The effects of the tree-to-ground sleep transition in the evolution of cognition in early Homo

Just as the study of great apes aids in the understanding of early hominid evolution, so too can contemporary dream research help in the understanding of ancient hominid dream life and cognitive evolution. It has been proffered that a major leap forward in the cognitive evolution of hominoids may first have occurred in the building of nests, and a second major step may have been the full transition to ground sleep. These two changes may have begun a modification of the quality and quantity of hominid sleep, which in turn may have enhanced waking survival skills through priming, promoted creativity and innovation, and aided the consolidation of procedural memories. Current dream research with children, adults, animals and the dreams of modern hunter-gatherers, appears to support the hypothesis that these changes in sleep may have been important to the cognitive evolution of hominids from Homo habilis to Homo erectus. The evolution of sleep and dreaming has recently received serious treatment in journals of psychology and cognitive science, but these efforts do not typically incorporate paleoanthropological evidence (eg, Franklin & Zyphur 2005; Revonsuo 2000; Walker 2005). However, just as the study of great apes aids in the understanding of early hominid evolution, so too can contemporary dream research help palaeoanthropologists understand evolutionary developments in hominid sleep and perhaps even aspects of cognition. It has been previously proffered (Fruth & Hohmann 1996) that one major leap forward in the evolution of the cognitive abilities of hominoids may first have occurred in the building of nests. We intend to build on this argument and will suggest that the hominid switch from arboreal to ground sleep prompted selection for and against specific aspects of sleep and that features of Homo erectus niche selected for others. We will present evidence from contemporary sleep research and from the palaeoanthropological record that changes in the sleep of hominoids about 2 million years ago, during the tree-to-ground sleep transition, may have aided the evolution of cognition. 1 Characteristics of sleep Researchers typically characterise sleep by five distinct EEG (electroencephalogram) stages. Stages 1 and 2 are considered light stages of sleep. Stage 1 is typically brief and usually occurs at the onset of sleep. Stage 2 accounts for about 50% of sleep and is distributed fairly evenly throughout a typical 8-hour sleep period. The brain waves during these stages are of lower amplitude than other stages but higher in frequency (ie, > 15 Hz). Stage 2 is also characterised by sleep spindles, which are periodic brief bursts of 12-16 Hz, occurring at a rate of about two per minute. Humans are typically awakened easily from these two stages and can report thoughts, ideas, and dreams after awakening from these stages. Stages 3 and 4 are considered deeper sleep stages, and are also referred to as slow-wave sleep (referring to its wave frequency). Slow-wave sleep has a frequency of 0.5 Hz to 3 Hz (called delta waves) and high amplitude (compared to other sleep stages). In humans, it is the stage in which it is least likely that