The First Major Leap in Cognition: The Tree-to-Ground Sleep Transition

1. The First Major Leap in Cognition: The Tree-to-Ground Sleep Transition -Chapter 8

2. Homo Erectus • Reduced Sexual Dimorphism and more helpless, altricial infants • What caused the change? • Homo Erectus’ home range was 10 times larger than those of Australopithecines. • Required spatial relations, territory mapping, memorization of important locations • Mobile in new areas with fewer trees • Slept on the ground, instead of trees

3. First Leap in Cognition • Australopithecines and early Homo demonstrated similar behavior. • Light body, long arms, must have lived/slept in trees (safer, but prevented further evolution). • The demands of life on the ground caused a real grade shift in evolution. • The tree-to-ground sleep transition allowed for height and weight increases. • Prevented the danger of falling out of trees.

4. Characteristics of Sleep Light Sleep • 5 different types based on EEG readings: • Light Sleep • Stage 1: brief, occurs at onset of sleep. • Stage 2: 50% of all sleep, evenly distributed. Brain waves lower in amplitude, but higher in frequency (>15 Hz). • Thoughts, ideas, dreams commonly remembered if awakened during these stages.

5. Characteristics of Sleep • Slow-wave sleep (in terms of frequency) • If awakened during stages 3 and 4, unlikely to remember any coherent dreams • Activation of parasympathetic nervous system, reduced heart rate, and increased gastrointestinal system • Most difficult stage to awaken from Slow-wave sleep

6. Characteristics of Sleep REM • Rapid Eye-Movement (5th stage) • Relatively new • 140 million years ago (compared to 200 for slow-wave) • Like Stage 1, low amplitude and high frequency • But there are bursts of eye movements (hence the name) • Sleepers report vivid dreams if awakened during REM sleep. • Last 1/3 of 8-hour sleep cycle • 25% of sleep

7. Characteristics of Sleep • The onset of sleep in most primates (including humans) involves a hypnagogic jerk. • Sudden muscle reflexive movement that oftentimes awakens the sleeper • Cause is unknown • But it is theorized that the brain is misinterpreting the muscle relaxation as falling out of a tree • Also, this reflex may have caused the sleeper to readjust himself in a nest or tree to prevent falling

8. Characteristics of Sleep • All sleep involves a form of muscle relaxation, but the deepest muscle relaxation occurs at the onset of REM sleep. • This muscle atonia (paralysis) seems to prevent us from acting out our dreams. • Some people sense this muscle atonia, and their dreams reflect it. • It has been theorized that the interpretation of atonia is the reason falling is a common theme in dreams.

9. Characteristics of Sleep • Along with motor paralysis, some of the central executive’s functions are specifically deactivated during REM sleep. • The Dorsolateral Prefrontal Cortex (complex problem-solving, etc.) is deactivated; while the Ventromedial Prefrontal Cortex (decisions related to emotions), amygdala (fear), and anterior cingulate gyrus (attention/interpersonal decisions) are activated. • This deactivation may prevent the sleeper from selecting and attending to dreams or external stimuli

10. Characteristics of Sleep • There is a great deal of evidence for the functions of sleep: • Lima et al., (2005)  restorative and anti-predator function of sleep • Since hominins have relatively poor night vision, it might be better for them to be sleeping away out of harm’s way rather than stumbling about. • Believed that the different stages of sleep represent an evolutionary trade-off between more and less vigilant stages of sleep. • Coolidge, (2006); Walker, (2005)  Mnemonitive and innovative functions

11. Primate and Hominin Nesting • Monkeys and lesser apes select sleeping sites, but do not modify them (nests). • These sites tend to involve the risk of falling but are generally safe from predators. • Construction of nests is a derived feature for the great apes. • The Chimpanzees, Orangutans, and bonobos build their nests in trees, while gorillas build their nests on the ground. • Great apes often nest near food sources. • Vocal exchange is common after settling down for the night.

12. Primate and Hominin Nesting • Sabater et al. (1997) proposed that early hominins also nested in trees. • Limb features of fossils suggest climbing anatomy: • Longer arms relative to legs • Length of forearm relative to upper arm • Headward-oriented shoulder cavity • Curved fingers • Early hominins spent more time and traveled longer distances on the ground. • But they stayed away from treeless savannahs before Homo Erectus. • Unlikely early hominins slept on ground • Therefore, they retained an ape-like pattern of sleep.

13. Early Homo and Homo erectus • Ways that Homo erectus was different than previous hominins: • Nariokotome’s anatomy was very similar to humans’. He had modern bipedalism and no remnants of climbing anatomy in his forelimbs. • His maturation pattern was more like modern humans and may have suggested secondary altriciality. • Sexual Dimorphism is smaller in Homo erectus. • All of these changes suggest an adaptation to hot, open environments and longer traveling on the ground. There are no remnants of climbing anatomy. • This is where hominins began to sleep on the ground.

14. Early Homo and Homo erectus • Fruth and Hohmann (1996) • “Great leap forward” in cognition • Nests aided the transfer of information and may have helped establish memories through increases of slow-wave and REM sleep • Coolidge and Wynn add to this, claiming that sleep on the ground further increased slow-wave and REM sleep by preserving the general integrity of sleep.

15. Early Homo and Homo erectus • The writers of the book propose 3 major benefits regarding the transition to sleep on the ground. • Threat simulation, social rehearsal, priming • Creativity and innovation • Procedural memory consolidation and enhancement, including procedural memories for visual-motor skills and visual-spatial locations

16. Threat Simulation, Social Rehearsal, and Priming • Modern children’s dreams may provide possible insight into the life of early hominins. • Van de Castle (1983) and Domhoff (1996) found that animal characters made up the largest proportion of children’s dreams. • But these animals were not ones normally encountered in day-to-day life. • Children’s dreams tend to be more aggressive than adult dreams, as well, including aggression involving animals. • Revonsuo (2000) interprets these findings in terms of a threat-simulation theory. • Threat-simulation theory = the present dream-production system simulates threatening events in the ancestral environment.

17. Threat Simulation, Social Rehearsal, and Priming • Recurring dreams (especially nightmares) in adults may shed some light on ancestral life, as well. • Robbins and Houshi (1983) found that the number one recurring theme in dreams of college students was anxiety as it pertained to being pursued or threatened. • Revonsuo said it is unlikely that these anxieties are relevant to one’s daily life, but rather they reveal ancestral concerns. • He claimed that dreams evolved to rehearse threat perception and threat avoidance. The advantage would come into play when repeating dream themes enhance and prepare waking threat-avoidance skills (priming).

18. Threat Simulation, Social Rehearsal, and Priming • Even Freud inadvertently provides evidence for the threat-simulation hypothesis. • He noted 2 dreams that may have been related to ancestral anxieties: • Examination Dream • Embarrassment of Being Naked Dream • They are both intended to prime the dream to prepare him for his waking life.

19. Threat Simulation, Social Rehearsal, and Priming • Franklin and Zyphur (2005) proposed that deactivation and activation of particular areas of the brain served not only to keep dreamers asleep but also to remain unaware that they are dreaming. • They believe that dreams prepare us for our daily lives by rehearsing real-life scenarios, and that they influence our waking encounters in positive ways. • This could happen subconsciously, so that we don’t even know when it happens. • This may happen most significantly in social situations, as more streamlined interaction would provide access to resources. • Evidence is the increased activity in the parts of the brain that mediate social/interpersonal functions.

20. Creativity and Innovation • There is a great deal of anecdotal evidence linking dreaming to creativity: • Goya, Blake, Rousseau, Dali, and Magritte are all artists who claimed their work was based on a dream. • The musical riff to “[I Can’t Get No] Satisfaction” by the Rolling Stones supposedly came from a dream. • Robert Louis Stevenson dreamt the story for Dr. Jeckyll and Mr. Hyde while pondering the duality of humans. • The Russian chemist Dmitri Mendeleyev claimed he conceived of the periodic table in a dream.

21. Creativity and Innovation • Hartmann (1998) speculated that dreaming allows the dreamer to make connections between disparate and often contradictory ideas. • Believed dreams contextualize emotions and create an explanatory metaphor for the dreamer’s emotional state. • Dement (1972) • Undergraduates were given a problem to solve before sleeping. Less than 1 % of the answers came in dreams. • There is not much empirical evidence linking creativity and dreams.

22. Creativity and Innovation • But Wagner et al. (2004) recently revived the idea. • They gave participants a cognitive task that required the subject to gain insight into a hidden abstract rule. • Participants either slept or stayed awake. • Twice as many participants who slept figured out the hidden rule. • The researchers concluded that sleep had caused a novel restructuring of the original representations of the procedural memory. • Probably mediated by the hippocampus, related medial temporal lobe structures, and prefrontal cortex. • Sleep may serve as a catalyst for insight.

23. Procedural Memory Consolidation and Enhancement • Memories are stabilized and consolidated during sleep. • Winson (1990) reasoned one purpose of REM sleep might be the strengthening and consolidation of visual-spatial (procedural) memories. • In terms of early hominins, increases in REM sleep due to nesting may have strengthened waking memories critical to survival. • The hippocampus probably played a large role in consolidating skills such as nest building and tool building/use.

24. Procedural Memory Consolidation and Enhancement • There is not much evidence for the enhancement of declarative memories (facts/verbal material) during sleep. • But an important factor for the effect sleep has on declarative memories is the emotional valence of the stimuli. • Wagner et al. (2001) found that memories with strong emotional valence were retained better than neutral memories. • However, there is plenty of evidence that procedural memory is enhanced by sleep. • Walker (2005) claimed that the initial stabilization stage of procedural memory (formation of memory representations) does not rely on sleep, but the second stage (consolidation-based enhancement) does rely on sleep.

25. Procedural Memory Consolidation and Enhancement • Other studies: • Stickgold et al. (2000) found that consolidation enhancement of procedural memories occurred during sleep, but only the first night. • Smith and MacNeill (1994) found that deprivation of sleep (especially stage 2) impaired retention. • Aubrey et al. (1999) proposed that less complex procedural memories are consolidated in stage 2, while more complex procedural memories are consolidated in REM sleep.

26. Procedural Memory Consolidation and Enhancement • Summary: • There is little evidence for the enhancement or consolidation of emotionally-neutral declarative memories in sleep. • Some types of procedural memories may be consolidated/enhanced by particular stages of sleep or the sequence of the stages of sleep. • The most recent research suggests that visual-spatial procedural tasks are enhanced by slow-wave sleep and hippocampal reactivation post-training.

27. Procedural Memory Consolidation and Enhancement • According to Walker and Stickgold (2004), ground sleep may have caused greater success in visual-motor procedural memories, waking behaviors, harm avoidance, better mate selection, enhanced exploration abilities, and even creativity.

28. Procedural Memory Consolidation and Enhancement • 2 Conclusions concerning the evolution of sleep: • Strong Conclusion—Selective pressure against lengthened stage 4 and REM sleep was released with the advent of ground sleep. • Weaker Conclusion—Features of the Homo erectus adaptive niche selected for lengthened Stage 4 and REM sleep.

29. Conclusion • Mode 2 tools appeared rather suddenly, as if because of a creative burst. • It is possible that this creative burst came about from the transition to ground sleep, and therefore, the lengthening of sleep.