The primary curves of actually develop when the fetus is still growing inside the womb. The secondary curves occur when the baby is born. The cervical curvature starts to form when the head is lifted while the lumbar curvature forms when people walk.
It is important for people to have the right posture whether they are sitting, standing, or walking because this will make a huge difference in the curvature that will be formed. Those who have better posture will also have better curvature and will also be less prone to various diseases and problems that are related to posture.