July 28, 2005

When Does Human Life Begin?

Many arguments put forward for when human life begins. To simplify the debate, some claim human life begins at fertilization, while others say that human life begins at implantation. While both events are significant in the early development of human life, neither offers a complete answer to the question of the beginning of human life.

I will argue for a definition of the beginning of human life that uses concepts taken from systems biology, and will apply this definition to the current debate on somatic cell nuclear transfer and embryonic stem cell research.

Systems biology, an emerging field of research that seeks to understand the fundamental principles of living systems, has sought to distinguish an organism from a cell. In so doing, it offers us two important insights that are particularly helpful in determining when human life begins. First, systems biology recognizes that an organism is an independent, embodied process; that is, a single unified whole that manifests itself in various ways over time. Second, systems biology holds that an organism is a determined system that actively follows a particular trajectory. It is not passive, and does not require outside intervention to develop. Together, these two insights help differentiate static cells from dynamic organisms. An organism, then, can be defined as a distinct embodied process that actively follows a particular trajectory. If that trajectory is ever manifest in ways considered human, then the organism from the beginning is human.

When, then, does human life begin?

Human life begins when it first appears as a determined embodied process. This embodied process, from the outset, has an active capacity to be manifest in human ways. Thus, we speak not of a potential human, but of a human with potential.

Fertilization is the usual event that gives rise to a human organism in nature. It is a moment when a distinct embodied process appears that has the active capacity to develop along a human trajectory. Not all fertilized ova, however, have such a capacity. Hydatidiform moles are a case in point. They have a genome made up of human material and a trajectory that is distinct from its parents. But moles have a genetic make-up that is so different from a diploid zygote that they do not, and will not ever, have the capacity to be manifest in human ways.

Further, fertilization is not the only event that produces a human organism. Twinning, for example, is a natural event where an early embryo divides into two separate organisms. A new independent embodied process appears, which can develop along on its own distinct path. Twinning, an event like fertilization, defines the beginning of a new human life. Fertilization, then, is neither necessary nor sufficient to define the beginning of all human life.

Implantation marks a significant point in the development of an embryo, since it demonstrates a particular stability of development. It is a clinical marker for the development of the primitive streak. This is significant because it is the point after which twinning does not occur. Implantation, then, is α defining moment of a human since it marks developmental individuality. But, it is not the moment when the embodied process first appears. The same process, which was initiated at some earlier time, only continues its development along a determined path. The embryo is, in essence, no different before or after the appearance of the primitive streak. Nothing is added, and nothing is taken away. The appearance of the primitive streak, or its clinical marker implantation, is not the beginning of human life. At best, it gives confirmation that an embodied process is developing along a human trajectory.

To summarize, then, fertilization is the moment when most human life begins, but not all. Implantation cannot be the moment that human life begins. Systems biology, instead, provides a definition for the beginning of human life that is complete and applicable to natural or artificial processes. It also shows the continuity of an organism in early development with a mature organism. Human life begins at the moment when it first appears a distinct embodied process.

This definition for the beginning of human life is relevant to the current debate on therapeutic cloning for embryonic stem cell research. Some claim that the product of somatic cell nuclear transfer (SCNT), formed by the implantation of the nucleus of a somatic cell into an enucleated ovum, can be treated differently from a zygote, formed by the fusion of sperm and egg. The argument is made that the product of SCNT, called a "clonote," is different from a zygote because they are created differently and because they are intended for different purposes. Systems biology denies, however, that one can know what something is if one knows only where it comes from. It is also inaccurate to define something based upon its intended use. Scientifically, the key to knowing what something is, is to know what determined trajectory that something will actively follow. A zygote is clearly a determined embodied process with a human trajectory as known by the way it is manifest.

What is a "clonote," then?

A clonote is also clearly an organism, since it is a distinct embodied process that actively follows a particular trajectory. However, we honestly do not know if a clonote is human, since we do not know what that trajectory is and the ways in which it will be manifest. While no clonote has ever matured to become an adult human, the recommendation that a clonote not be allowed to exist beyond 14 days indicates that it could. Of course, if it ever developed a primitive streak, or implanted in a uterus, it would be highly suggestive that it has a human trajectory.

In the face of this lack of full knowledge, the only prudent course of action is to treat the clonote as if it were human. In fact, it is precisely because the clonote seems to have a human trajectory that its stem cells are thought to be useful for therapy. It should therefore be given the respect deserving of human life, and not destroyed for the sake of another human life.

Few would dispute the idea that respect for human dignity imposes certain moral directives on scientific research and medical care. However, it does not follow that respecting human life, from its very beginning, will deny patients needed care or restrict scientific progress. In fact, it is the only way to ensure its success. Adult stem cell research, so far, is the only area of stem cell research that has produced concrete results. Perhaps mother nature is telling us something. Surely, one should ask, if a clonote were not human, how effective will it be for therapy?


Ford, N. When Did I Begin? Cambridge University Press, 1988, 217 pgs.

Kitano H. Systems Biology: A Brief Overview. Science 295;5560 (March 1, 2002), pp 1662-1664.

Austriaco, N. On Static Eggs and Dynamic Embryos: A Systems Perspective. NCBQ Winter 2002 2;4, pp 659-683.

McHugh P. Zygote and "Clonote" - The Ethical Use of Embryonic Stem Cells. N Engl J Med 351;3, pp 209-211.


PatrickYeung, Jr., M.D., has a Bachelor of Science in Biophysics and Physiology from the University of Toronto, and is a board- certified Family Practice physician. Currently, he is a resident in Obstetrics and Gynecology at Georgetown University, Washington, D.C., USA.

PatrickYeung, Jr., M.D., has a Bachelor of Science in Biophysics and Physiology from the University ofToronto, and is a board- certified Family Practice physician. Currently, he is a resident in Obstetrics and Gynecology at Georgetown University, Washington, D.C., USA.

Copyright Bioethics Press Summer 2005