genes and eliminate them from an individual. It acts on entire organisms, determining how much their particular collection of genes is replicated and retained in the population. Therefore, we can define an organism as the unit that plays this role in the process of evolution. It is a fundamental particle in the process of evolution — the component of a population on which natural selection acts to determine the genetic composition of the population.”

This definition of organism can also be referred to as an “evolutionary individual.” Physiological individuals and evolutionary individuals are organisms. As we will see later, human embryos are both physiological and evolutionary individuals.

There are other definitions of an organism which are used in biology textbooks, classrooms, online discussions, and everyday language. You might notice most of these common definitions fall under the umbrella of the long-standing general, historical definition, which places more emphasis on physiology rather than whether natural selection can act on the entity. However, you may also notice (or have already noticed) that many examples of organisms can be considered both physiological and evolutionary individuals. The overlap between different definitions of an organism can indeed be very broad.

Here are some other definitions of “organism” you may find commonly used online, in textbooks, and in journals:

“Specifically, something is an organism if the parts work together for the integrated whole, with high cooperation and low conflict. This means the organism is the largest unit of near-unanimous design. The near’ is required because there can always be some conflict, even within the strongest examples of organisms, such as genetic conflicts within animals.” i.e. organismality is a spectrum, with things which are all organisms being more or less organismal than others.”

This same paper also states as obvious fact that mammalian single-celled embryos are organisms:

“In animals such as mammals, each individual cell develops from a single fertilised cell, leading to genetic uniformity, with minimal conflict and high cooperation. This is clearly an organism.”
“An organism refers to a living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis.”
“All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, growth and development, regulation, homeostasis, energy processing, and evolution. When viewed together, these nine characteristics serve to define life.”

Why Does it Matter if an Embryo is an Organism?

If a human embryo is an organism, this would mean the second premise of the most basic pro-life argument is scientifically accurate. While scientific facts and use of the scientific method cannot make moral conclusions, they can — and many times, should — inform moral conclusions.  

As Scott Klusendorf puts it, the most basic pro-life argument is:

“Premise #1: It is wrong to intentionally kill innocent human beings.
Premise #2: Abortion intentionally kills innocent human beings.
Conclusion: Abortion is morally wrong.”

The second premise states, “Abortion intentionally kills innocent human beings.” Human beings are, in the most basic, scientific (non-philosophical), unemotional terms, individual members of the species Homo sapiens (see here, here, and here). From our earlier discussion of organisms, we can conclude an individual member of a species (i.e., an organism) is a physiological and/or evolutionary individual with that species’ DNA. So a human being is a genetically human organism, an organism reproduced from a human parent (cloning) or parents (IVF, sexual reproduction) who has distinctly human DNA. And if a human embryo is an organism, that would make him/her an individual of the species Homo sapiens, and therefore a human being. To summarize:

1. Human organisms are human beings (A = B)

2. All human embryos are human organisms (All C are A)

3. Therefore, all human embryos are human beings (All C are B) by identity

It is scientifically accurate, then, to claim abortion intentionally kills innocent human beings, if even from the earliest of stages — the embryonic stage — what is growing and developing in the womb of a pregnant woman is a human organism. Note, of course, this article is not about whether the second premise is philosophically accurate; that is an argument for another time. From the standpoint of science, however, we can confidently state elective abortion intentionally kills human organisms after looking at the scientific findings from the early 1900s- onward.

Is a Single-Celled Human Embryo Actually an Organism?

How can something so small, so young, and requiring such a specific environment to grow and develop be an organism? Whether something is an organism or not never has anything to do with how old or large something is or what environment it has to be in to live. You do not see those traits mentioned in any discussion of the definition of an organism. Single-celled E. coli bacteria are considered organisms, and they are also tiny and require very specific growing conditions to stay alive.

A human embryo fits any definition of organism we have, and the vast majority of biologists affirm fertilization (Carnegie Stage 1a) is the mark of the existence of a new human organism. While many aspects of very early human development are still unknown in precise detail, many details and basic aspects are well-known, and there is no serious scientific doubt over whether a human embryo is a human organism. There are so many details to consider, but below I will briefly go over some of the biological evidence and expert sources which support the fact that even early human embryos are human organisms; in fact, they are both physiological and evolutionary individuals. There will be several sources included for you to read over on your own which will supply much greater detail.

Neurobiologist Dr. Maureen Condic provides a fairly comprehensive but also fairly easy to understand overview of the early details of the formation of a human embryo, and why human embryos are human organisms from fertilization. She even addresses common concerns and challenges to that point in time, like syngamy (Carnegie Stage 1c), clones, hydatidiform moles, and whether a human embryo actually controls their own development or not. The American College of Pediatricians acknowledges human life begins at fertilization and gives a good, brief history of embryology as well as several citations from more scientific sources concerning fertilization marking the beginning of a mammalian organism’s life. Numerous medical and scientific textbooks standard to universities, as well as scientific journal articles acknowledge the scientific fact that a human organism begins to exist at fertilization.

Human Embryos are “Physiological Individuals” (Organisms)

A human embryo is a physiological individual because s/he is “a functionally integrated and cohesive whole made of interdependent and interconnected parts.” A human embryo displays high cooperation and low conflict in development and growth and displays all 7-9 characteristics commonly cited for living organisms. These are all true from Carnegie Stage 1a (fertilization).

Carnegie Stage 1a is defined as first contact between sperm cell and egg cell. As soon as that contact is made, the two separate, distinct human haploid cells (cells which only contain 23 chromosomes) which were part of other organisms begin to work together as a single unit in an entirely new way. The processes which start at first contact are irreversible and interdependent. This marks the beginning of the existence of a new human organism as a single human diploid cell (cell containing 46 chromosomes). This single-celled stage of human development (Carnegie stages 1a-1c) lasts only 24 hours or so in total before rapid division and growth begins. The zygotic stage only exists at Carnegie Stage 1c. Stage 1c occurs about 20 hours after in vitro insemination but is elusive since it exists for only a brief period” before cell division starts. This is important to note because many times pro-life people will state a zygote (a specific term for a very young embryo) is when human life begins, when actually a new human was in existence about 20 hours before that.

Human embryos self-direct their own growth and development from fertilization, even in the absence of maternal tissues, and past the time they would implant if they were in the presence of maternal tissues. While the status of the human embryo as an organism was never in question by the two groups who did the research linked below, the extent to which the human embryo directs their own development did come as a bit of a surprise, to the authors of this paper and to the ones of this paper. As Dr. Morris, Assistant Professor of Developmental Biology and Genetics at Washington University School of Medicine in St. Louis commented in a scientific review article concerning these papers:

“The studies of the two groups showed that human embryos developing in vitro possess all the key landmarks of normal development...the critical remodelling events at this stage of human development are autonomous, highlighting their remarkable and unanticipated self-organizing properties.”

These findings clearly show young human organisms are not strictly dependent on their mothers’ bodies to direct their own growth and development; they are only, like us all, dependent on some source of nutrition and sustainable living conditions (like temperature, air pressure, shelter from the elements, etc).

Human Embryos are Evolutionary Individuals (Organisms)

Human embryos are evolutionary individuals because natural selection acts on human embryos; a human embryo is “the unit” which determines the genetic composition of a population. Carnegie stage 1a is when the genotype of a new organism is determined. Very shortly after fertilization, epigenetic markers from paternal and maternal DNA are removed (to a low level, not completely erased) and new epigenetic markers unique to the new human organism are made, which regulate gene expression. This organism is subject to evolutionary processes and can influence future evolutionary processes by passing down his/her genes to their own progeny.

We Have Known About Human Embryology for a Long Time

This knowledge of human development — specifically, the fact that a new human organism begins to exist at fertilization — is not a secret! Embryology has been an area of study since the classical Greek philosophers. In the early 1900s, the Carnegie Institute began collecting and classifying human embryos. The Carnegie Stages of Human Embryological Development were instituted in 1942 — 30 years before Roe v. Wade was decided. An international committee of human embryologists regularly updates and reviews the nomenclature of the Carnegie Stages. Yet the majority opinion in Roe v. Wade constantly used language like “potential human life” and “potentiality of life” to refer to prenatal human beings, which are not at all “potential” human lives, but actual living, respiring human organisms existing in real space and time.

Answering Objections

Many objections to the argument that fertilization marks the beginning of existence of a new human organism are not rooted in scientific evidence or definitions. People may argue no human organism exists at such an early age because an embryo or fetus cannot think or feel or is not self-aware. They may say because a prenatal human depends on the mother’s body for survival or is not viable outside of the uterus, s/he is not actually a living organism. Some people may argue first breath or implantation are actually what determine the beginning of human life.

Objections such as these may be conflating valuable human life with biological human life; it is worth asking someone to clarify their statement to find out if this conflation is being made. However, defining any other point in human development but fertilization as when a new human organism begins to exist places modifiers on biological definitions — modifiers people attempt to only apply to humans and no other species for the express purpose of defending abortion. The definition of what makes an organism does not depend on environment, viability inside or outside of an environment, consciousness, self-awareness, higher brain function, dependence on another organism, or method of respiration.

However, some objections to the fact that fertilization marks the beginning of existence of a new human organism are rooted in scientific concerns. Most scientific objections center on two characteristics of organisms not as easily seen in human embryos. That is to 1) maintain homeostasis or 2) direct their own development. However, many people conflate maintaining homeostasis with being viable outside of the uterus, but this is a poor understanding of homeostasis. To be fair, homeostasis can be hard to understand because it has a rather broad definition and it is taught inconsistently in common undergraduate physiology and biology textbooks. The general concept is that internal conditions of individual cells and an organism as a whole are passively and actively regulated by a variety of processes to stay within the narrow range needed to maintain life.

Just because an organism needs a specific environment or types of nutrients to grow (which all organisms require, by the way) does not mean the organism cannot maintain homeostasis. As explained above, human embryos do actually direct their own development and maintain homeostasis even without the maternal tissue present. This common objection is just not grounded in scientific evidence.

A couple of other common scientific concerns which could give rise to an objection to the fact that human embryos are organisms include thinking about cancer and teratomas, as well as twinning.

Cancer Has Unique DNA

For example, someone might respond to a pro-life person who states human organisms begin at fertilization and have unique human DNA with, “Cancer has unique DNA!” or “Teratomas have human parts and tissue just like an embryo or young fetus.” These objections do not address the core point the pro-life person has made, which is that a new human life begins at fertilization, and of course a new human organism will have his/her own unique DNA. But unique DNA is not the core point; DNA alone is not enough to state whether something is a living organism or not.

Cancer DNA is unique in that it is your DNA, mutated in some way to make your cells ignore normal growth and/or dying signals. This mutated DNA does not give rise to a new organism. Cancer cells are a part of you (your being), not a new (other) being. Cancer cells will never develop into a human embryo or fetus or toddler or adult, even if you take them out of the body and keep them alive in a dish. They will remain cancer cells. Similarly, most teratomas arise from a person’s own body and are not a separate organism, but are cell development within an organism gone wrong. The only possible exception is the very rare case of fetus in fetu, which may possibly be an instance of one organism dying before reaching maturity and becoming enclosed in another developing organism, or twinning gone wrong.

What About Twinning?

As far as twinning goes, some pro-choice people will say a human organism cannot exist at fertilization because a monozygotic twin (commonly known as an identical twin) does not exist at fertilization; the twinning happens at some point after fertilization. This is really not as strong an argument as people may think. Yes, twinning most likely happens after fertilization. The exact time is still unknown; it can likely happen at any point in a certain period of early embryonic development, though there is one model which argues for it happening specifically at the zygotic stage. All this really means is that an asexually reproduced human organism comes into existence hours to days after a sexually reproduced human organism does. It does not mean there was no organism to start with.


Knowing the basic science of human embryology is important to the abortion debate, because  science can play a key role in informing our philosophy and morals. Both pro-choice and pro-life people should be able to acknowledge the reality of human development in making their case for or against induced abortion.

And just to be absolutely clear, pro-life people understand “life” is a continuum. We understand sperm cells and egg cells are alive prior to meeting, and they contain human DNA, so that makes them living human cells. We understand human life on earth began at some point in the past, so human life started many years ago when talking from a historical and evolutionary standpoint. But we also know that a living cell with human DNA may not be a human, as in, it may not be a whole human organism. We are not the ones trying to conflate human skin cells with human organisms. We are not the ones trying to conflate an individual human organism’s beginning of life with the evolutionary beginning of human life on earth. For pro-choice people to make those conflations or say pro-life people are the ones doing so is highly disingenuous.

Embryology directly supports the claim that abortion kills a human being. This simply cannot be denied unless you want to also deny science.

For further reading on the science of human embryology:

The views and opinions expressed in this article are those of the author and do not necessarily reflect the official position of Human Defense Initiative.

The Moment A Human Life Begins
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