The human zygote is the first developmental stage in a human life cycle.
A zygote is different in kind from sperm and unfertilized eggs, which are gametes, not organisms. The zygote is different from any random cell that has human DNA, such as your skin cells (also not organisms). The zygote is a separate and biologically unique human.
In the abortion debate, people treat this statement as if it were a belief, rather than a fact. They seem to assume the demarcation of the zygote as a human's beginning is just one belief of many, brought up only to support an anti-abortion agenda.
Here is my toddler's ABCs of Science book, part of the Baby University series. I couldn't help but notice that the final page states "Z is for Zygote. A zygote is the first stage of development in living things."
Similarly, here's a page out of the (pretty delightful) children's book You Are Stardust, which reads "You started life as a single cell. So did all other creatures on planet earth."
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Here's a photo from Chicago's Museum of Science and Industry (MSI) in late 2018. MSI had a room you can walk through that shows prenatal human development using real specimens. Just outside the exhibit is a display which reads in part "All of us start as a single cell and then begin a wondrous journey of change in—and with—our moms."
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We begin as zygotes. This is a basic (and typically readily acknowledged) biological fact. It doesn't necessarily follow that human zygotes are morally relevant or are "people"—the personhood debate is an important but separate discussion. But before we can discuss which human organisms count as "people," we need to have a shared understanding of when human organisms begin in the first place.
- Our Experience at the OMSI Prenatal Exhibit Displaying Real Preserved Children, Equal Rights Institute, April 30, 2015
- Human Beings Begin as Zygotes: Refutations to 8 Common Pro-Choice Arguments, Secular Pro-Life Perspectives, August 11, 2017
- Are viruses 'alive'? Secular Pro-Life Perspectives, February 13, 2018
- Pro-choice embryologist contradicts his own biology textbook. Secular Pro-Life Perspectives, August 31, 2018
- Hydatidiform moles and molar pregnancies, Secular Pro-Life Perspectives, November 28, 2018
- Nearly half of all fertilized eggs fail to implant, Secular Pro-Life Perspectives, December 3, 2018
- Monozygotic Twinning: Weasley brothers, flatworms, and cow clones, Secular Pro-Life Perspectives, December 5, 2018