If REM was so evolutionary important it would be the first type of sleep humans get once they enter the sleep period, but it is not. Slow Wave Sleep is the first type of sleep and the overall largest component of sleep over the first half of the sleep period, compared to REM. The two types of sleep exchange distribution over night with SWS giving way to REM sleep over the course of the sleep period.
I explain sleep as being Prepairative.
It REPAIRS the body during SWS (that is why it is the first type of sleep gotten during the sleep period and where we excrete growth hormone) allowing the repair from the physical exertion and incorporates our physical learned behavior patterns as it PREPARES the organism to function the next day. REM sleep is where we incorporate our new sensory experiences into our memory and historical behavior patterns.
What I find utterly confusing is what evolutionary advantage does having less and less sense of smell offer us as a species? Why did we lose our sense of smell? I think this too feeds the narrative into why we have such consolidated sleep. Smell is a non-cortex sense and as such has its own needs of incorporation and this was found to disrupt the other types of sleep, which were more evolutionary advantageous in that memory and physical ability gave us more ability to manipulate the world around us than smell gives us protection from warning or mate identification.