It is true that biological investigation can only tell you who is a member of the human species, meaning humans of all ages and stages of development, both sexes, and all sizes, colors, and races. Biological investigation cannot tell you why it matters.
Gone are the days when great theorists of government like Thomas Jefferson could say that it is "self-evident" that "all men are created equal" and that they are "endowed by their Creator with certain inalienable rights" --- although this is, and ought to be, the foundation of American political philosophy.
Unfortunately, modern philosophy has degerated to the point where, to many people, it just means "personal opinions dressed up in verbal pomposity."
A philosophy worthy of the name will be free of internal contradictions, coherent, reasonable, and based upon observable truths, including social, historical, sociological and psychological. When it is rooted in identifying what causes human nature to flourish and be perfected, this tradition is called "Natural Law."
Religious thinkers would say that. since human beings were created in the image and likeness of God, we can only discern what would constitute "the perfection of the human nature" by looking to God in Christ.
However, for those who don't believe in God, there is still a Natural Law based on what "most people" are convinced constitutes individual human virtue and excellence, and the collective flourishing of human communities over time.
OK. The bottom line, according to the foundations of American political philosophy, is this: to prove that an entity is a human being, suffices to prove that this entity is endowed with rights, the protection of which is the basic function of government.
"you" don't look like "you" did 10 years ago, or will look like in another 10. "you" became "you" at the moment of conception.