Actually, I stand corrected - not the multicellular life evolved once, but the eucaryotic cell. According to the book, eucaryotic cell evolved by acquisition of mitochondria. Energy generated by mitochondria allowed the eucaryotic cells to be much bigger than procaryotic ones, and to support multiple different organelles.
Yup! The formation of the first Eukaryotic cell is a mystery. Eukaryotes have features of both bacteria and archea, yet doesn't have a clear origin in either of them. Here's a book that goes into more depth on the important stages of evolution: http://www.amazon.com/Major-Transitions-Evolution-Maynard-Sm...
On a tangent, my dismissiveness of creationists has only increased since reading The Major Transitions in Evolution. There are some really perplexing things in early evolution that we don't understand at all. And yet creationists like to talk about chimps and rocks.
