We know when life begins.
It began billions of years ago and has continued in unbroken seccession ever since.
When a human life begins is not so certain. Being human is a state of mind, not DNA, not physical form, not whether our hearts beat or we take a breath. We aren't human until we can stand up (even if only figuratively) and declare ourself to BE human. A fetus can't do that. Not even a newborn has that capacity. Their "human rights" are provisional and dependent on the desires of their parents or guardians. When a child defiantly says, "I'm a person too!" then they've crossed the line. Yes, you are, and I'm proud of you.