When life begins is, of course, the central disagreement that fuels the controversy over abortion. Attacks on abortion rights are now more veiled and indirect—like secret videos pointing to Planned Parenthood’s fetal tissue donations, or state legislation that makes operating abortion clinics so onerous they have to shut down. But make no mistake, the ultimate question is, when does a fetus become a person—at fertilization, at birth, or somewhere in between?

Here, modern science offers no clarity. If anything, the past century of scientific advances have only made the answer more complicated. As scientists have peered into wombs with ultrasound and looked directly at sperm entering an egg, they’ve found that all the bright lines they thought existed dissolving.