Sleep intensity and the evolution of human cognition

Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers

Sleep is essential for survival, yet it also represents a time of extreme vulnerability to predation, hostile conspecifics and environmental dangers. To reduce the risks of sleeping, the sentinel hypothesis proposes that group-living animals share the task of vigilance during sleep, with some individuals sleeping while others are awake. To investigate sentinel-like behaviour in sleeping humans, we investigated activity patterns at night among Hadza hunter-gatherers of Tanzania. Using actigraphy, we discovered that all subjects were simultaneously scored as asleep for only 18 min in total over 20 days of observation, with a median of eight individuals awake throughout the night-time period; thus, one or more individuals was awake (or in light stages of sleep) during 99.8% of sampled epochs between when the first person went to sleep and the last person awoke. We show that this asynchrony in activity levels is produced by chronotype variation, and that chronotype covaries with age. Thus, asynchronous periods of wakefulness provide an opportunity for vigilance when sleeping in groups. We propose that throughout human evolution, sleeping groups composed of mixed age classes provided a form of vigilance. Chronotype variation and human sleep architecture (including nocturnal awakenings) in modern populations may therefore represent a legacy of natural selection acting in the past to reduce the dangers of sleep.

The evolution of human sleep: Technological and cultural innovation associated with sleep-wake regulation among Hadza hunter-gatherers

Sleep is necessary for the survival of all mammalian life. In humans, recent investigations have generated critical data on the relationship between sleep and ecology in small-scale societies. Here, we report the technological and social strategies used to alter sleep environments and influence sleep duration and quality among a population of hunter-gatherers, the Hadza of Tanzania. Specifically, we investigated the effects that grass huts, sound levels, and fire had on sleep. We quantitatively compared thermal stress in outdoor environments to that found inside grass hut domiciles to test whether the huts function as thermoregulated microhabitats during the rainy season. Using physiological equivalent temperature (PET), we found that the grass huts provide sleep sites with less overall variation in thermal stress relative to outside baseline environments. We also investigated ambient acoustic measures of nighttime environments and found that sound significantly covaried with sleep-wake activity, with greater sound levels associating with less sleep. Finally, after controlling for ecological variables previously shown to influence sleep in this population, fire was shown to neither facilitate nor discourage sleep expression. Insofar as data among contemporary sub-tropical foragers can inform our understanding of past lifeways, we interpret our findings as suggesting that after the transition to full time terrestriality, it is likely that early Homo would have had novel opportunities to manipulate its environments in ways that could have significantly improved sleep quality. We further conclude that control over sleep environment would have been essential for migration to higher latitudes away from equatorial Africa.

Cancer: A disease at the crossroads of trade-offs

Central to evolutionary theory is the idea that living organisms face phenotypic and/or genetic trade-offs when allocating resources to competing life-history demands, such as growth, survival, and reproduction. These trade-offs are increasingly considered to be crucial to further our understanding of cancer. First, evidences suggest that neoplastic cells, as any living entities subject to natural selection, are governed by trade-offs such as between survival and proliferation. Second, selection might also have shaped trade-offs at the organismal level, especially regarding protective mechanisms against cancer. Cancer can also emerge as a consequence of additional trade-offs in organisms (e.g., eco-immunological trade-offs). Here, we review the wide range of trade-offs that occur at different scales and their relevance for understanding cancer dynamics. We also discuss how acknowledging these phenomena, in light of human evolutionary history, may suggest new guidelines for preventive and therapeutic strategies.

Human grooming in comparative perspective: People in six small-scale societies groom less but socialize just as much as expected for a typical primate

OBJECTIVES

Grooming has important utilitarian and social functions in primates but little is known about grooming and its functional analogues in traditional human societies. We compare human grooming to typical primate patterns to test its hygienic and social functions.

MATERIALS AND METHODS

Bayesian phylogenetic analyses were used to derive expected human grooming time given the potential associations between grooming, group size, body size, terrestriality, and several climatic variables across 69 primate species. This was compared against observed times dedicated to grooming, other hygienic behavior, and conversation among the Maya, Pumé, Sanöma, Tsimane', Yanomamö, and Ye'kwana (mean number of behavioral scans = 23,514).

RESULTS

Expected grooming time for humans was 4% (95% Credible Interval = 0.07%-14%), similar to values observed in primates, based largely on terrestriality and phylogenetic signal (mean λ = 0.56). No other covariates strongly associated with grooming across primates. Observed grooming time across societies was 0.8%, lower than 89% of the expected values. However, the observed times dedicated to any hygienic behavior (3.0%) or "vocal grooming," that is conversation (7.3%), fell within the expected range.

CONCLUSIONS

We found (i) that human grooming may be a (recent) phylogenetic outlier when defined narrowly as parasite removal but not defined broadly as personal hygiene, (ii) there was no support for thermoregulatory functions of grooming, and (iii) no support for the "vocal grooming" hypothesis of language having evolved as a less time-consuming means of bonding. Thus, human grooming reflects decreased hygienic needs, but similar social needs compared to primate grooming.

Does selection for short sleep duration explain human vulnerability to Alzheimer's disease?

Compared with other primates, humans sleep less and have a much higher prevalence of Alzheimer 's disease (AD) pathology. This article reviews evidence relevant to the hypothesis that natural selection for shorter sleep time in humans has compromised the efficacy of physiological mechanisms that protect against AD during sleep. In particular, the glymphatic system drains interstitial fluid from the brain, removing extra-cellular amyloid beta (eAβ) twice as fast during sleep. In addition, melatonin - a peptide hormone that increases markedly during sleep - is an effective antioxidant that inhibits the polymerization of soluble eAβ into insoluble amyloid fibrils that are associated with AD. Sleep deprivation increases plaque formation and AD, which itself disrupts sleep, potentially creating a positive feedback cycle. These and other physiological benefits of sleep may be compromised by short sleep durations. Our hypothesis highlights possible long-term side effects of medications that reduce sleep, and may lead to potential new strategies for preventing and treating AD.

An Introduction to Cognitive Archaeology

Cognitive archaeology studies human cognitive evolution by applying cognitive-science theories and concepts to archaeological remains of the prehistoric past. After reviewing the basic epistemological stance of cognitive archaeology, this article illustrates this interdisciplinary endeavor through an examination of two of the most important transitions in hominin cognitive evolution—the appearance of Homo erectus about 2 million years ago, and the recent enhancement of working-memory capacity within the past 200,000 years. Although intentionally created stone tools date to about 3.3 million years ago, Homo erectus produced a bifacial, symmetrical handaxe whose design then persisted for nearly the next 2 million years. An enhancement in working-memory capacity may have been responsible for the relative explosion of culture within the past 50,000 years, which included personal ornamentation, highly ritualized burials, bow-and-arrow technology, depictive cave art, and artistic figurines.

Evolutionary change in physiological phenotypes along the human lineage

BACKGROUND AND OBJECTIVES

Research in evolutionary medicine provides many examples of how evolution has shaped human susceptibility to disease. Traits undergoing rapid evolutionary change may result in associated costs or reduce the energy available to other traits. We hypothesize that humans have experienced more such changes than other primates as a result of major evolutionary change along the human lineage. We investigated 41 physiological traits across 50 primate species to identify traits that have undergone marked evolutionary change along the human lineage.

METHODOLOGY

We analysed the data using two Bayesian phylogenetic comparative methods. One approach models trait covariation in non-human primates and predicts human phenotypes to identify whether humans are evolutionary outliers. The other approach models adaptive shifts under an Ornstein-Uhlenbeck model of evolution to assess whether inferred shifts are more common on the human branch than on other primate lineages.

RESULTS

We identified four traits with strong evidence for an evolutionary increase on the human lineage (amylase, haematocrit, phosphorus and monocytes) and one trait with strong evidence for decrease (neutrophilic bands). Humans exhibited more cases of distinct evolutionary change than other primates.

CONCLUSIONS AND IMPLICATIONS

Human physiology has undergone increased evolutionary change compared to other primates. Long distance running may have contributed to increases in haematocrit and mean corpuscular haemoglobin concentration, while dietary changes are likely related to increases in amylase. In accordance with the pathogen load hypothesis, human monocyte levels were increased, but many other immune-related measures were not. Determining the mechanisms underlying conspicuous evolutionary change in these traits may provide new insights into human disease.

Shining evolutionary light on human sleep and sleep disorders

Sleep is essential to cognitive function and health in humans, yet the ultimate reasons for sleep—i.e. ‘why’ sleep evolved—remain mysterious. We integrate findings from human sleep studies, the ethnographic record, and the ecology and evolution of mammalian sleep to better understand sleep along the human lineage and in the modern world. Compared to other primates, sleep in great apes has undergone substantial evolutionary change, with all great apes building a sleeping platform or ‘nest’. Further evolutionary change characterizes human sleep, with humans having the shortest sleep duration, yet the highest proportion of rapid eye movement sleep among primates. These changes likely reflect that our ancestors experienced fitness benefits from being active for a greater portion of the 24-h cycle than other primates, potentially related to advantages arising from learning, socializing and defending against predators and hostile conspecifics. Perspectives from evolutionary medicine have implications for understanding sleep disorders; we consider these perspectives in the context of insomnia, narcolepsy, seasonal affective disorder, circadian rhythm disorders and sleep apnea. We also identify how human sleep today differs from sleep through most of human evolution, and the implications of these changes for global health and health disparities. More generally, our review highlights the importance of phylogenetic comparisons in understanding human health, including well-known links between sleep, cognitive performance and health in humans.

Why are there apes? Evidence for the co-evolution of ape and monkey ecomorphology

Apes, members of the superfamily Hominoidea, possess a distinctive suite of anatomical and behavioral characters which appear to have evolved relatively late and relatively independently. The timing of paleontological events, extant cercopithecine and hominoid ecomorphology and other evidence suggests that many distinctive ape features evolved to facilitate harvesting ripe fruits among compliant terminal branches in tree edges. Precarious, unpredictably oriented, compliant supports in the canopy periphery require apes to maneuver using suspensory and non-sterotypical postures (i.e. postures with eccentric limb orientations or extreme joint excursions). Diet differences among extant species, extant species numbers and evidence of cercopithecoid diversification and expansion, in concert with a reciprocal decrease in hominoid species, suggest intense competition between monkeys and apes over the last 20 Ma. It may be that larger body masses allow great apes to succeed in contest competitions for highly desired food items, while the ability of monkeys to digest antifeedant-rich unripe fruits allows them to win scramble competitions. Evolutionary trends in morphology and inferred ecology suggest that as monkeys evolved to harvest fruit ever earlier in the fruiting cycle they broadened their niche to encompass first more fibrous, tannin- and toxin-rich unripe fruits and later, for some lineages, mature leaves. Early depletion of unripe fruit in the central core of the tree canopy by monkeys leaves a hollow sphere of ripening fruits, displacing antifeedant-intolerant, later-arriving apes to small-diameter, compliant terminal branches. Hylobatids, orangutans, Pan species, gorillas and the New World atelines may have each evolved suspensory behavior independently in response to local competition from an expanding population of monkeys. Genetic evidence of rapid evolution among chimpanzees suggests that adaptations to suspensory behavior, vertical climbing, knuckle-walking, consumption of terrestrial piths and intercommunity violence had not yet evolved or were still being refined when panins (chimpanzees and bonobos) and hominins diverged.