Male strategies and Plio-Pleistocene archaeology

A case of suspected chimpanzee scavenging in the Issa Valley, Tanzania

Like humans, chimpanzees (Pan troglodytes) are well known for their vertebrate and invertebrate hunting, but they rarely scavenge. In contrast, while hunting and meat consumption became increasingly important during the evolution of the genus Homo, scavenging meat and marrow from carcasses of large mammals was also likely to be an important component of their subsistence strategies. Here, we describe a confrontational scavenging interaction between an adult male chimpanzee from the Issa Valley and a crowned eagle (Stephanoaetus coronatus), which resulted in the chimpanzee capturing and consuming the carcass of a juvenile bushbuck (Tragelaphus scriptus). We describe the interaction and contextualize this with previous scavenging observations from chimpanzees.

Life history impacts on infancy and the evolution of human social cognition

Greater longevity, slower maturation and shorter birth intervals are life history features that distinguish humans from the other living members of our hominid family, the great apes. Theory and evidence synthesized here suggest the evolution of those features can explain both our bigger brains and our cooperative sociality. I rely on Sarah Hrdy's hypothesis that survival challenges for ancestral infants propelled the evolution of distinctly human socioemotional appetites and Barbara Finlay and colleagues' findings that mammalian brain size is determined by developmental duration. Similar responsiveness to varying developmental contexts in chimpanzee and human one-year-olds suggests similar infant responsiveness in our nearest common ancestor. Those ancestral infants likely began to acquire solid food while still nursing and fed themselves at weaning as chimpanzees and other great apes do now. When human ancestors colonized habitats lacking foods that infants could handle, dependents' survival became contingent on subsidies. Competition to engage subsidizers selected for capacities and tendencies to enlist and maintain social connections during the early wiring of expanding infant brains with lifelong consequences that Hrdy labeled "emotionally modern" social cognition.

Somatic maintenance/reproduction tradeoffs and human evolution

The authors propose that many morbidities higher in women than men are adaptations protecting survival, selected because survival has been especially crucial to mothers' reproductive success. Following their lead, I pursue variation in tradeoffs between reproduction and survival recognized by Darwin that were likely central to the evolution of many traits that distinguish us from our great ape cousins.

What Are the “Costs and Benefits” of Meat-Eating in Human Evolution? The Challenging Contribution of Behavioral Ecology to Archeology

Despite the omnivorous diet of most human populations, meat foraging gradually increased during the Paleolithic, in parallel with the development of hunting capacities. There is evidence of regular meat consumption by extinct hominins from 2 Ma onward, with the first occurrence prior to 3 Ma in Eastern Africa. The number of sites with cut-marked animal remains and stone tools increased after 2 Ma. In addition, toolkits became increasingly complex, and various, facilitating carcass defleshing and marrow recovery, the removal of quarters of meat to avoid carnivore competition, and allowing the emergence of cooperative (i.e., social) hunting of large herbivores. How can we assess the energy costs and benefits of meat and fat acquisition and consumption for hunter-gatherers in the past, and is it possible to accurately evaluate them? Answering this question would provide a better understanding of extinct hominin land use, food resource management, foraging strategies, and cognitive abilities related to meat and fat acquisition, processing, and consumption. According to the Optimal Foraging Theory (OFT), resources may be chosen primarily on the basis of their efficiency rank in term of calories. But, could other factors, and not only calorific return, prevail in the choice of prey, such as the acquisition of non-food products, like pelts, bone tools or ornaments, or symbolic or traditional uses? Our main goal here is to question the direct application of behavioral ecology data to archeology. For this purpose, we focus on the issue of animal meat and fat consumption in human evolution. We propose a short review of available data from energetics and ethnographic records, and provide examples of several various-sized extant animals, such as elephants, reindeer, or lagomorphs, which were some of the most common preys of Paleolithic hominins.

No sustained increase in zooarchaeological evidence for carnivory after the appearance of Homo erectus

The appearance of Homo erectus shortly after 2.0 Ma is widely considered a turning point in human dietary evolution, with increased consumption of animal tissues driving the evolution of larger brain and body size and a reorganization of the gut. An increase in the size and number of zooarchaeological assemblages after the appearance of H. erectus is often offered as a central piece of archaeological evidence for increased carnivory in this species, but this characterization has yet to be subject to detailed scrutiny. Any widespread dietary shift leading to the acquisition of key traits in H. erectus should be persistent in the zooarchaeological record through time and can only be convincingly demonstrated by a broad-scale analysis that transcends individual sites or localities. Here, we present a quantitative synthesis of the zooarchaeological record of eastern Africa from 2.6 to 1.2 Ma. We show that several proxies for the prevalence of hominin carnivory are all strongly related to how well the fossil record has been sampled, which constrains the zooarchaeological visibility of hominin carnivory. When correcting for sampling effort, there is no sustained increase in the amount of evidence for hominin carnivory between 2.6 and 1.2 Ma. Our observations undercut evolutionary narratives linking anatomical and behavioral traits to increased meat consumption in H. erectus, suggesting that other factors are likely responsible for the appearance of its human-like traits.

A case of hominin scavenging 1.84 million years ago from Olduvai Gorge (Tanzania)

Meat eating is one of the hallmarks of human evolution. It has been linked to the beginning of stone tool use, to physiological changes leading to crucial anatomical transformations defining our genus, and to new socioreproductive and cognitive behaviors. Uncontroversial evidence of meat eating goes back to 2.6 million years ago; however, little is known about the frequency and timing with which early hominins acquired animal resources. Here, we show that the combination of hunting and scavenging documented in some modern human foragers may have a long evolutionary trajectory. Using a new set of artificial intelligence methods for objective identification, we present direct evidence of an episode of hominins scavenging from large felids-probably lions-discovered at Olduvai Gorge (DS site, Bed I). This casts a new perspective on the diversity of hominin carcass acquisition behaviors and survival strategies, and places some early Pleistocene hominins in ecological proximity to African large carnivore guilds.

The cost of cooking for foragers

Cooked food provides more calories to a consumer than raw food. When our human ancestors adopted cooking, the result was an increase in the caloric value of the diet. Generating the heat to cook, however, requires fuel, and accessing fuel was and remains a common problem for humanity. Cooking also frequently requires monitoring, special technology and other investments. These cooking costs should vary greatly across multiple contexts. Here I explain how to quantify this cooking trade-off as the ratio of the energetic benefits of cooking to the increased cost in handling time and examine the implications for foragers, including the first of our ancestors to cook. Ethnographic and experimental return rates and nutritional analysis about important prey items exploited by ethnohistoric Numic foragers in the North American Great Basin provide a demonstration of how the costs of cooking impact different types of prey. Foragers should make choices about which prey to capture based on expectations about the costs involved to cook them. The results indicate that the caloric benefit achieved by cooking meat is quickly lost as the cost of cooking increases, whereas many plant foods are beneficially cooked across a range of cooking costs. These findings affirm the importance of plant foods, especially geophytes, among foragers, and are highly suggestive of their importance at the onset of cooking in the human lineage.

Seasonality and Oldowan behavioral variability in East Africa

The extent, nature, and temporality of early hominin food procurement strategies have been subject to extensive debate. In this article, we examine evidence for the seasonal scheduling of resource procurement and technological investment in the Oldowan, starting with an evaluation of the seasonal signature of underground storage organs, freshwater resources, and terrestrial animal resources in extant primates and modern human hunter-gatherer populations. Subsequently, we use the mortality profiles, taxonomic composition, and taphonomy of the bovid assemblages at Kanjera South (Homa Peninsula, Kenya) and FLK-Zinj (Olduvai Gorge, Tanzania) to illustrate the behavioral flexibility of Oldowan hominins, who were targeting different seasonally vulnerable demographics. In terms of the lithic assemblages, the specific opportunities and constraints afforded by dry season subsistence at FLK-Zinj may have disincentivized lithic investment, resulting in a more expedient toolkit for fast and effective carcass processing. This may have been reinforced by raw material site provisioning during a relatively prolonged seasonal occupation, reducing pressures on the reduction and curation of lithic implements. In contrast, wet season plant abundance would have offered a predictable set of high-quality resources associated with low levels of competition and reduced search times, in the context of perhaps greater seasonal mobility and consequently shorter occupations. These factors appear to have fostered technological investment to reduce resource handling costs at Kanjera South, facilitated by more consistent net returns and enhanced planning of lithic deployment throughout the landscape. We subsequently discuss the seasonality of freshwater resources in Oldowan procurement strategies, focusing on FwJj20 (Koobi Fora, Kenya). Although more analytical studies with representative sample sizes are needed, we argue that interassemblage differences evidence the ability of Oldowan hominins to adapt to seasonal constraints and opportunities in resource exploitation.

Multiple Genomic Events Altering Hominin SIGLEC Biology and Innate Immunity Predated the Common Ancestor of Humans and Archaic Hominins

Human-specific pseudogenization of the CMAH gene eliminated the mammalian sialic acid (Sia) Neu5Gc (generating an excess of its precursor Neu5Ac), thus changing ubiquitous cell surface “self-associated molecular patterns” that modulate innate immunity via engagement of CD33-related-Siglec receptors. The Alu-fusion-mediated loss-of-function of CMAH fixed ∼2–3 Ma, possibly contributing to the origins of the genus Homo. The mutation likely altered human self-associated molecular patterns, triggering multiple events, including emergence of human-adapted pathogens with strong preference for Neu5Ac recognition and/or presenting Neu5Ac-containing molecular mimics of human glycans, which can suppress immune responses via CD33-related-Siglec engagement. Human-specific alterations reported in some gene-encoding Sia-sensing proteins suggested a “hotspot” in hominin evolution. The availability of more hominid genomes including those of two extinct hominins now allows full reanalysis and evolutionary timing. Functional changes occur in 8/13 members of the human genomic cluster encoding CD33-related Siglecs, all predating the human common ancestor. Comparisons with great ape genomes indicate that these changes are unique to hominins. We found no evidence for strong selection after the Human–Neanderthal/Denisovan common ancestor, and these extinct hominin genomes include almost all major changes found in humans, indicating that these changes in hominin sialobiology predate the Neanderthal–human divergence ∼0.6 Ma. Multiple changes in this genomic cluster may also explain human-specific expression of CD33rSiglecs in unexpected locations such as amnion, placental trophoblast, pancreatic islets, ovarian fibroblasts, microglia, Natural Killer(NK) cells, and epithelia. Taken together, our data suggest that innate immune interactions with pathogens markedly altered hominin Siglec biology between 0.6 and 2 Ma, potentially affecting human evolution.

The Centrality of Ancestral Grandmothering in Human Evolution

When Fisher, Williams, and Hamilton laid the foundations of evolutionary life history theory, they recognized elements of what became a grandmother hypothesis to explain the evolution of human postmenopausal longevity. Subsequent study of modern hunter-gatherers, great apes, and the wider mammalian radiation has revealed strong regularities in development and behavior that show additional unexpected consequences that ancestral grandmothering likely had on human evolution, challenging the hypothesis that ancestral males propelled the evolution of our radiation by hunting to provision mates and offspring. Ancestral grandmothering has become a serious contender to explain not only the large fraction of post-fertile years women live and children's prolonged maturation yet early weaning; it also promises to help account for the pair bonding that distinguishes humans from our closest living evolutionary cousins, the great apes (and most other mammals), the evolution of our big human brains, and our distinctive preoccupation with reputations, shared intentionality and persistent cultural learning that begins in infancy.

Cognitive consequences of our grandmothering life history: cultural learning begins in infancy

Postmenopausal longevity distinguishes humans from our closest living evolutionary cousins, the great apes, and may have evolved in our lineage when the economic productivity of grandmothers allowed mothers to wean earlier and overlap dependents. Since increased longevity retards development and expands brain size across the mammals, this hypothesis links our slower developing, bigger brains to ancestral grandmothering. If foraging interdependence favoured postmenopausal longevity because grandmothers' subsidies reduced weaning ages, then ancestral infants lost full maternal engagement while their slower developing brains were notably immature. With survival dependent on social relationships, sensitivity to reputations is wired very early in neural ontogeny, beginning our lifelong preoccupation with shared intentionality. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

The emergence of emotionally modern humans: implications for language and learning

According to the Cooperative Breeding Hypothesis, apes with the life-history attributes of those in the line leading to the genus Homo could not have evolved unless male and female allomothers had begun to help mothers care for and provision offspring. As proposed elsewhere, the unusual way hominins reared their young generated novel phenotypes subsequently subjected to Darwinian social selection favouring those young apes best at monitoring the intentions, mental states and preferences of others and most motivated to attract and appeal to caretakers. Not only were youngsters acquiring information in social contexts different from those of other apes, but they would also have been emotionally and neurophysiologically different from them in ways that are relevant to how humans learn. Contingently delivered rewards to dependents who attracted and ingratiated themselves with allomothers shaped their behaviours and vocalizations and transformed the way developing youngsters learned from others and internalized their preferences. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Warfare in an evolutionary perspective

The importance of warfare for human evolution is hotly debated in anthropology. Some authors hypothesize that warfare emerged at least 200,000-100,000 years BP, was frequent, and significantly shaped human social evolution. Other authors claim that warfare is a recent phenomenon, linked to the emergence of agriculture, and mostly explained by cultural rather than evolutionary forces. Here I highlight and critically evaluate six controversial points on the evolutionary bases of warfare. I argue that cultural and evolutionary explanations on the emergence of warfare are not alternative but analyze biological diversity at two distinct levels. An evolved propensity to act aggressively toward outgroup individuals may emerge irrespective of whether warfare appeared early/late during human evolution. Finally, I argue that lethal violence and aggression toward outgroup individuals are two linked but distinct phenomena, and that war and peace are complementary and should not always be treated as two mutually exclusive behavioral responses.

Social evolution in Plio-Pleistocene hominins: Insights from hamadryas baboons and paleoecology

Reconstructions of hominin evolution have long benefited from comparisons with nonhuman primates, especially baboons and chimpanzees. The hamadryas baboon (Papio hamadryas) is arguably one of the best such models, as it exhibits both the male kin bonding and the cross-sex pair bonding thought to have been important in hominin evolution. Here we link processes of behavioral evolution in hamadryas baboons with those in a Plio-Pleistocene hominin, provisionally identified as Homo erectus (sensu lato) - a pivotal species in that its larger body and brain size and wider ranging patterns increased female costs of reproduction, increasing the importance of sociality. The combination of these higher costs of reproduction and shifts in diet and food acquisition have previously been argued to have been alleviated either via strengthening of male-female bonds (involving male provisioning and the evolution of monogamy) or via the assistance of older, post-reproductive females (leading to post-reproductive longevity in females, i.e., the grandmother hypothesis). We suggest that both arrangements could have been present in Plio-Pleistocene hominins if they lived in multilevel societies. Here we expand on our earlier scenario with two sets of recent data in support of it, (1) archaeological data from the 2 million year old Oldowan site of Kanjera South, Kenya and other sites that are suggestive of tool dependent foraging on nutrient dense resources (animal carcasses and plant underground storage organs), cooperation, and food sharing; and (2) a pattern of genetic variation in hamadryas baboons that suggests the operation of kin selection among both males and females at multiple levels of society. Taken together, these two sets of data strengthen our model and support the idea of a complex society linked by male-male, male-female, and female-female bonds at multiple levels of social organization in Plio-Pleistocene hominins.

Wild chimpanzees deprived a leopard of its kill: Implications for the origin of hominin confrontational scavenging

This study reports the first observed case of wild chimpanzees (Pan troglodytes) obtaining animal prey freshly killed by a sympatric leopard (Panthera pardus) and scavenging it with the leopard still nearby. This observation has important implications for the emergence of confrontational scavenging, which may have played a significant role in human evolution. Many scholars agree that eating meat became important during human evolution, and hominins first obtained meat by scavenging. However, it is debatable whether scavenging behavior was "passive" or "confrontational (power)." The latter is more dangerous, as it requires facing the original predator, and it is thus considered to have been important for the evolution of several human traits, including cooperation and language. Chimpanzees do scavenge meat, although rarely, but no previous evidence of confrontational scavenging has hitherto emerged. Thus, it was assumed that they are averse to confrontation with even leopard-sized predators. However, in the observed case the chimpanzees frequently emitted waa barks, which indicated that they were aware of the leopard's presence but they nevertheless continued to eat the scavenged meat. In addition, we compiled and reviewed 49 cases of chimpanzee encounters with animal carcasses in the Mahale Mountains of Tanzania in 1980-2017. Chimpanzees scavenged meat in 36.7% of these cases, and tended to eat the meat when it was fresh or if the animal species was usually hunted by chimpanzees. However, no evidence indicated that carcasses were avoided when leopard involvement was likely. These results suggest that chimpanzee-sized hominins could potentially confront and deprive leopard-size carnivores of meat.

The meta-group social network of early humans: A temporal-spatial assessment of group size at FLK Zinj (Olduvai Gorge, Tanzania)

Humans are the only primates that maintain regular inter-group relationships and meta-group social networks that enable the inter-group flow of individuals. This is the basis of the band/tribe concept in the anthropology of modern foragers. The present work is a theoretical approach to the development of analytical tools to understand group size and the temporal scale of site occupation in the archaeological record. We selected FLK Zinj as one of the oldest examples of a taphonomically-supported anthropogenic site in which both variables (group size and time) could be modelled using a combination of modern forager regression estimates from their camp sizes and estimates derived from the combined use of modern African foragers' meat consumption rates per day per capita during the dry season and minimum estimates of flesh yields provided by the carcass parts preserved at FLK Zinj. This approach provides the basis for a testable hypothesis which should be further tested in other Oldowan sites. An estimate of 18-28 individuals occupying FLK Zinj was made, which is similar to the estimated 16 individuals of one of the 1.5 Ma Ileret Homo erectus footprint trails. It also shows a similar proportional distribution to Dunbar's equations (group size to neocortex ratio) as documented in modern foragers, which suggest that most of the social network of H. erectus was in the meta-group level as is the case of modern foragers. Irrespective of the range of variation discussed for both variables (group size and length of time represented), it is argued that neither small estimates of time nor small group sizes can account for the formation of FLK Zinj.

Hunter-gatherer studies and human evolution: A very selective review

The century long publication of this journal overlapped major changes in the sciences it covers. We have been eyewitnesses to vast changes during the final third of the last century and beginning of this one, momentous enough to fundamentally alter our work separately and collectively. One (NBJ) from animal ethology, another from western North American archaeology (JOC), and a third (KH) from cultural anthropology came to longtime collaboration as evolutionary ecologists with shared focus on studying modern hunter-gatherers to guide hypotheses about human evolution. Our findings have radically revised hypotheses each of us took for granted when we began. Our (provisional) conclusions are not the consensus among hunter-gatherer specialists; but grateful that personal reflections are invited, we aim to explain how and why we continue to bet on them.

Zoon politikon: The evolutionary origins of human socio-political systems

We deploy the most up-to-date evidence available in various behavioral fields in support of the following hypothesis: The emergence of bipedalism and cooperative breeding in the hominin line, together with environmental developments that made a diet of meat from large animals adaptive, as well as cultural innovations in the form of fire, cooking, and lethal weapons, created a niche for hominins in which there was a significant advantage to individuals with the ability to communicate and persuade in a moral context. These forces added a unique political dimension to human social life which, through gene-culture coevolution, became Homo ludens-Man, the game player-with the power to conserve and transform the social order. Homo sapiens became, in the words of Aristotle's Nicomachean Ethics, a zoon politikon.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

Loss of CMAH during Human Evolution Primed the Monocyte-Macrophage Lineage toward a More Inflammatory and Phagocytic State

Humans and chimpanzees are more sensitive to endotoxin than are mice or monkeys, but any underlying differences in inflammatory physiology have not been fully described or understood. We studied innate immune responses in Cmah^-/- mice, emulating human loss of the gene encoding production of Neu5Gc, a major cell surface sialic acid. CMP-N-acetylneuraminic acid hydroxylase (CMAH) loss occurred ∼2-3 million years ago, after the common ancestor of humans and chimpanzees, perhaps contributing to speciation of the genus HomoCmah^-/- mice manifested a decreased survival in endotoxemia following bacterial LPS injection. Macrophages from Cmah^-/- mice secreted more inflammatory cytokines with LPS stimulation and showed more phagocytic activity. Macrophages and whole blood from Cmah^-/- mice also killed bacteria more effectively. Metabolic reintroduction of Neu5Gc into Cmah^-/- macrophages suppressed these differences. Cmah^-/- mice also showed enhanced bacterial clearance during sublethal lung infection. Although monocytes and monocyte-derived macrophages from humans and chimpanzees exhibited marginal differences in LPS responses, human monocyte-derived macrophages killed Escherichia coli and ingested E. coli BioParticles better. Metabolic reintroduction of Neu5Gc into human macrophages suppressed these differences. Although multiple mechanisms are likely involved, one cause is altered expression of C/EBPβ, a transcription factor affecting macrophage function. Loss of Neu5Gc in Homo likely had complex effects on immunity, providing greater capabilities to clear sublethal bacterial challenges, possibly at the cost of endotoxic shock risk. This trade-off may have provided a selective advantage when Homo transitioned to butchery using stone tools. The findings may also explain why the Cmah^-/- state alters severity in mouse models of human disease.

The pyrophilic primate hypothesis

Members of genus Homo are the only animals known to create and control fire. The adaptive significance of this unique behavior is broadly recognized, but the steps by which our ancestors evolved pyrotechnic abilities remain unknown. Many hypotheses attempting to answer this question attribute hominin fire to serendipitous, even accidental, discovery. Using recent paleoenvironmental reconstructions, we present an alternative scenario in which, 2 to 3 million years ago in tropical Africa, human fire dependence was the result of adapting to progressively fire-prone environments. The extreme and rapid fluctuations between closed canopy forests, woodland, and grasslands that occurred in tropical Africa during that time, in conjunction with reductions in atmospheric carbon dioxide levels, changed the fire regime of the region, increasing the occurrence of natural fires. We use models from optimal foraging theory to hypothesize benefits that this fire-altered landscape provided to ancestral hominins and link these benefits to steps that transformed our ancestors into a genus of active pyrophiles whose dependence on fire for survival contributed to its rapid expansion out of Africa.

Variable postpartum responsiveness among humans and other primates with "cooperative breeding": A comparative and evolutionary perspective

This article is part of a Special Issue "Parental Care".Until recently, evolutionists reconstructing mother-infant bonding among human ancestors relied on nonhuman primate models characterized by exclusively maternal care, overlooking the highly variable responsiveness exhibited by mothers in species with obligate reliance on allomaternal care and provisioning. It is now increasingly recognized that apes as large-brained, slow maturing, and nutritionally dependent for so long as early humans were, could not have evolved unless "alloparents" (group members other than genetic parents), in addition to parents, had helped mothers to care for and provision offspring, a rearing system known as "cooperative breeding." Here I review situation-dependent maternal responses ranging from highly possessive to permissive, temporarily distancing, rejecting, or infanticidal, documented for a small subset of cooperatively breeding primates. As in many mammals, primate maternal responsiveness is influenced by physical condition, endocrinological priming, prior experience and local environments (especially related to security). But mothers among primates who evolved as cooperative breeders also appear unusually sensitive to cues of social support. In addition to more "sapient" or rational decision-making, humankind's deep history of cooperative breeding must be considered when trying to understand the extremely variable responsiveness of human mothers.

What's burning got to do with it? Primate foraging opportunities in fire-modified landscapes

OBJECTIVES

Anecdotal and formal evidence indicate that primates take advantage of burned landscapes. However, little work has been done to quantify the costs and benefits of this behavior. Using systematic behavioral observations from a population of South African vervet monkeys (Chlorocebus aethiops pygerythrus), we evaluate differences in food availability and energetics before and after controlled burns altered vegetation near their home range. We aim to determine whether burned habitats offer improved foraging opportunities.

METHODS

We collected feeding data from foraging individuals and analyzed common plant foods for their energetic content. We then used the feeding and energetic data to calculate postencounter profitabilities and encounter rates for food types. Using negative binomial and mixed linear regression models we compared data from burned and unburned habitats.

RESULTS

Our results show significantly improved encounter rates in burned landscapes for two prey items, invertebrates and grasses. However, postencounter profitabilities in burned areas were not significantly different than those achieved in unburned areas.

CONCLUSIONS

Results suggest that improved encounters alone can motivate changes in foraging behavior. These foraging benefits enable the exploitation of burned savanna habitats, likely driving postburn range expansions observed among populations of vervet monkeys. Thus quantified, these results may serve as a foundation for hypotheses regarding the evolution of fire-use in our own lineage.

Assessing digestibility of Hadza tubers using a dynamic in-vitro model

OBJECTIVES

Bioaccessibility is a useful measure for assessing the biological value of a particular nutrient from food, especially foods such as tubers. The wild tubers exploited by Hadza foragers in Tanzania are of interest because they are nontoxic, consumed raw or briefly roasted, and entail substantial physical barriers to consumers. In this study, we attempted to elucidate the biological value of Hadza tubers by measuring the absorption of glucose through in-vitro digestion.

METHODS

We quantified digestibility using data from 24 experimental trials on four species of Hadza tuber using a dynamic in-vitro model that replicates digestion in the stomach and small intestine. Analysis of glucose in the input meal and output dialysate revealed the accessible glucose fraction. We also conducted assays for protein, vitamin, and mineral content on whole tubers and meal fractions.

RESULTS

Bioaccessibility of glucose varies depending on tuber species. Holding effects of chewing constant, brief roasting had negligible effects, but high intraspecific variation precludes interpretive power. Overall, Hadza tubers are very resistant to digestion, with between one- and two-thirds of glucose absorbed on average. Glucose absorption negatively correlated with glucose concentration of the tubers.

CONCLUSIONS

Roasting may provide other benefits such as ease of peeling and chewing to extract edible parenchymatous tissue. A powerful factor in glucose acquisition is tuber quality, placing emphasis on the skill of the forager. Other nutrient assays yielded unexpectedly high values for protein, iron, and iodine, making tubers potentially valuable resources beyond caloric content.

The significance of cooking for early hominin scavenging

Meat scavenged by early Homo could have contributed importantly to a higher-quality diet. However, it has been suggested that because carrion would normally have been contaminated by bacteria it would have been dangerous and therefore eaten rarely prior to the advent of cooking. In this study, we quantified bacterial loads on two tissues apparently eaten by hominins, meat and bone marrow. We tested the following three hypotheses: (1) the bacterial loads on exposed surfaces of raw meat increase within 24 h to potentially dangerous levels, (2) simple roasting of meat on hot coals kills most bacteria, and (3) fewer bacteria grow on marrow than on meat, making marrow a relatively safe food. Our results supported all three hypotheses. Our experimental data imply that early hominins would have found it difficult to scavenge safely without focusing on marrow, employing strategies of carrion selection to minimize pathogen load, or cooking.

Embodied niche construction in the hominin lineage: semiotic structure and sustained attention in human embodied cognition

Human evolution unfolded through a rather distinctive, dynamically constructed ecological niche. The human niche is not only generally terrestrial in habitat, while being flexibly and extensively heterotrophic in food-web connections. It is also defined by semiotically structured and structuring embodied cognitive interfaces, connecting the individual organism with the wider environment. The embodied dimensions of niche-population co-evolution have long involved semiotic system construction, which I hypothesize to be an evolutionarily primitive aspect of learning and higher-level cognitive integration and attention in the great apes and humans alike. A clearly pre-linguistic form of semiotic cognitive structuration is suggested to involve recursively learned and constructed object icons. Higher-level cognitive iconic representation of visually, auditorily, or haptically perceived extrasomatic objects would be learned and evoked through indexical connections to proprioceptive and affective somatic states. Thus, private cognitive signs would be defined, not only by their learned and perceived extrasomatic referents, but also by their associations to iconically represented somatic states. This evolutionary modification of animal associative learning is suggested to be adaptive in ecological niches occupied by long-lived, large-bodied ape species, facilitating memory construction and recall in highly varied foraging and social contexts, while sustaining selective attention during goal-directed behavioral sequences. The embodied niche construction (ENC) hypothesis of human evolution posits that in the early hominin lineage, natural selection further modified the ancestral ape semiotic adaptations, favoring the recursive structuration of concise iconic narratives of embodied interaction with the environment.

Early Pleistocene aquatic resource use in the Turkana Basin

Evidence for the acquisition of nutritionally dense food resources by early Pleistocene hominins has implications for both hominin biology and behavior. Aquatic fauna may have comprised a source of highly nutritious resources to hominins in the Turkana Basin at ∼1.95 Ma. Here we employ multiple datasets to examine the issue of aquatic resource use in the early Pleistocene. This study focuses on four components of aquatic faunal assemblages (1) taxonomic diversity, (2) skeletal element proportion, (3) bone fragmentation and (4) bone surface modification. These components are used to identify associations between early Pleistocene aquatic remains and hominin behavior at the site of FwJj20 in the Koobi Fora Fm. (Kenya). We focus on two dominant aquatic species: catfish and turtles. Further we suggest that data on aquatic resource availability as well as ethnographic examples of aquatic resource use complement our observations on the archaeological remains from FwJj20. Aquatic food items provided hominins with a valuable nutritional alternative to an exclusively terrestrial resource base. We argue that specific advantages afforded by an aquatic alternative to terrestrial resources include (1) a probable reduction in required investment of energy relative to economic return in the form of nutritionally dense food items, (2) a decrease in the technological costs of resource acquisition, and (3) a reduced level of inter-specific competition associated with carcass access and an associated reduction of predation risk relative to terrestrial sources of food. The combined evidence from FwJj20 suggests that aquatic resources may have played a substantial role in early Pleistocene diets and these resources may have been overlooked in previous interpretations of hominin behavior.

Household and kin provisioning by Hadza men

We use data collected among Hadza hunter-gatherers between 2005 and 2009 to examine hypotheses about the causes and consequences of men's foraging and food sharing. We find that Hadza men foraged for a range of food types, including fruit, honey, small animals, and large game. Large game were shared not like common goods, but in ways that significantly advantaged producers' households. Food sharing and consumption data show that men channeled the foods they produced to their wives, children, and their consanguineal and affinal kin living in other households. On average, single men brought food to camp on 28% of days, married men without children at home on 31% of days, and married men with children at home on 42% of days. Married men brought fruit, the least widely shared resource, to camp significantly more often than single men. A model of the relationship between hunting success and household food consumption indicates that the best hunters provided 3-4 times the amount of food to their families than median or poor hunters. These new data fill important gaps in our knowledge of the subsistence economy of the Hadza and uphold predictions derived from the household and kin provisioning hypotheses. Key evidence and assumptions backing prior claims that Hadza hunting is largely a form of status competition were not replicated in our study. In light of this, family provisioning is a more viable explanation for why good hunters are preferred as husbands and have higher fertility than others.

Investigating the signature of aquatic resource use within Pleistocene hominin dietary adaptations

There is general agreement that the diet of early hominins underwent dramatic changes shortly after the appearance of stone tools in the archaeological record. It is often assumed that this change is associated with dietary expansion to incorporate large mammal resources. Although other aspects of the hominin diet, such as aquatic or vegetal resources, are assumed to be a part of hominin subsistence, identifying evidence of these adaptations has proved difficult. Here we present a series of analyses that provide methodological support for the inclusion of aquatic resources in hominin dietary reconstructions. We suggest that bone surface modifications in aquatic species are morphologically distinguishable from bone surface modifications on terrestrial taxa. We relate these findings to differences that we document in the surface mechanical properties of the two types of bone, as reflected by significant differences in bone surface microhardness values between aquatic and terrestrial species. We hypothesize that the characteristics of bone surface modifications on aquatic taxa inhibit the ability of zooarchaeologists to consistently diagnose them correctly. Contingently, this difficulty influences correspondence levels between zooarchaeologists, and may therefore result in misinterpretation of the taphonomic history of early Pleistocene aquatic faunal assemblages. A blind test using aquatic specimens and a select group of 9 experienced zooarchaeologists as participants was designed to test this hypothesis. Investigation of 4 different possible explanations for blind test results suggest the dominant factors explaining patterning relate to (1) the specific methodologies employed to diagnose modifications on aquatic specimens and (2) the relative experience of participants with modifications on aquatic bone surfaces. Consequently we argue that an important component of early hominin diets may have hitherto been overlooked as a result of (a) the paucity of referential frameworks within which to identify such a component and (b) the inability of applied identification methodologies to consistently do so.

Earliest archaeological evidence of persistent hominin carnivory

The emergence of lithic technology by ∼2.6 million years ago (Ma) is often interpreted as a correlate of increasingly recurrent hominin acquisition and consumption of animal remains. Associated faunal evidence, however, is poorly preserved prior to ∼1.8 Ma, limiting our understanding of early archaeological (Oldowan) hominin carnivory. Here, we detail three large well-preserved zooarchaeological assemblages from Kanjera South, Kenya. The assemblages date to ∼2.0 Ma, pre-dating all previously published archaeofaunas of appreciable size. At Kanjera, there is clear evidence that Oldowan hominins acquired and processed numerous, relatively complete, small ungulate carcasses. Moreover, they had at least occasional access to the fleshed remains of larger, wildebeest-sized animals. The overall record of hominin activities is consistent through the stratified sequence – spanning hundreds to thousands of years – and provides the earliest archaeological evidence of sustained hominin involvement with fleshed animal remains (i.e., persistent carnivory), a foraging adaptation central to many models of hominin evolution.

Early hominin diet included diverse terrestrial and aquatic animals 1.95 Ma in East Turkana, Kenya

The manufacture of stone tools and their use to access animal tissues by Pliocene hominins marks the origin of a key adaptation in human evolutionary history. Here we report an in situ archaeological assemblage from the Koobi Fora Formation in northern Kenya that provides a unique combination of faunal remains, some with direct evidence of butchery, and Oldowan artifacts, which are well dated to 1.95 Ma. This site provides the oldest in situ evidence that hominins, predating Homo erectus, enjoyed access to carcasses of terrestrial and aquatic animals that they butchered in a well-watered habitat. It also provides the earliest definitive evidence of the incorporation into the hominin diet of various aquatic animals including turtles, crocodiles, and fish, which are rich sources of specific nutrients needed in human brain growth. The evidence here shows that these critical brain-growth compounds were part of the diets of hominins before the appearance of Homo ergaster/erectus and could have played an important role in the evolution of larger brains in the early history of our lineage.

Colloquium paper: how grandmother effects plus individual variation in frailty shape fertility and mortality: guidance from human-chimpanzee comparisons

In the first paper to present formal theory explaining that senescence is a consequence of natural selection, W. D. Hamilton concluded that human postmenopausal longevity results from the contributions of ancestral grandmothers to the reproduction of their relatives. A grandmother hypothesis, subsequently elaborated with additional lines of evidence, helps explain both exceptional longevity and additional features of life history that distinguish humans from the other great apes. However, some of the variation observed in aging rates seems inconsistent with the tradeoffs between current and future reproduction identified by theory. In humans and chimpanzees, our nearest living relatives, individuals who bear offspring at faster rates do not cease bearing sooner. They continue to be fertile longer instead. Furthermore, within both species, groups with lower overall mortality rates have faster rates of increase in death risk with advancing age. These apparent contradictions to the expected life history tradeoffs likely result from heterogeneity in frailty among individuals. Whereas robust and frail alike must allocate investments between current and future reproduction, the more robust can afford more of both. This heterogeneity, combined with evolutionary tradeoffs and the key role of ancestral grandmothers they identify, helps explain aspects of human aging that increasingly concern us all.