‘Platypuses are unique in that they’re part of a group of mammals called “monotremes,” which diverged from the rest of mammals very early in the evolution of the mammalian line. Because their REM is so similar to our own, Siegel’s findings “suggest that the immediate reptilian ancestors of the early mammals either had REM sleep or had a state with many of the neural correlates of REM sleep, or that REM sleep evolved very rapidly in the mammalian line.”‘