Sentence:  It is likely that the long evolutionary trajectory of "Mycoplasma" went from a reductive autotroph to oxidative heterotroph to a cell-wall–defective degenerate <WSD>parasite</WSD>. This evolutionary trajectory assumes the simplicity to complexity route of biogenesis, a point of view that is not universally accepted. 
Meaning of the word marked with <WSD> tags:  An organism that lives on or in another organism of a different species, deriving benefit from living on or in that other organism, while not contributing towards that other organism sufficiently to cover the cost to that other organism.