How would people's bellys look thousands of years ago?

Our ancestor species likely bit the umbilical cord much like other animals today do in the wild. They would of course still have belly buttons as this is simply the point of attachment of the umbilical cord. Today, doctors do cut the umbilical cord but only at a certain point. Newborns still have a length of umbilical cord attached which is pinched by a plastic piece for the first several days after birth. Eventually it dries up an falls off. The same thing would happen in our ancestors.