The Human Person’s Beginning: The Biological Evidence

Angela Smith

There is little controversy in stating that a newborn baby is a human being. Less obvious is our earlier beginning: at which point before birth can we say that the conceptus constitutes a new human being?

Each of us originates from two reproductive cells: a sperm (spermatozoa) and an egg (oocyte), known collectively as gametes (from the Greek for husband and wife, "gamete" and "gametes"). When, after these gametes join, we can define the resultant combination as a unique organism?

The sperm-egg entity rapidly progresses from two distinct cells to one new, unique cell. Dr. Maureen Condic, Associate Professor of Neurobiology and Anatomy at the University of Utah School of Medicine, explains "Scientific distinctions are made between various cell types based on two relatively simple criteria: cells are known to be different from each other because they have different composition (i.e., different genes are expressed, different proteins produced, etc.) and because they exhibit distinct types of cell behavior."¹ Condic notes, for example, that embryonic neural crest cells are said to have differentiated into a new type of cell (sensory neural progenitors) when they "stop migrating, begin a period of more active cell proliferation, begin to express different genes,"² and take on a new cell shape. The sperm-egg entity, then, can be said to have differentiated into a new type of cell when it has established a unique composition and unique behavior.

Within the first second after the sperm penetrates the egg’s outer layer, known as the zona pellucida (from the Latin for “transparent zone”), and fuses to the egg, the membranes of these two cells join. All components of what were previously the sperm and the egg are now contained together within one cell wall. Due to its distinct molecular and genetic composition, biologists give this new cell a new name: a zygote.

Minutes after membrane fusion, the zygote differentiates itself behaviorally from its predecessors. Within half an hour, chemical changes render the surface of the zona pellucida inhospitable to further sperm entry. The goal of the zygote’s parent cells, the sperm and egg, was to achieve fusion of two cells into one. The goal of the new cell, the zygote, is the complete reverse: to prevent fusion of other sperm, since a new cell has already been formed. In Condic’s words, “the prior trajectories of sperm and egg have been abandoned, and a new developmental trajectory-- that of the zygote-- has taken their place.”³

Given its new contents and behavior, then, the zygote can be considered a distinct cell within one to thirty minutes after sperm-egg merging. Yet the question remains: does this new cell also constitute an entirely new organism?

One characteristic of a living organism universally accepted by biologists is the capacity to reproduce.⁴ No gamete possesses this ability independently. Left to itself, a sperm is programmed to remain a sperm, and an egg to remain an egg, until death. By contrast, without adding anything other than nutrition, a zygote is programmed to grow into an adult human being who does have the capacity to reproduce. Additionally,the zygote possesses the other six traits which define an organism: nutrition, respiration, movement, excretion, growth, and sensitivity.⁵

But when does it possess these traits? Is there a defining moment at which the zygote transitions from two cells, neither of which is an organism, into one cell, which is an organism?

A human somatic cell (from the Greek “soma” for body), the name for all cells in the body other than gametes, contains 46 chromosomes (coiled strands of DNA). Gametes (reproductive cells) are different. A sperm contains only 23 chromosomes. Its combination with the 23 maternal chromosomes from an oocyte results in an entirely new combination of 46. At insemination, DNA from sperm and egg are inside the same cell wall, but each is contained separately, in two membrane bound pronuclei. The zygote must grow from a one celled to a multicellular organism, so each pronucleus replicates its DNA in preparation. The two pronucleus membranes break down and then finally fuse, in a process known as syngamy (from “syn” for synthesis and “gam” for gametes). The resulting 92 chromosomes separate into two sets, each containing 23 maternal and 23 paternal chromosomes. Next, the zygote cleaves into two identical cells. All of this occurs within the first 24 hours after sperm and egg make initial contact.⁶

Dr. Condic states that at the moment of initial sperm-egg fusion, the zygote constitutes a new human life because it contains an entirely new set of DNA and exhibits organismic behavior. Some argue that a new human being does not originate until syngamy, because this is when the maternal and paternal DNA leave their pronuclear membranes and arrive at the final combination which will direct the human being for the rest of its life. After reviewing the arguments for when new life occurs in this timeline from sperm penetration to syngamy, Dr. Francis Beckwith, Associate Professor of Philosophy and Jurisprudence at Baylor University, concludes, “It may be that one cannot, with confidence, pick out the precise point at which a new being comes into existence between the time at which the sperm penetrates the ovum and a complete and living zygote is present. But how does it follow from that acknowledgement of agnosticism that one cannot say that zygote X is a human being?” He warns against committing the “fallacy of the beard”⁸: “just because I cannot tell you when stubble ends and a beard begins does not mean that I cannot distinguish bearded faces from clean shaven ones.”⁹

Whether we accept sperm penetration, syngamy, or some point in between as the beginning of a new human being, we can at least say that at the conclusion of syngamy--that is, within the first 24 hours of its existence-- the zygote constitutes a new organism.

Some still claim the zygote is simply part of the mother’s body. How is this cell so different from other human cells that we can call it a new organism?

To find out, in addition to recalling that the zygote possesses completely new DNA and the seven traits universally accepted as defining an organism, we can examine the zygote’s behavior. Merriam Webster’s Medline Plus medical dictionary defines an organism as “an individual constituted to carry on the activities of life by means of organs separate in function but mutually dependent: a living being”.¹⁰ An organism can be identified similarly to the way a cell is identified: by examining its composition and behavior, then determining whether or not these fit the aforementioned definition.

A living skin cell contains human DNA. Its composition and activity are that of a human cell, but not of a human being, because it lacks the interdependent substructures present in an organism. Although, with care, “in the laboratory it will continue to live and will divide many times to produce a large mass of cells,“¹¹ these cells will be neither separate in function, nor mutually dependent. The result will never be anything more than a group of skin cells.

By contrast, mutually interdependent substructures are programmed into the zygote from its inception. After syngamy, the final act of the fertilization process, the zygote undergoes a series of divisions known as cleavage, ending with a sixteen cell entity called the morula. Cells form an outer layer, the trophoblast, forerunner of the placenta, and an inner cell mass, the embryoblast, precursor to the embryo. Differentiation has begun within the first two days of existence. By the fourth day, the primitive streak, antecedent to the nervous system, has developed. There are now three cell layers: the ectoderm, which will grow into the nervous system; the mesoderm, which will become bone, muscles, and connective tissue; and the endoderm, which will give rise to the lining of the gastrointestinal and respiratory tracts.¹² Even within the first few days of its existence, the embryo is not a mere clump of independent cells, but an entity of increasing subspecialization and interdependence: an organism known as a human being.

At fertilization, a new cell, which is also a new organism, originates. This young one celled organism directs itself to grow through all of the human life stages. We know from theology that what sets this human person apart from other organisms is its immortal intellectual soul. We understand that, in the words of Pope Francis, “All life has inestimable value: even the weakest and most vulnerable, the sick, the old, the unborn and the poor, are masterpieces of God’s creation, made in his own image, destined to live forever, and deserving of the utmost respect.”¹³

Yet we need not convince those who disagree with us of these moral and theological facts. Reason and scientific evidence are common ground to all of humanity, regardless of religious beliefs. A 2010 article from Nature, the most frequently cited scientific journal in the world¹⁴, states that “The life cycle of mammals begins when a sperm enters an egg.”¹⁵ In the public square, we can assert the value of the human person from conception by reasoning from universally known biological evidence. Reason is, as always, consistent with our faith, as eloquently expressed by Pope John Paul II: “Faith and reason are like two wings on which the human spirit rises to the contemplation of truth; and God has placed in the human heart a desire to know the truth-- in a word, to know himself-- so that, by knowing and loving God, men and women may also come to the fullness of truth about themselves.”¹⁶

Glossary of Terms¹⁷

cleavage: the series of cell divisions of the fertilized egg which change the single-celled zygote into a multicellular embryo

fertilization: the process of union of two gametes whereby the development of a new individual is initiated

gamete:a mature male or female sex cell capable of initiating formation of a new individual by fusion with a gamete of the opposite sex

somatic cell: any of the cells of the body that compose the tissues, organs, and parts of that individual other than the sex cells

syngamy¹⁸: a method of reproduction in which two individuals (gametes) unite permanently and their nuclei fuse

zona pellucida: the transparent outer layer of a mammalian ovum

zygote: a cell formed by the union of two gametes, or the developing individual produced from such a cell