Evolutionary Sleep - more polyphasic? | Polyphasic Sleep Experiment Discussion | Forum

Much more recently than evolutionary timescales, what we think of as "normal" monophasic sleep was not at all the norm.  See, for example, the interesting article Sleep we have lost. Doesn't really speak to your question, but is nonetheless interesting.  More on point, a couple of anthropological studies among hunter-gatherer tribes throughout the world are cited at the beginning of Stampi's book:

It is worth mentioning that anthropological studies conducted in tribes 
active at night show that human sleep can be highly polyphasic in certain 
cultures. Although they have different cultures and ways of life, both the 
Temiars of Indonesia and the Ibans of Sarawak have similar polyphasic 
sleep-wake behaviors (Petre-Quadens, 1983). Their average nocturnal sleep
episode duration ranges between 4 and 6 hr, and nighttime activities 
(fishing, cooking, watching over the fire, rituals) at any one time involve 
approximately 25% of the adult members. Daytime napping is very 
common in both tribes: at almost any time of day, about 10% of the adult 
members are asleep. Whatever the cause of these polyphasic sleep patterns,
whether the expression of an inborn ultradian rest-activity tendency or 
other factors, such populations exhibit extremely flexible and fragmentary
sleep-wake cycles. The minimal contact with modern civilization could be 
one of the reasons for the preservation of this possibly ancestral sleep 
pattern. (pp. 8-9)

This doesn't speak directly to the question of how our paleolithic ancestors lived, because each society has to adapt to its own climate and circumstances, but it is evidence in favor of at least a greater degree of flexibility in sleep/wake cycles than we currently observe in industrialized cultures.