Jack Black stars as what appears to be an early homo sapiens.
In all honesty, I think this intends to be a ripoff of 10000BC
http://www.youtube.com/watch?v=IMgGnhPnSF4
I've never seen 10,000 B.C. nor was I able to watch the video because I'm not at my dad's house and he's the one with fast internet. ;) But I dunno. Kind of a weird thing for me, whenever the trailer for a movie looks stupid, it's usually pretty good and whenever it looks like one of those "Oh my God! I MUST see that!", then I see it, and it sucks. So, who knows, you might like it. :)
He also made a guest appearance on Yo Gabba Gabba!, a really trippy kids' show (which I watch while I am, ah, yeah).
It was a good episode.
Did he sing and make musical instruments with his voice?
He did indeed, I'm looking up the episode online for you to see it as we speak.