I always thought I was hearing the word incorrectly - but I don't think the song was written for the show; I'll find out and post. Also, could the song writer be refering to the evolution of the autotroph - as in, they didn't drool, then they "beg...
The theme song is about the passage of time, not about zoology. Autotrophs were are not just capable of converting sunlight into food. Blue-green algae is a good example. For the purpose of this song, autotrophs began and then evolved very quickly...