Jack learned to crawl before he learned to sit. He was very different from my other kids in this way. The others learned how to sit well first, and then learned to crawl and the later happened after eight months of age. Jack started crawling at six and a half months, and after crawling for two weeks, on his seven month birthday, I found him like this when I left him for a minute to go the bathroom. This was the very first time he sat by himself for more than five seconds. He sat for about three minutes, giving me plenty of time to get a few pictures.
Now, fours days later, look how much better his posture is.