The following link makes such a claim


The following references illustrate the fact that a new human embryo, the starting point for a human life, comes into existence with the formation of the one-celled zygote:

I suspect this might be cherry picking.

  • 84
    Scientists argue about whether viruses should be considered living organisms or not (and the only reasonable correct answer, from a scientific standpoint, is "it depends on your definition"). Going to science looking for a binary decision on something you intend to answer a moral question is not a good idea, because nuance is an integral part of the scientific method. May 17, 2019 at 19:10
  • 30
    @BarryHarrison If your definition of "living organism" is "they can reproduce by themselves without a host" then you are correct (and this is often the trivia answer). However, there are lots of examples of things considered living organisms that fail that definition, including quite complex ones (most parasites need a host to reproduce, for example). Same thing for the title question: if you define human development as beginning at the point of fertilization, then that is the starting point, by definition. That doesn't give you any more information, it's just a definition. May 17, 2019 at 19:19
  • 23
    Science can't make moral judgements. But it can, sometimes, provide relevant facts that inform those judgements. Such as the percentage of viable conceptions that result in established pregnancies or the percentage of implanted embryos that lead to established pregnancies. If full human life is said to begin at conception most people die before birth.
    – matt_black
    May 17, 2019 at 20:43
  • 34
    Whether or not that clump of cells constitutes life is trivial. The cells are inarguably human, but whether or not they are a separate being who has achieved "personshood" is the central argument between pro/anti abortion activists.
    – PC Luddite
    May 17, 2019 at 21:01
  • 21
    @Fizz that article doesn't make a good distinction between simply life and personhood. Science can (mostly) answer the former while the latter is a philosophical question that can never be addressed scientifically.
    – PC Luddite
    May 17, 2019 at 21:07

2 Answers 2


Interestingly there's a survey: "Biologists' Consensus on 'When Life Begins'", 2018 of US biologists on this (the choice of profession/experts was motivated by a pre-survey of the US population at large):

Many Americans disagree on ‘When does a human’s life begin?’ because the question is subject to interpretive ambiguity arising from Hume’s is-ought problem. There are two distinct interpretations of the question: descriptive (i.e., ‘When is a fetus classified as a human?’) and normative (i.e., ‘When ought a fetus be worthy of ethical and legal consideration?’). To determine if one view is more prevalent today, 2,899 American adults were surveyed and asked to select the group most qualified to answer the question of when a human’s life begins. The majority selected biologists (81%), which suggested Americans primarily hold a descriptive view. Indeed, the majority justified their selection by describing biologists as objective scientists that can use their biological expertise to determine when a human's life begins.

A sample of 5,502 biologists from 1,058 academic institutions assessed statements representing the biological view ‘a human’s life begins at fertilization’. This view was used because previous polls and surveys suggest many Americans and medical experts hold this view. Each of the three statements representing that view was affirmed by a consensus of biologists (75-91%). The participants were separated into 60 groups and each statement was affirmed by a consensus of each group, including biologists that identified as very pro-choice (69-90%), very pro-life (92-97%), very liberal (70-91%), very conservative (94-96%), strong Democrats (74-91%), and strong Republicans (89-94%). Overall, 95% of all biologists affirmed the biological view that a human's life begins at fertilization (5212 out of 5502).

Historically, the descriptive view on when life begins has dictated the normative view that drives America's abortion laws: (1) abortion was illegal at ‘quickening’ under 18th century common law, (2) abortion was illegal at ‘conception’ in state laws from the late 1800’s to the mid-1900’s, and (3) abortion is currently legal before ‘viability’ due to 20th century U.S. Supreme Court cases Roe v. Wade and Planned Parenthood v. Casey. While this article’s findings suggest a fetus is biologically classified as a human at fertilization, this descriptive view does not entail the normative view that fetuses deserve legal consideration throughout pregnancy.

But do note (as the 3rd para [I added the breaks for readability] says) that that doesn't imply anything about viability/personhood. The fact that these are distinct questions is emphasized in other (scientific) sources, e.g. Kurjak et al.:

In this paper we show that the question, "When does human life begin?", is not one question, but three. The first question is, "When does human biological life begin?", and is a scientific question. A brief review of embryology is provided to answer this question. The second question is, "When do obligations to protect human life begin?", and is a question of general theological and philosophical ethics. A brief review of major world religions and philosophy is provided to answer this question but has no settled answer and therefore involves irresolvable controversy. The third question is, "How should physicians respond to disagreement about when obligations to protect human life begin?" and is a question for professional medical ethics.

The science in this latter paper is nothing unconventional, i.e. it's consistent with the majority view from the other one.

Some (interesting, I hope) details from the survey (first paper). They asked 4 question of biologists:

Q1 - Implicit Statement A: “The end product of mammalian fertilization is a fertilized egg (‘zygote’), a new mammalian organism in the first stage of its species’ life cycle with its species’ genome.”

Q2 - Implicit Statement B: “The development of a mammal begins with fertilization, a process by which the spermatozoon from the male and the oocyte from the female unite to give rise to a new organism, the zygote.”

Q3 - Explicit Statement “In developmental biology, fertilization marks the beginning of a human's life since that process produces an organism with a human genome that has begun to develop in the first stage of the human life cycle.”

Q4 - Open-Ended Essay Question: “From a biological perspective, how would you answer the question ‘When does a human's life begin?’”

And some charts for the responses (for the first two questions there was no [graphical] breakdown of political orientation; there is some in a table, but it's two pages long):

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For the last question, there was manual coding of the free-form responses into those categories.

The "95% consensus" may be a little exaggerated because it was derived by evaluating whether each subject affirmed at least one of Q1-Q3 (i.e. logical ORing).

LangLangC asks some interesting (terminology) questions. One way to solve these is as in the 2nd paper I mentioned (Kurjak et al.):

A human being originates from two living cells: the oocyte and the spermatozoon, transmitting the torch of life to the next generation. [...] After syngamy, the zygote undergoes mitotic cell division as it moves down the fallopian tube toward the uterus. A series of mitotic divisions then leads to the development of the preembryo. [...]

The pre-embryo is the structure that exists from the end of the process of fertilization until the appearance of a single primitive streak. Until the completion of implantation the pre-embryo is capable of dividing into multiple entities, but does not contain enough genetic information to develop into an embryo: it lacks genetic material from maternal mitochondria and of maternal and parental genetic messages in the form of messenger RNA or proteins.

A key stage in embryonic development is the emergence of an individual human being. ‘‘Individual’’ means that an entity (1) can be distinguished from other entities and (2) is indivisible, i.e., it cannot be divided or split into two members of the same species. An entity meeting the first criterion, but not the second, is a distinct but not individual entity. The pre-embryo, because it can divide into monozygotic twins is a distinct but not individual entity. The embryo, by contrast, no longer divides into monozygotic twins and so it meets both criteria for being an individual.

Distinct human life begins when there is a distinct entity, the pre-embryo, resulting from the process of conception. There is no ‘‘moment’’ of conception, a phrase that has no biological application. Individual human life begins later, with the emergence of the embryo. There is no ‘‘moment’’ at which this occurs either. The beginnings of human life involve complex biological processes that occur over time.

The latter terminology "distinct human life", "individual human life" is probably not so well-established... But in this sense, an (unfertilized) egg or sperm is not "distinct human life" from its host/producer.

Here is a contrary opinion in more detail, from a physician:

What is scientifically incorrect about saying that human life begins at fertilization? First, it is a categorical designation in conflict with the scientific observation that life is a continuum. The egg cell is alive, and it has the potential to become a zygote (a single-celled embryo) if it is appropriately fertilized and activated by a live sperm. If fertilization is successful and the genetic complement of the sperm is added to that of the egg, the resulting zygote is also alive. The zygote has the same size as the egg; other than for its new genotype, the cell (comprising the cytoplasm and the rest) is nearly identical to the egg cell. From a biological perspective, no new life has been created.

Second, “human life” implies individuality, which is also not consistent with scientific observations. In the clinical practice of IVF, we often speak of preimplantation embryos as individual entities, with distinct qualities like a specific genotype (mosaicism notwithstanding), and morphologic and developmental characteristics. But at the same time we realize that each of the totipotent cells that comprise these embryos is, at least theoretically, capable of producing a complete new individual. Indeed, multiple individuals can arise from the implantation of a single embryo, as in the case of identical twins. Therefore, we know that the preimplantation embryo is not actually an individual. The preimplantation embryo is essentially an aggregate of stem cells, which has the potential to produce a pregnancy, including placental and fetal tissues, assuming that it successfully implants in a receptive endometrium. It is only after implantation that the early embryo can further differentiate into the organized cell groups that enable the developing conceptus to progress further in embryonic and eventually fetal development.

The facts are basically the same as in the previous quote. It's a matter of interpretation/terminology.

Somewhat diverging on facts are some (mostly recent) studies which delve into syngamy. They found that in some non-human mammals and in some in-vitro human fertilizations it may take a few cell division for syngamy to (fully) complete; in the meantime the genetic materials may still be somewhat or mostly separated. These finding at the very least enhance the view that talking of a "moment" when life begins is a tenuous. From a 2018 journal editorial serving as a mini-review:

Syngamy has become an acceptable sentinel for the beginning of life. Nearly a century ago, a generation of cell and developmental biologists laid down the groundwork for the biology of fertilization in EB Wilson’s treatise of 1925. Compiled, congealed, and constitutional in nature, tales of syngamy based on the tools of cytology recognized the importance of that special moment in the life history of all sexually reproducing metazoans. That nuptial encounter between the genomes of mom and dad, with the subsequent and equivalent segregation of maternal and paternal chromosomes on a mitotic spindle, has become a building block for our understanding of how embryo development is launched on the pathway to implantation and beyond. That the sperm brings more than a genome to this BYOB (bring your own bottle) affair has long been accepted in the case of humans, emphasizing the role of the paternally inherited centrosome in construction of that first bipolar mitotic spindle.

Fast forward 75 years to the prescient work of Mayer and colleagues, where in an elegant series of experiments on mouse embryos, the stage of development when mom and dad consummate their genome merger is called into question (a probable reason why this work has been virtually buried in the literature). These papers suggested that maternal and paternal chromosomes retained a spatially exclusive location within blastomere nuclei through the first three cell cycles, after which both genomes became spatially integrated into one.

Since these studies were published, the development and application of live cell imaging techniques has blossomed in many biomedical research disciplines. And among those pushing the limits of conventional and novel microscopic techniques in human ARTs (assisted reproduction technologies), and willing to overcome some of the methodical obstacles associated with monitoring living human embryos, is the laboratory of Professor Mio and his collaborators [...]. Among their accomplishments has been the implementation of high resolution microscopy capable of revealing dynamics of cell motility in human embryos and the identification of a novel mechanism operative during the block to polyspermy. Supplementing this technology with spinning disk imaging of fluorescent reporters of chromosomes and spindle components, and differential labeling of maternal and paternal genomes, the present study aimed at evaluating the fate of single pronuclear stage embryos, and in the process uncovered much more. Could it be that, as in the mouse, parental genomes exhibit some degree of autonomy relative to each other?

Their findings are to be interpreted with caution given the source of embryos, the manipulations required to both express and track biomarkers, and the limited number of samples being investigated. However, if repeated and confirmed, these results extend an ongoing conversation suggesting that zygotes of several mammals, including the human, engage in processes that delimit the final integration of maternal and paternal genomes to some as yet undetermined stage within or beyond the initial cell cycles.

[...] in closing that we draw attention to the most recent work coming from the EMBO laboratories of Jan Ellenberg in Heidelberg for both the technological bravado and surprising findings in the mouse zygote regarding separation of maternal and paternal genomes. In essence, what they have now accomplished using light sheet microscopy is to reveal the gender-specific generation of spindles for mom and dad that ultimately converge into one before effecting anaphase on the way to the two-cell stage. Unlike the work of Mayer et al. cited above, their results detected genome integration in two-cell embryos.

Collectively, these observations are causing much head-scratching as we await the results of similar studies on the human conceptus. We have reached a point where foundational concepts such as syngamy may have to be re-visited, not only to deepen our understanding of early human development but also provide a science-based infrastructure upon which societal and ethical guidelines will be formulated based on solid observation and not historical bias.

The are probably quite a few other positions, but it's harder to tell how widespread they are; e.g. there's a 1984 paper (with ~50 citations in Google Scholar) in a medical ethics journal arguing that brain activity marks the beginning of human life:

In an attempt to provide some clarification in the abortion issue it has recently been proposed that since 'brain death' is used to define the end of life, 'brain life' would be a logical demarcation for life's beginning.

Actually the same view is taken in a 1985 paper which has more citations (~100). There's also some published criticism of this idea; it seems to center on the view that brain death is itself not well defined, i.e. there are multiple conceptions of that process/state too.

The whole brain definition of death refers to the loss of major brain regions, including the brain stem. Is there a parallel at the beginning of life? Employing the appearance of brain stem functioning as one's criterion, brain birth would be placed at around 6-8 weeks gestation. I shall refer to this as brain birth I, which is a vitalist interpretation, with its emphasis on biological integration and its stress on mere human biological life. In contrast, a second definition may be determined by the beginning of consciousness at 24-36 weeks gestation. This is brain birth II, which parallels the personalist overtones of the higher brain definition of death, with a sufficiently well-developed neural organization to serve as the substratum from which self-consciousness and personal life subsequently emerge.

And I suppose I should mention here that viability--currently the most medically relevant notion for abortion (but not for other contexts like "the day after" pill, IVF, etc.)--is not a purely biological notion, but rather it intersects with (medical) technology; quoting the obvious from Kurjak et al.:

Viability must be understood in terms of both biological and technological factors, because it is only by virtue of both factors that a viable fetus can exist ex utero [...]

And I'm not quoting more because that (2007) paper is somewhat obsolete in this respect; Wikipedia has a decent coverage of developments in the past decade in artificial uterus technology. More detailed discussions of viability are also concerned with the quality of life aspects, not mere survival, e.g.

in the study by Rysavy et al., the rate of survival without moderate or severe morbidity in those 22-week-old infants was 9%.

Consequently, there's criticism (in that paper) of fixed gestation-age cutoffs in abortion legislation.

  • 12
    Nice line. But getting a bit philosophical, after discarding all normative doctrine, "life begins": wouldn't this also imply that sperm and egg aren't 'alive', meaning 'dead'? – The normative angle cannot be cast away, as the question itself is loaded with its goals, and even more so: axioms, and biologically – 'problematic'? May 17, 2019 at 23:07
  • 9
    @LangLangC Not alive doesn't mean dead. Dead things are things that were once alive but no longer are. I cant think of any reasonable argument about why an egg/sperm would have once been alive but no longer are, while they are still viable for reproduction.
    – Matt
    May 18, 2019 at 2:48
  • 40
    Warning: the study "Biologists' Consensus on When Life Begins" has not been peer-reviewed or published in a reputable journal. The information contained in this answer is from the first draft of a study that is currently only published on a preprint site. Use at your own risk. May 18, 2019 at 8:42
  • 2
    Frankly, despite the high interest of this essay, it does not address the question "Does science define life as...". Instead, it is a survey of the opinions of scientists on the issue.
    – Lee Mosher
    May 19, 2019 at 16:31
  • 2
    This answet would benefit from a summary paragraph. May 19, 2019 at 18:21


Science does not define life as "beginning at conception", because science does not, to any satisfactory degree, define "life" in the general case:

There is no broadly accepted definition of 'life.' Suggested definitions face problems, often in the form of robust counter-examples. Here we use insights from philosophical investigations into language to argue that defining 'life' currently poses a dilemma analogous to that faced by those hoping to define 'water' before the existence of molecular theory. In the absence of an analogous theory of the nature of living systems, interminable controversy over the definition of life is inescapable.

(full paper by Cleland, Carol E.; Chyba, Christopher F., SETI, Universities of Stanford, Colorado)

Several fields of science (medicine, gerontology, biology, physics…) have working, ad hoc definitions of life and death that mostly serve, but something so apparently simple as a doctor calling a time of death can be difficult, unclear and contested; not because death is morally difficult, but because the issue of whether death has occurred is practically difficult to establish in some cases.

A woman lies in a bed at The Johns Hopkins Hospital. Aided by a ventilator, her lungs inflate, deflate, and fill again. Her heart beats and her skin is warm. But her eyes stay closed and she does not react to stimuli such as pain and light.

Is she alive or dead?

If you’re unsure, or if the question makes you uncomfortable, you’re not alone. The hypothetical case described here reflects a real problem: the inherent difficulties of diagnosing and accepting brain death.

(full article by Karen Nitkin, Johns Hopkins)

  • 5
    Please provide some references to support your claims. What does it mean to say a definition is "ad hoc" - what definitions in science are not? Who says the definitions are not satisfactory? There is a logical leap between "life is not well defined" and "beginning of life is not well defined".
    – Oddthinking
    May 19, 2019 at 0:41
  • 4
    Perhaps a useful thread to follow in improving this answer: from a microbiological standpoint, an egg cell is equally alive regardless of whether it has been fertilized or not, and the "beginning of life" was an event which occurred roughly 3.5 - 4 billion years ago.
    – user42059
    May 19, 2019 at 4:54
  • 1
    @Oddthinking Sources have been added to address the first part of your comment, but the second is pure nonsense. An undefined quantity cannot in any sense have a beginning, so life not being defined means, a prima facie, the beginning of life not being defined. May 19, 2019 at 9:39
  • Am almost sure that this "2nd part", "nonsense", "a prima facie" is handled in some high level philosophy of science or methodological epistemology paper or book as well. Find that, include it, please. It'll also make your A more attractive by naming the targets of your links with at least author/title. And corrcetion: the Princeton src is not full of misunderstandings of what's being said, readers of that page seem to misunderstand what's being said (or the reach and meaning of the quotes) May 19, 2019 at 9:53
  • Cleland et al. is an astrobiology paper, by the way. I'm a little doubtful their refutation has much to do with human life. Also, not knowing if viruses are alive for instance, doesn't mean that once you agree that some organism is alive you can't define/know when its life begins. What you're saying basically is that we can't know/define whether anything is alive. Which is probably too nihilistic for most scientists. Just because you found one paper saying that it doesn't most other scientist would agree.
    – Fizz
    May 19, 2019 at 18:02

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