In a historic address to Spain’s Parliament on June 8, Pope Leo XIV received a standing ovation after declaring that “the defense of human life is neither a partisan issue nor a confessional interest: it is a goal of civilization.”

“If life ceases to be recognized as a fundamental value, what future can our societies have?” he asked lawmakers, many of whom support abortion and euthanasia. The pope continued: “Can a community that casts into the shadows the unborn child, the elderly, the sick, those who suffer in silence, or those who depend entirely on the care of others be called fully just? Every human life must be recognized and safeguarded from conception to its natural end, in every circumstance of its existence.”

Leo’s words, echoing themes in his first encyclical Magnifica Humanitas, cut to the heart of what sustains just societies. Yet they also expose a critical weakness in our public discourse: Most people do not know that we can answer the question of when a human life starts with an objective scientific fact. Without this foundational knowledge, we lack the intellectual equipment to fully defend individual human lives or the civilization built upon them.

The biological science of human embryology, the specialized branch of biology that studies when human life begins and its early development, has been unequivocal for more than 80 years. Since 1942, the Carnegie Stages of Human Embryonic Development — the international gold standard, akin to the periodic table in chemistry — have documented this moment with precision.

At Carnegie Stage 1, fertilization begins with first contact between sperm and oocyte, marking the beginning of a new, whole, individual, and living human being. This new organism directs its own continuous, coordinated development from that moment forward. Ronan O’Rahilly, one of the world’s authorities on human embryology, states it simply: “Although life is a continuous process, fertilization is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed.” Human embryologist C. Ward Kischer similarly affirms that from this initial contact, “all subsequent development to birth of a living newborn is a fait accompli.”

In other words, we are talking about a seamless process in which stages overlap and blend, with no upgrade or second trigger required at implantation, the onset of the heartbeat, “viability,” birth, or any other milestone.

Human embryology assigns no reduced status to the unborn (the disabled, the sick, or the elderly). The embryo, fetus, infant, toddler, and so on, is the same human organism progressing through the natural stages of development along the continuum of human life.

Every human being is a human person by virtue of existing as a member of our species — not by achieving certain developmental milestones such as size, location, consciousness, sentience, or “independence.” This intrinsic reality applies equally from the tiny beginnings of one’s biological existence to natural death. The claim that biomarkers somehow change the essence of a human being (rendering someone “more fully human” later) is not science. It is a politically charged fabrication.

Many people today nevertheless embrace this destructive myth, dehumanizing the vulnerable at every stage. The comedian Bill Maher captured this mindset in 2024 when he acknowledged that pro-lifers view abortion as murder “and it kind of is,” while admitting he is “OK with that,” a casual acceptance of ending what he and others frame as a less-than-fully-human life. (Maher is the most intellectually honest of today’s talk-show hosts, and it’s therefore all the more disturbing that he should hold these sinister views.)

Our inhumanity is seen at its most stark and shameless in the case of abortion. Decades of treating the unborn as disposable “potential” humans, mere tissue, or “pre-embryos” have led to the deaths of more than 65 million unborn children in the United States alone since 1973. Many Americans shy away from the issue precisely because this dehumanization has made the unborn seem subhuman.

Consider people with Down syndrome: In the United States, an estimated 67-74% of pregnancies prenatally diagnosed with Down syndrome end in abortion. Rates climb even higher in Europe, with Iceland close to 100% and Denmark reaching 98%. The same flawed thinking underlies assisted reproductive technologies (ARTs) such as IVF, in which millions of embryonic human beings are created, often frozen indefinitely, discarded as “surplus,” or used in research, further severing early human life from its rightful place among humankind.

Not surprisingly, this horrifying, conditional view of humanity and personhood also extends to the elderly and disabled, fueling growing enthusiasm for euthanasia.

This ideology of pre- and post-human personhood is not science. It is a narrative that justifies discarding the fragile. As Pope Leo warned, when this certainty is obscured, “the most vulnerable are the first victims, and the law loses its deepest meaning: to serve and protect every person.”

How did we reach this grave place? Public perception lags badly behind scientific consensus. Many Americans still believe that the status of a human life is a subjective political, religious, or personal matter. Younger Americans are especially uninformed; nearly 40% think that a new human life begins at birth.

But people cannot think about what they do not know. Our public policies and laws must be anchored in objective truth. Recognizing all human beings as human persons from the very beginning of their biological existence is not an imposition of ideology: it is the foundation of equality and justice. It aligns faith’s witness with reason’s clearest evidence, building the “civilization of love” the pope offers us as an alternative to throwaway culture and technological reductionism.

In an era of profound bioethical challenges, from chemical abortion to advances in artificial intelligence and biotechnology, Pope Leo XIV reminds us that protecting human life at every stage is civilization’s litmus test. Until we stop ignoring the objective scientific (embryological) reality of human reproduction and human development, we will fail.

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Brooke Stanton is the co-author of “The First 56 Days of You: How Your Human Journey Begins” and the chief executive officer of Contend Projects, a registered 501(c)(3) education organization spreading the basic, accurate scientific facts about when a human life starts and the biological science of human embryology.

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