Still, trying this in a monkey is the closest approximation to what could happen in a human. “This shows that you can get a pregnancy started or at least trigger the macaque’s hormonal system to think that it’s pregnant,” says Hank Greely, director of the Center for Law and the Biosciences at Stanford School of Medicine. “It says there’s some evidence that maybe it could give rise to a baby monkey.”

Embryo models give researchers an alternative without having to rely on the real thing. But as they get more sophisticated, they are raising concerns of their own. “I think what we’d really like to know is, can an embryo model give rise to a living organism?” Greely says. “If it can, then it should be treated like an embryo. If it can’t, then it doesn’t need to be treated like an embryo.”

To answer that question, Greely is in favor of scientists conducting the kind of experiments that the new paper outlines. He feels it would be unethical to do so in people, because any resulting babies could end up with birth defects or genetic disorders.