Nothing.
As soon as the baby is breathing on its own, the cord stops pulsing. There is no more blood transfer from mother to baby via the cord.
The mother will then deliver the placenta, which now hangs off the baby like a lump of meat.
Early humans probably didn’t clamp the cord but they probably bit it, followed by tying it. Even if they did nothing, within a week or so, the cord will drop off the baby and it’s all good.