Caring for our newborn child has led to some thoughts on evolutionary optimization.
First, humans have big heads to accommodate our large brains, but we must be born through a rather tightly constricted canal. Thus, heads cannot be too large at birth, and thus humans are born premature and with soft heads. We cannot even crawl on our own and are entirely physically helpless. Most other mammals have offspring capable of walking from almost the moment of their birth. Marsupials are an exception, but “birth” is a kind of nebulous concept when your offspring spend an extended amount of time in a placental pouch on the outside of your body.
If humans were not born so premature, the incidence of both maternal and child death during birth would be huge. Infants would get stuck, mothers would bleed to death. And trust me, there are enough complications and risks during birth as it is in the current situation. Tilting the scales toward larger babies would not help. On the other hand, being born too premature would result in increased infant mortality.
Since humans are born premature, they require excessive amounts of care and attention. The average newborn must eat every 2 or 3 hours, including overnight. This means that her caregivers must wake up every 2 or 3 hours as well. This is near the limit of what adult humans can do. If the child had to wake up every 45 minutes to eat, then there would be two possible results: Either her parents would die from exhaustion, or we would have a very strong emphasis on communal child raising, multiple wet nurses, etc.
If the child only ate every 6 to 8 hours, which is in line with an adult sleeping pattern, then infant mortality would rise dramatically during the early months of life. This is too long for such a small person to frequently go without food and raises the risk of malnutrition, sickness, and death.
I’m also convinced that infant amnesia is an adaptation, though its probable that it has more to do with the very early stages of brain development than anything else. But being an infant is tough. Your only mode of communication is screaming. You cannot crawl or walk, you cannot even turn yourself over. You cannot lift up your own head, at least not for long. You depend entirely on caregivers for food, hygiene, entertainment, and warmth. You can barely see anything for the first couple months. You have no theory of mind and don’t understand that your caregivers are other people with their own needs, so you get upset if they have to put you down to tend to their needs. You go through drastic digestive and other bodily changes in the early months, often causing great pain (look up colic or infant acid reflux for more). Your sleep is restless and shallow. You are susceptible to many more diseases. You grow so fast that it must be uncomfortable (at least, our child has grown 2 inches per month for 2 months). If you cry out because you’re hungry or in pain, and no one comes, then you can do nothing.
I’m not sure how strongly infants feel fear, but I’d be scared out of my mind most of the time. I don’t think anyone would want to clearly remember this period of their life. Instead, we learn without conscious recall.
Finally, parents (and people in general) are programmed to find babies cute and to want to protect them. The reason for this is that babies are exhausting and demanding. Caring for one full time requires huge amounts of patience, drains your energy, and, especially if you’re a nursing mother, requires you to relinquish your bodily autonomy. If we didn’t find them so cute and hilarious, if we didn’t have some baked-in affinity for neotony, we’d have problems.
Even as it is, there are still problems. The optimizations above lead to lots of stress. Every single parent I know has had at least one minor breakdown, and many have had major ones. When you become a parent, suddenly other parents start opening up to you. Its like a club that you kind of new existed, but you had no idea what it was like to be a member. Women rarely tell other women the gory details of pregnancy beforehand, because if they did then a lot of people wouldn’t have kids in the first place. But then once they have a kid, many of them are actively planning for the next one. Couples rarely tell their unmarried friends that they broke down and cried last night because their child screamed for 5 hours and they haven’t slept at all. Etc. Everyone has some form of postpartum depression or anxiety, even if in most cases it is not severe enough for a medical diagnosis or to put anyone’s health in danger.
For all that, its almost crazy that we do have kids. But I know why we do. Despite all the exhaustion and stress, when my two month old sees me, smiles, and starts waving his arms around and cooing nonsense sounds at me to come play with him, I think “yeah, I’d be happy with having another one of these guys.”
If any of the above were pushed significantly in one direction or another, human reproduction would be disastrous. We’re in a very roughly optimized spot.