Humanity as Idea Virus
There are very few cases of humans who somehow made it to adulthood completely without human contact, but the few we have highlight a very important aspect of human beings.
Examples of such are Caspar Hauser, the random 'wolf child' legend, and more recently, a poor child here in the USA whose mother severely neglected her. This poor creature, left with minimal human contact, was found severely malnourished, lying alone in a room egregiously filthy and in disrepair. She had no reaction to human input, nearly non-existent motor skills, and a look in her eye not unlike a simple farm animal. Even if we only have her as the example to draw from, the implications of failing to be raised by humans is devastating- I do not have further data that declares her perfectly normal, so, the following is conjecture, but I wouldn't suggest otherwise even if the child were certainly physiologically "normal".
We are well aware that a human infant is an extremely complex learning machine, and that there are distinct stages in human development where certain types of learning flourish.
I would run further with this line of thinking, perhaps off the map and into the darkness, and suggest that there are interactions between child and parent- involved, critical, and absolutely essential interactions and lessons- which take a human animal and, over time, mold it into a functioning member of society. Maybe you can think of these subtle things as the breath between words in a song- the paper the words of the song are written on, etc.
On an assembly line, a bare vehicle frame rolls out, and this is the beginning. It glides along its path, and at each stop essential parts are added. Without these properly constructed and attached parts, at the end of the process, we do not have a complete vehicle.
If the car example were a human child- there is no complete list of required "parts" in human development, so we can't say for sure what part is missing when a child is "at the end of the process".
My feeling is this- humanity is an idea virus your parents give you, and if they are not around you enough to properly transmit it to you, your copy is incomplete and you end up with some trouble or other, or perhaps you luck out and have no problems at all.
If your parents don't have a complete copy of the virus, you won't get it no matter what they do. So you spend years trying to figure stuff out on your own, while other people waltz through life as if they knew what to do at each step along the way.
This leaves a lot of fun things to think about- and so I will. More later, perhaps.