Since you asked, I will try to answer.
First of all, I reject the notion that "human beings" emerge from anything other than human beings/ organisms.
It's a scientific fact that human beings (mammals) do not undergo metamorphosis as frogs and butterfiles do.
Unlike frogs and butterflies, human being (mammals) are the same organism from conception until death.
So, it's obvious (and a biological fact) that a human being - even in the zygote stage of their life is (biologically) a human organism.
2nd Scientific fact is that all living organisms are "beings." They exist.
So, a human organism in ANY stage of their life is BIOLOGICALLY a "human being." That's not my opinion, it's scientific fact.
Gamtes cells (sperm and eggs) are hapoloid reproductive cells that exist solely to merge and to produce new members of their species.
A Conception, their individual lives as reproductive cells are over. The life of the new organism they merged to create has began.
Look at the synonyms for the word "
conception." They include "originate, origin, start, beginning, and even birth" for good reason.
Conception is the biological point when your parents first became your biological parents.
Think about it. What more did your biological father do to become YOUR biological parent. . . than to contribute a single sperm cell to the moment of YOUR conception?
It is illogical to conclude that conception did not begin YOUR life but that conception did begin his status as YOUR biological parent.
Furthermore, Conception is when an organisms starts to age.
"Aging is a very natural process. It begins at conception and continues throughout the life cycle."
Any human being's life can be biologically traced all the way back to the moment of conception but no further.
You spoke in your post about what "qualifies" as " human being."
Scientifically / Biologically - the only qualifications are that it be a living human organism.
". . . the scientific evidence supports the conclusion that a zygote is a human organism and that the life of a new human being commences at a scientifically well defined “moment of conception.” This conclusion is objective, consistent with the factual evidence, and independent of any specific ethical, moral, political, or religious view of human life or of human embryos. "- Maureen L. Condic Senior Fellow Westchester Institute for Ethics & the Human Person Associate Professor of Neurobiology and Anatomy at the University of Utah School of Medicine
I could go on. . . but I think you get the point.
My answer is Conception.
And it's also the point now recognized in our Fetal Homicde laws.