Objectives
Cross‐cultural sleep research is critical to deciphering whether modern sleep expression is the product of recent selective pressures, or an example of evolutionary mismatch to ancestral ...sleep ecology. We worked with the Hadza, an equatorial, hunter‐gatherer community in Tanzania, to better understand ancestral sleep patterns and to test hypotheses related to sleep segmentation.
Methods
We used actigraphy to analyze sleep‐wake patterns in thirty‐three volunteers for a total of 393 days. Linear mixed effects modeling was performed to assess ecological predictors of sleep duration and quality. Additionally, functional linear modeling (FLM) was used to characterize 24‐hr time averaged circadian patterns.
Results
Compared with post‐industrialized western populations, the Hadza were characterized by shorter (6.25 hr), poorer quality sleep (sleep efficiency = 68.9%), yet had stronger circadian rhythms. Sleep duration time was negatively influenced by greater activity, age, light (lux) exposure, and moon phase, and positively influenced by increased day length and mean nighttime temperature. The average daily nap ratio (i.e., the proportion of days where a nap was present) was 0.54 (SE = 0.05), with an average nap duration of 47.5 min (SE = 2.71; n = 139).
Discussion
This study showed that circadian rhythms in small‐scale foraging populations are more entrained to their ecological environments than Western populations. Additionally, Hadza sleep is characterized as flexible, with a consistent early morning sleep period yet reliance upon opportunistic daytime napping. We propose that plasticity in sleep‐wake patterns has been a target of natural selection in human evolution.
The function of dreams is a longstanding scientific research question. Simulation theories of dream function, which are based on the premise that dreams represent evolutionary past selective ...pressures and fitness improvement through modified states of consciousness, have yet to be tested in cross-cultural populations that include small-scale forager societies. Here, we analyze dream content with cross-cultural comparisons between the BaYaka (Rep. of Congo) and Hadza (Tanzania) foraging groups and Global North populations, to test the hypothesis that dreams in forager groups serve a more effective emotion regulation function due to their strong social norms and high interpersonal support. Using a linear mixed effects model we analyzed 896 dreams from 234 individuals across these populations, recorded using dream diaries. Dream texts were processed into four psychosocial constructs using the Linguistic Inquiry and Word Count (LIWC-22) dictionary. The BaYaka displayed greater community-oriented dream content. Both the BaYaka and Hadza exhibited heightened threat dream content, while, at the same time, the Hadza demonstrated low negative emotions in their dreams. The Global North Nightmare Disorder group had increased negative emotion content, and the Canadian student sample during the COVID-19 pandemic displayed the highest anxiety dream content. In conclusion, this study supports the notion that dreams in non-clinical populations can effectively regulate emotions by linking potential threats with non-fearful contexts, reducing anxiety and negative emotions through emotional release or catharsis. Overall, this work contributes to our understanding of the evolutionary significance of this altered state of consciousness.
Widespread cooperation is a defining feature of human societies from hunter-gatherer bands to nation states 1, 2, but explaining its evolution remains a challenge. Although positive assortment of ...cooperators is recognized as a basic requirement for the evolution of cooperation, the mechanisms governing assortment are debated. Moreover, the social structure of modern hunter-gatherers, characterized by high mobility, residential mixing, and low genetic relatedness 3, undermines assortment and adds to the puzzle of how cooperation evolved. Here, we analyze four years of data (2010, 2013, 2014, 2016) tracking residence and levels of cooperation elicited from a public goods game in Hadza hunter-gatherers of Tanzania. Data were collected from 56 camps, comprising 383 unique individuals, 137 of whom we have data for two or more years. Despite significant residential mixing, we observe a robust pattern of assortment that is necessary for cooperation to evolve; in every year, Hadza camps exhibit high between-camp and low within-camp variation in cooperation. We find little evidence that cooperative behavior within individuals is stable over time or that similarity in cooperation between dyads predicts their future cohabitation. Both sets of findings are inconsistent with models that assume stable cooperative and selfish types, including partner choice models. Consistent with social norms, culture, and reciprocity theories, the strongest predictor of an individual’s level of cooperation is the mean cooperation of their current campmates. These findings underscore the adaptive nature of human cooperation—particularly its responsiveness to social contexts—as a feature that is important in generating the assortment necessary for cooperation to evolve.
•Assortment on cooperation is a characteristic feature of hunter-gatherer life•Assortment persists despite substantial migration and residential mixing•No evidence for stable social types or a preference to live with cooperators•Individuals respond in kind to the cooperative behavior of their group members
For cooperation to evolve, cooperators must interact with other cooperators. Smith et al. use panel data from a population of extant hunter-gatherers to show how assortativity in cooperation is maintained.
The appearance of the Acheulean is one of the hallmarks of human evolution. It represents the emergence of a complex behavior, expressed in the recurrent manufacture of large-sized tools, with ...standardized forms, implying more advance forethought and planning by hominins than those required by the precedent Oldowan technology. The earliest known evidence of this technology dates back to c. 1.7 Ma. and is limited to two sites (Kokiselei Kenya and Konso Ethiopia), both of which lack functionally-associated fauna. The functionality of these earliest Acheulean assemblages remains unknown. Here we present the discovery of another early Acheulean site also dating to c. 1.7 Ma from Olduvai Gorge. This site provides evidence of the earliest steps in developing the Acheulean technology and is the oldest Acheulean site in which stone tools occur spatially and functionally associated with the exploitation of fauna. Simple and elaborate large-cutting tools (LCT) and bifacial handaxes co-exist at FLK West, showing that complex cognition was present from the earliest stages of the Acheulean. Here we provide a detailed technological study and evidence of the use of these tools on the butchery and consumption of fauna, probably by early Homo erectus sensu lato.
Recent archaeological work at BK has uncovered abundant taphonomic evidence of megafaunal exploitation by 1.34 Ma hominins. Butchery of small, medium-sized and large carcasses at the site indicate ...that meat consumption was a crucial adaptive element in the behavior of Homo erectus. Current debates on the role played by meat in this early stage of the evolution of the genus Homo confront cost signaling interpretations against dietary/physiological interpretations of meat eating and its relation to brain evolution. BBK (including all the archaeological levels) contains the largest amount of hominin-modified bones and butchered animals documented in the Early Pleistocene archaeological record. This evidence supports that meat consumption was tightly linked to the physiology that shaped the evolution of our genus. Hunting was an integral part of the adaptive behavior of H. erectus although megafaunal exploitation may have included more opportunistic behaviors. Site organization also suggests that this species may have exhibited a different within-site spatial organization, which differed from previous hominins, as documented at sites such as FLK Zinj. This unveils the need of new behavioral models to explain the functionality of Acheulian central-place sites.
Humans are motivated to compete for access to valuable social partners, which is a function of their willingness to share and ability to generate resources. However, relative preferences for each ...trait should be responsive to socioecological conditions. Here, we test the flexibility of partner choice psychology among Hadza hunter-gatherers of Tanzania. Ninety-two Hadza ranked their campmates on generosity and foraging ability and then shared resources with those campmates. We found Hadza with greater exposure to other cultures shared more with campmates ranked higher on generosity, whereas Hadza with lower exposure showed a smaller preference for sharing with generous campmates. This moderating effect was specific to generosity-regardless of exposure, Hadza showed only a small preference for sharing with better foragers. We argue this difference in preferences is due to high exposure Hadza having more experience cooperating with others in the absence of strong norms of sharing, and thus are exposed to greater variance in willingness to cooperate among potential partners increasing the benefits of choosing partners based on generosity. As such, participants place a greater emphasis on choosing more generous partners, highlighting the flexibility of partner preferences.
FLK West (Bed II, Olduvai Gorge) contains the oldest association of Acheulian stone tools and exploitation of fauna (including megafauna) by hominins in the Pleistocene. Recently, the FLK West ...paleolandscape has been intensively studied, unveiling a spatial association between archaeological materials and hydrothermal resources. A new type of landscape use by hominins has also been documented around the area where the penecontemporaneous FLK West and HWK site complex were formed, resulting in an array of habitats spanning thousands of square meters covered with large amounts of lithic artefacts. Here, we show how the intensive use of certain environments by hominins resulted in these megasites, in which hominins engaged in a variety of activities, complementary to those performed at discrete archaeological clusters like FLK West. Despite using these habitats redundantly as quarries, hominins performed extensive core reduction of several types of raw materials indicating a dexterity and careful planning undocumented in earlier periods. Here, we also show how palaeoecological reconstructions must be based on fine-scale geological analyses, given the palimpsestic nature of both geological and anthropogenic processes. The research reported here also uncovered an additional unknown source of metamorphic rocks for hominins at Olduvai, which questions a large part of previous modeling based on hominin provisioning exclusively at Naibor Soit. We also show that the manufacture of handaxes was slightly older than documented at FLK West and that they occur in isolation on the landscape in addition to being clustered at sites. This implies that at the beginning of the Acheulian, hominins were not only using and discarding handaxes at specific loci, but they also transported these tools for various activities across the landscape as part of their strategized adaptation to those environments. The occurrence of these intensively-used megasites hints at some territorial behaviors by early Acheulian hominins.
Teaching is cross-culturally widespread but few studies have considered children as teachers as well as learners. This is surprising, since forager children spend much of their time playing and ...foraging in child-only groups, and thus, have access to many potential child teachers. Using the Social Relations Model, we examined the prevalence of child-to-child teaching using focal follow data from 35 Hadza and 38 BaYaka 3- to 18-year-olds. We investigated the effect of age, sex and kinship on the teaching of subsistence skills. We found that child-to-child teaching was more frequent than adult-child teaching. Additionally, children taught more with age, teaching was more likely to occur within same-sex versus opposite-sex dyads, and close kin were more likely to teach than non-kin. The Hadza and BaYaka also showed distinct learning patterns; teaching was more likely to occur between sibling dyads among the Hadza than among the BaYaka, and a multistage learning model where younger children learn from peers, and older children from adults, was evident for the BaYaka, but not for the Hadza. We attribute these differences to subsistence and settlement patterns. These findings highlight the role of children in the intergenerational transmission of subsistence skills.
In this experimental study, we continue investigating an association between facial morphology and individual psychological characteristics. The study was conducted in the population of Maasai ...(Ngorongoro, Tanzania) in two stages during field trips in 2016 and 2021. At the first stage, we collected anthropological photo portraits from 305 individuals (123 women and 182 men) and interviewed them to identify the propensity to help others. Six generalized morphed portraits of Maasai (men and women) were created based on individual propensity to help. At the second stage of the study, portraits were presented to 200 Maasai, men and women from the same population. The portraits were evaluated according to the degree of expression of four qualities: physical strength, proneness to help others, a "good friend", and self-confidence. The results of the study revealed that representatives of the same population are able to distinguish altruistic men by their facial shape. Men who were prone to provide assistance to others were perceived as physically stronger. No such association was found for female portraits. The results are interpreted from the standpoint of evolutionary psychology.
Настоящим экспериментальным исследованием мы продолжаем работу по выявлению связи между морфологическими чертами лица человека и его психологическими характеристиками. Исследование проводили в популяции масаев, проживающих в Нгоронгоро, Танзания, в два этапа на протяжении двух полевых выездов в 2016 и 2021 гг. На первом этапе участниками являлись 305 человек (123 женщины и 182 мужчины). Мы проводили опрос на выявление склонности помогать окружающим и антропологическую фотосъемку, а также создали шесть обобщенных морфированных портретов масаев (мужчин и женщин) по критерию склонности к оказанию помощи другим масаям не родственникам. На втором этапе исследования портреты предъявляли 200 масаям, мужчинам и женщинам из той же популяции. Портреты оценивались по степени выраженности четырех качеств: физическая сила, склонность помогать окружающим, «хороший друг», самоуверенность. Результаты исследования показали, что представители той же популяции способны отличать по форме лица альтруистов-мужчин. Мужчины, склонные оказывать помощь другим, воспринимались экспертами как физически более сильные. Для женских портретов такая связь не обнаружена. Результаты интерпретированы с позиций эволюционной психологии.
Cross-cultural sex differences in mobility and harm avoidance have been widely reported, often emphasizing fitness benefits of long-distance travel for males and high costs for females. Data emerging ...from adults in small-scale societies, however, are challenging the assumption that female mobility is restricted during reproduction. Such findings warrant further exploration of the ontogeny of mobility. Here, using a combination of machine-learning, mixed-effects linear regression, and GIS mapping, we analyze range size, daily distance traveled, and harm avoidance among Hadza foragers during middle childhood and adolescence. Distance traveled increased with age and, while male adolescents had the longest daily ranges, average daily distance traveled by each sex was similar. We found few age- or sex-related patterns in harm-avoidant responses and a high degree of individual variation. When queried on the same issues, children and their parents were often in alignment as to expectations pertaining to harm avoidance, and siblings tended to behave in similar ways. To the extent that sex differences in mobility did emerge, they were associated with ecological differences in physical threats associated with sex-specific foraging behaviors. Further, we found no strong association between harm avoidance and mobility. Young Hadza foragers of both sexes are highly mobile, regardless of how harm avoidant they are. Taken together, our findings indicate that the causal arrows between harm avoidance and mobility must be evaluated in ecologically specific frameworks where cultural expectations of juvenile mobility can be contextualized.
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