Most mammals have mammaries that are purely functional and only round out due to filling with milk. Human female breasts are full for other reasons, too full in fact to be baby-friendly. Most other mammals also feed their offspring in plain view of others of their species, which means there is plenty of opportunity to observe how it is done. In human cultures, we debate the morality of -- gasp! -- breastfeeding in public. When it is done in public at all, it is handled very discretely so no one gets a glimpse of someone's breast or, god forbid, nipple. Not exactly a learning opportunity for any other women who happen to be around who might someday have a baby themselves and might want to breastfeed.

Part of my experience with this subject: I breastfed my oldest until he was nearly 2 and my youngest rejected the breast very early (due to lactose intolerance), breastfeeding full-time for only 10 days and part-time until he was 6 weeks old. And I helped my sister address some issues she had with her baby so she could successfully breastfeed.