Sunday, January 8, 2017

Yes, Zygotes And Embryos Are Individual Organisms: A Rebuttal To Common Claims

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  • In this article, I will go over several arguments I have found on the Internet arguing that zygotes and embryos are not individual organisms in their own right, but, rather, merely cells. In other words, they argue that a human zygote or embryo is not an individual human, or that a whale zygote or embryo is not an individual whale, but merely a cell or clump of cells of human/whale origin, no different from shed human/whale skin cells, for example.
  • I will examine these arguments to try to see if they hold any veracity. Overall, I am satisfied with the conclusions I reach, as I feel that they are the result of my application of logic and critical thinking to these arguments.
  • Alright, without further ado, let us delve right on in.

  • One of the most basic arguments I could find was that a human (to utilize the example of the species for which this issue is the most controversial) zygote or embryo does not resemble a mature human, and lacks none of the physical characteristics, or autapomorphies, necessary for classification as a member of the species Homo sapiens. As a human zygote is a unicellular organism, indeed, it resembles a protozoan, such as an amoeba, far more than an adult human. However, I find this argument incredibly myopic and spurious, due to the fact that a simple glance at the rest of the animal kingdom shows much diversity in physical forms at various stages of ontogeny. Most kindergartners are probably aware (I know I was) of the metamorphosis by which a caterpillar transmogrifies into a butterfly, and possibly also that by which a tadpole transmogrifies into a frog. Indeed, it shall be observed that, in many ways, a caterpillar bears far more resemblance, physically, to an annelid worm than it does to its older self, just as how a human zygote bears far more resemblance to an amoeba than to its older self. Yet it is uncontroversially accepted that a caterpillar and the butterfly it becomes are the same individual, and members of the same species, who only look that much different due to being at different ontogenetic stages in their life cycle. A human, at the earliest stage of life, is a unicellular organism, but it is still entirely a Homo sapiens, just as a Monarch butterfly, when it was a caterpillar, and lacked wings, was still entirely a Danaus plexippus.

  • Additionally, it shall be noted that many organisms are classified as members of various clades, be they species, genera, families, orders, classes, phyla, kingdoms, domains, or any kind of clade, while lacking key autapomorphies defining membership in said clade. For example, humans, due to their mammalian provenance, are classified in the clade Synapsida, membership in which is defined by the presence of one fenestra (literally translated as window, meaning an orifice, or a hole, in the skull) in the skull, as compared with anapsids, which have no fenestrae in their skulls, and diapsids, which have two. Humans lack this single fenestra -- they are secondarily fenestra-less -- yet they are still classified as synapsids because they are descended from ancestors that possessed it. To use yet another example, snakes and caecilians are classified in the clade Tetrapoda, defined as encompassing verebrate animals with four limbs, despite the fact that they lack limbs entirely in the adult ontogenetic stage of their life cycles.

  • Another argument I could find was that, as it is not genes alone, but genes in addition to environmental influences, that make an individual who they are, a zygote cannot be an individual of the species it belongs to. This argument asserts that saying that a zygote, immediately after a spermatozo√∂n fertilizes an oocyte, is an individual of the species it belongs to is a form of genetic reductionism that discounts the role of the environment in shaping the individual, and compares the assertion that a zygote is a member of the same species as the adult organism it will later become to a form of preformationism, the erroneous belief that the physical features the zygote will later have are already present within it. 
  • First, I need to make it clear that I am no preformationist; I acknowledge that the phenotype of the zygote is drastically different from the phenotype of the adult creature that it will eventually become. Second, I am no genetic reductionist, either; I acknowledge the key role that environmental influences, in addition to genetics, play in molding an individual.

  • The difference is that I am aware that the environmental influence on an individual's phenotype is a process that continues throughout life, until the individual's death. The phenotype of an individual organism, and humans are no exception, changes drastically as they age, up to the time of their death, and environmental influences continue to play a significant role in these changes up until death. One notable example is height, or body size. This is undoubtedly one of the most noticeable and characteristic physical features of any organism. And it is well-known that it is affected by environmental factors, such as nutrition, throughout childhood and adolescence. Let's say a human child, seven years after their birth, is now four feet tall. They could have a range of possible tallest adult heights in their future, depending upon environmental influences in the intervening years. They might have the genetic potential to grow to be six feet tall, but if they spend the next years of their life in a war zone, deprived of nutrition, they might end up reaching an adult height of only five feet, eight inches, while, if they live a healthy lifestyle, they could end up reaching the height of six feet. Since the child's final height seven years after birth is still indeterminate, and is, at least partially, contingent upon environmental influences, does this mean that the child, at seven years after birth, is not yet an individual person, as their environmental influences have not yet fully molded them into their adult form alongside their genes?

  • To furnish more examples, I can easily think of cases in which the environment continues to modify an individual's phenotype well into adulthood. For example, let's say a forty-year-old runner with unnaturally flat feet due to a lifetime of wearing unhealthy shoes decides to take up barefoot running, and subsequently experiences elevation of the arches in their feet. It is also well-known that the aging process comes with numerous physical changes, many of which seem to be due to the environment interacting with genetics. A thirty-year-old adult human whose height is 5'6'', with black hair, a mouth full of teeth, and high bone mineral density looks significantly different from an eighty-year-old adult adult human whose height is 5'2'', with white hair devoid of pigmentation, no teeth in an edentulous mouth, and low bone mineral density, yet, it is entirely possible, and, in fact, is true in a lot of cases, that these two disparate descriptions are of the same individual at different ontogenetic stages in their life history.

  • As it is established that the environment acting in concert with genes to shape an individual is a lifelong process that does not truly cease until death, it is totally arbitrary to draw a line at any age and say that "this is when enough of these environmental influences have accumulated to say that you are now you." To take this argument the only logical way, one would have to say that we are never truly ourselves, and, therefore, never truly alive as individuals, until the very end of our lives, when the environmental forces finally cease exerting their influence upon our genomes, which is, of course, absurd.

  • Since it is acknowledged that environmental influences upon genomes are an important aspect of shaping an individual, it also must be pointed out, at this juncture, that it has been shown that this process actually starts at conception. Recent embryological research has demonstrated that environmental influences are thought to be an important part of causing the zygote to undergo its first mitotic cleavage, through which it reaches its two-cell stage. So the assertion that the life of an individual organism has to begin later than at the time of its conception because only genes are present at this time is rendered moot, as environmental influences, as well as genes -- the whole package -- are actually present from conception onwards.

  • Finally, another argument is that, although a human zygote, for example, is human biological material in the same sense that a human somatic cell is, it is not an individual human in the same sense that a human adult is. This argument fails to comprehend and take into account fully the distinction between living matter and living organisms. The truth is that, subtle though it may seem, there is, indeed, a distinctive difference that separates cells that are organisms in their own right from cells that constitute parts of organisms. That is the ability to behave in an organized manner, with the different parts of the cell functioning together as a unified whole. A somatic cell, if removed from its owner and taken to a locality with conditions conducive for growth, will grow in a random, chaotic manner, unlike the organized, coordinated manner of a zygote as it undergoes repeated mitotic cleavages to become a morula, a blastula, a blastocyst, a gastrula, and so on and so forth. It is often asserted that, as some cells that sometimes form in a mature organism's body, like cancer cells, have genomes that are different from the other cells in the body, but do not constitute organisms in their own right, that it cannot be said that the zygote constitutes an organism in its own right due to having its own unique genome. However, this argument overlooks the crucial fact that a zygote displays behavior characteristic of an organism from the moment that the sperm penetrates the zona pellucida of the ovum, unlike somatic cells of any type, including cancer cells. It is not merely the fact that the zygote possesses its own unique genome that makes it an organism, but the way it displays a self-organizing, coordinated pattern of behavior, with the parts of the cell working together to allow the entity to function as a whole, to facilitate the transition to the following multicellular stages. For example, different regions of cytoplasm within the zygote coordinate their varying activities to allow the organism, as a whole, to function.

  • So the zygote not only has a unique genome of its own from the time of its conception, but environmental forces are already affecting it and exerting their influence on its genes from the time of its conception, and it displays a pattern of behavior characteristic of living organisms, as opposed to mere living biological material, from the time of its conception.

  • And, yes, it is true that the sperm cell and the egg cell that formed the zygote were just as alive as it was. However, it needs to be clarified that, unlike the zygote they gave rise to, they were not individual organisms in their own right, but, rather, parts of organisms -- the ovum to the adult female animal which produced it, and the sperm to the adult male animal which produced it. This is not entirely because they lacked a distinct set of genes, but mainly because, unlike a zygote, they do not exhibit organismal patterns of behavior.

  • Another argument, which that, as human cloning is now possible, and as any somatic cell from an individual's body could be used to produce a clone of them, this renders these somatic cells the equivalent of zygotes, can also be summarily rejected due to one critical flaw. This argument makes the mistake of overlooking the fact that, before said somatic cells can produce a clone, genetic information from them first needs to be transplanted into an egg cell, rendering the new combination equivalent to a zygote in terms of its molecular composition and its properties.

  • Overall, the argument from physical appearance can be refuted by a simple cursory glance at the various ontogenetic stages numerous species pass through during their life cycles, the argument from environmental influences can be refuted by the fact that environmental influences that shape individuals, such as epigenetic processes, start acting on an individual's genome at conception and do not cease doing so until death, the argument from false equivocation of zygotes and embryos with somatic cells and tissues can be refuted by the fact that the former display clearly organismal behavioral patterns, whilst the latter do not, and the argument from cloning, closely connected to the last argument, can be refuted by the crucial fact that, in order for cloning to occur, an ovum must be involved, as in the formation of a zygote.

  • I must arrive at the conclusion that the embryological evidence seems to point firmly to the conclusion that, from the moment that the sperm and the ovum form a zygote, it is a living individual member of the species to which it belongs. Indeed, this seems to me to be the predominant view in most of the field of embryology. I penned this article because I felt the need to clear up the numerous misconceptions surrounding this subject that I kept repeatedly encountering on the Internet. I felt a duty to set the record straight, and allow the world to know the truth.

  • References:
  • Condic, Maureen L. (2014). Confusions About Totipotency: Stem Cells Are Not Embryos. (http://www.thepublicdiscourse.com/2014/03/12361/)

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