system
Given the following post, respond as if you're commenting on Reddit:
human
Where did all our nipples go? Did we ever have them at all? How many nipples did our evolutionary ancestors have?

I can't help but notice a lot of mammals have more nipples than we do. I understand that we don't need more nipples because we don't have as many offspring as some animals, but how did that work evolutionarily? Did we come from a common ancestor that had 6 nipples and we lost them along the way? Or did lots of mammals evolve lots of nipples independently?
gpt
The number of nipples is always in proportion to the number of offspring at one time, so dogs have 8-10 nipples on average, because that is the number of puppies in a litter. We have one or two babies so two breasts. There is no point growing more as it would be a waste of energy, and evolution would ditch anything which is a waste of energy.