Not humans, humans always gave birth with a placenta. Millions of years ago, a mammal was infected with a virus that ended up resulting in a structure that became the placenta.

We can look at the non-placental mammals around to deduce how our non-placental ancestor would have spawned offspring. The obvious is egg laying, like almost every animal does today besides placental mammals. We see this in monotremes like the platypus. The other option is to do it like marsupials where offspring is born ridiculously underdeveloped, and spends its early life in a pouch drinking it’s mothers’ milk.

Live birth is observed in some non-mammals, like sharks. Those shark species sustain themselves by eating their siblings while still in the mother