The Evolution of sex

I am by no means an Expert on plants so Help me out here
Recently I was studying Plants(I don't normaly do this) and I got to thinking about angiosperms and how the reproduce By Sexual means.
Correct me if I am wrong but wasn't our last Common ancestor asexual or thought to be? Did both Evolve independent from each other? It seems we must have shared a common ancestor.... to me it seems unlikely a very similar Mutation like that would happen again... But I guess it could happen though.. However aren't those plants thought to have evolved from gymnosperms which are asexual aren't they?
thanks in advance