Where Does Bending Really Come From?

Twitter this morning prompted some really interesting discussions about the world in Avatar: The Last Airbender and The Legend of Korra. People were mainly discussing the relationships at first which prompted a thought from myself, "Perhaps Tenzin is with Pemma instead of Lin because maybe Lin didn't want children." Now, there's no firm evidence to suggest that Lin wouldn't have wanted children, necessarily, but what if she didn't?
Incompatibility over the desire for children is an unfortunate reason to not be with someone, but it's very important to many people. I can especially imagine why it would be important for Tenzin, since he would have been (at the time) the only Airbender left. It's possible that Tenzin felt incredible pressure to pass on Airbending. Or perhaps, he was worried that if he were with Lin and if they had children, that none of them might have been Airbenders. It's impossible to know why, but it's something to think about.

This left me wondering about what it takes to becomes a bender. Is it innate or is it something that can be learned? The show certainly portrays bending as an innate skill. Benders must descend from the particular tribe that has their skill and it even seems as if a bender must have a bender for a parent (I've been trying to find proof or counter to this point and haven't been able to).

The show does seem to contradict itself w/ bending being an innate skill. For example, all of the bending origin stories are of people learning the skill from something else. Firebenders learned it from the dragons. Waterbenders learned it from the moon and ocean. Earthbenders learned it from the badgermoles. Airbenders learned it from the sky bisons. If the original benders were able to learn it from something else, then what would keep a non-bender from learning it? Are these stories false/folklore?

The "learned" aspect of bending is reinforced by the Airbenders, though. From their own description, the Air Nomads are so spiritual as a people that all of them can Airbend. If, as the origin stories and the implications of the Air Nomads' spirituality are true, then bending doesn't actually seem to be an innate skill, because then why would it be more prevalent with the Air Nomads that with the other tribes? This would appear to be at odds with many of the themes of the show. The first season contained a big panic because Aang was The Last Airbender. If bending isn't some hereditary trait then that's not as big of a deal as the show portrays it; somebody could relearn it from the extant texts. Furthermore, if bending is something that can be learned, that undermines the Equalist movement.

The spiritual connection aspect of the Airbenders and the spirit bridge aspect of the Avatar seem to imply that bending isn't something that was learned from animals or by watching the tides but has to do with one's connection to the spirits/spirit world. This is affirmed by the Waterbenders' weakness when the moon (moon spirit) is killed. But the existence of the dragons Ran and Shaw confuses this. Do the bending animals have a connection to the spirit world?

The existence of the Avatar is the biggest check mark in the spiritual connection column. Not only is this because of his role as a bridge between the spirit world and the physical world, but the way it passes from tribe to tribe upon death. Such a thing traveling around the world upon death is not something that would happen naturally. If bending comes from the spirits, keep in mind that it could still have a hereditary aspect, it just would have originated with the spirits. Something I wonder is what would have happened if Aang died a natural death with no children. Would the Avatar cycle have continued and a) skipped over air b) stalled and ended or c) created a new airbender?

Whether bending is hereditary/innate or learned is something that the show lands on both sides of. The show runs with the idea that it's hereditary/innate but so many of the clues and event in the show indicate otherwise. It's an important distinction because it definitely colors and changes so much of the tension in the show. I'm wondering/hoping that The Legend of Korra will shine some more light on this issue.