Additionally, Chameleon isn't a song where you need to be overly concerned about the key it's in. You just play the bassline and keep it funky. That's all you need to do as a bassist which makes it a great song to learn to jam. You just keep the pocket. There are better examples to use if you want to worry about key signature.
You're getting at the reason why I distinguished between "soloing" and "jamming" in my comments above.
I define "soloing" as stepping in to the spotlight and playing a part that draws attention to yourself, becoming the temporary "musical leader." Your solo could be improvised on the spot, or it could be a pre-composed solo that you worked out ahead of time. On the other hand I think of "jamming" as casual music-making with a strong element of spontaneity. Maybe there is no rehearsal, or you are playing with new musicians for the first time, or it could even just be getting together in someone's garage and playing for fun. Sometimes in a "jamming" situation, you aren't even playing "songs" in the strict sense of the word, you are just loosely improvising.
A lot of times we are both "jamming" and "soloing" at the same time. But (according to my definitions, at least) it is certainly possible to have one without the other!
I think if we are "soloing" on Chameleon, then you're right, it's not terribly important to know what "key" the song is in. So long as your solo sounds good, who cares the intellectual process you used to create that solo? There are great soloists who think in terms of note names, soloists who think in terms of key centers, soloists who think in terms of shapes & patterns, soloists who think in terms of sound color & texture, and so forth. All of these approaches to soloing can be valid, so long as they result in a solo that sounds good.
But if we are "jamming" on the other hand, then I think it's vitally important to know the key of the song, otherwise you could have a musical train wreck. If the leader of the jam calls out "Chameleon in E" then you'd darn well better understand what that means. Seems at least once a week, we have a thread on TalkBass where someone got fired or failed an audition because the bassist was playing in a different key than the rest of the band. And that pretty much never sounds good.
