Behavior and Its Evolution

What makes humans unique?

Bipedalism, cooking, cooperation and tolerance, language, social cognition, general intelligence

Hominin timeline

7 mya - Divergence from LCA with chimps; 5 mya - Ardipithecus genus; 4-3 mya - Australopithecus genus; 2-1 mya split between Homo (erectus) and Paranthropus

Ardipithecus ramidus

4.4 mya; found in Ethiopia; chimp-like teeth, derived features suggestive of bipedalism (probably still very good at climbing); chimp-like cranium

Australopithecus afarensis

3.8 mya; 'Footprints in the ash' at Laetoli; definitely bipedal but not entirely; combination of big toe and short pelvis suggests part-time bipedalism and arborealism, disproving the Savannah Hypothesis; teeth become more human like including smaller incisors and canines and bigger molars suggesting dietary shift to more fallback foods (e.g. with more high, repetitive chewing like tubers), less frugivory, and some meat

Australopithecus africanus

About 3 mya; larger brains; first direct evidence of stone tool use at Lomekwi → 'chicken and the egg' situation; dispersal to Southern Africa

Homo habilis

2.4 mya; 'HandyMan' Oldowan stone tools; first deliberate stone tools and hammers, including bifacial, acute-edged and polyhedral; short legs, long arms; lived in Southern and Eastern Africa

Homo rudolfensis

1.9 mya; larger brains and smaller teeth as compared to H. habilis

Homo erectus

~2 mya; larger brains, small teeth, small jaw muscle, long legs; had fire and naked skin; narrow pelvis first suggests the Obstetrical dilemma; first hominin to leave Africa; major expansion in brain size for Homo genus

Carrying hypothesis

Theory that bipedalism emerged so that hominins could use their hands to carry items; supported by Carvalho et al., 2012; some suggest that environmental change may have increased the selection pressure for carrying as climate change at the end of the Miocene shifted the environment from forest to Savannah

Carvalho et al., 2012

Experiment that suggested chimpanzee carrying behavior increased in high competition settings and for more valuable/rare foods (e.g. they preferred the coula nuts to palm nuts)

Thermoregulation hypothesis

Theory that bipedalism helped hominins regulate their body temperature

Savannah hypothesis

"Man did not leave the trees, the trees left them" (Weinert, 1932)Movement from the rainforest into the Savannah drove bipedalism; walking on two feet would be advantageous in an open grassland because it allowed hominins to spot predators and prey

Energy economy hypothesis

Theory that the energy 'cost' for walking was lower in early hominins than it is in extant primates; supported mathematical models from Sockol et al., 2007

Sockel et al., 2007

Suggested that human bipedalism is 4 times more energy efficient than chimp knuckle-walking

Chimp bipedalism (relative to humans)

Short hindlimbs; flexed knees and hips; pelvic tilt; feet and knees lateral to center of mass (essentially chimps look like they're squatting)

Everted nasal margin

A derived (unique) feature of Homo; external nose helps to humidify inhaled air and recapture moisture from exhaled air (helping prevent dehydration); might have been an adaptation to prevent dehydration

Multi-step transition summarized

LCA --> Australopiths - Canine reduction, postcanine expansion, bipedal walkers & climbers; Australopiths --> H. habilis - slight brain size increase, more vertical face; H. habilis --> H. erectus - body size increases, postcanine reduction, bigger brain, bipedal walkers & runners

Oldowan stone tools

Typified by 'choppers,' stone cores with flakes removed from part of the surface creating a sharp edge, from 2.5 mya made in what is now Tanzania (Carvalho and Beardmore-herd, 2019); previously oldest known stone tool industry

Acheulian stone tools

Made starting around 1.7 mya; these so called 'handaxes' were first made by H. erectus followed by H. heidelbergensis; 'revolution in stone age technology' as this industry lasted over 1 million years and is found in Europe, Africa, and Asia

Semaw et al., 1997

Dikkia, Ethiopia 2.5 mya bones with stone tool markings on them - shows meat and marrow consumption; previous oldest indirect evidence for tool-use

Toth et al., 1993

Kanzi, a bonobo in captivity (who do not typically make tools) was taught how to make stone tools in captivity; was even able to teach his sister; these tools were not as good as Oldowan stone cools, but he had little evolutionary pressure as he was a captive monkey

Proffitt et al., 2016

The capuchins of Serra da Capivara National Park flake stone tools in the wild; similar (but not as good) to Oldowan relics

What were early stone tools used for?

Cutting up meat before eating it (spend less time chewing; increasing the efficiency of digestion and retaining more energy; this is evidenced best by the reduced chewing apparatus found in H. erectus); access to new environments (Dmanisi hominins) with new resources; impact on social learning & social flexibility

Persistence hunting

A combination of running, walking and tracking to pursue prey until it is exhausted; quadrupeds cool themselves by panting, but they can't pant while galloping whereas humans cool themselves by sweating -- this allows humans to chase their prey until heat exhaustion

Levi-Strauss, 1969

People don't have to cook their food, they do so for symbolic reasons; people believed that most human foods are edible raw and thus cooking was truly unnecessary

Koebnick C. et al., 1999

Suggests that a human raw-food diet leads to loss of fertility; and thus doesn't make sense for human evolution; also hunter-gathers actually cook most of their evening meals

Benefits of cooking

Cooking allows for a higher energy gain from all major macronutrients of around 50% by increasing digestibility and decreasing digestion costs

Direct evidence of cooking

Late Middle Paleolithic, around 44-50 ka; gelatinized starch grains found in Neanderthal teeth (Henry et al., 2010)

Indirect evidence of cooking

At least 1 mya; evidence of controlled fire (Alperson-Afil, 2007); H. erectus had smaller guts, rounded molars and weak jaws, which also suggests cooked food

Schoenemann, 2006

Multi-stage hypothesis that suggested that hominin brains grew first from the digestion of meat, and then from cooking; but this hypothesis is problematic because there is no association between meat-eating and brain size; and there is a lack of physiological adaptation in the fossil record (e.g. enzyme production, gut kinetics)

Herculano-Heuzel, 2016

Argues that brain size grew first with the advent of bipedality and hunting-gathering; and then grew exponentially with the advent of cooking (and potentially more meat with the cooking commitment of H. erectus)

Bobe and Carvalho, 2019

Suggest that there may have been 3-4 different hominin species coexisting around 1.5 mya in the Koobi Fora Formation--including H. habilis and H. rudolfensis--which seems unusually high; thus suggests that H. habilis and H. rudolfensis may have been one species

Savannah-Mosaic Theory

Argues that bipedalism evolved on a variety of environments from woodland to grassland including a gradual environmental transition; potentially would enhance the carrying hypothesis

Wadley et al., 2020

200,000 years ago Homo sapien was using fire and grass to create comfortable areas for sleeping; one of the first instances of material culture

Homo floresiensis

Lived 100kya-50kya on the Island of Flores, Indonesia; known for being an example of island dwarfism (around 3.5 ft tall adults)

Homo heidelbergensis

Lived 700kya-200kya in Africa and Europe, LCA of H. sapiens and H. neanderthalensis, descended from H. erectus after they left Africa; very similar to H. erectus with an expanded brain and a bigger face

Multiregionalism model

Suggests that ancient H. sapiens, Neanderthals, and other Homo groups were an interbreeding network of one species, exchanging gene flow in both directions and hybridizing into modern H. sapiens; meaning that H. sapiens did not evolve 'out-of-Africa' but rather across multiple continents (Wolpoff, Thorne, & Wu, 1984)

Recent African Origin Theory (RAO)

Proposes that 60,000 years ago, H. sapiens left Africa and began to disperse around the world, without any interbreeding (Stringer & Andrews 1988)

Recent African Origin Theory with Hybridization (RAOWH)

Suggests that H. sapiens did interbreed with other hominin groups, but for a relatively short time with a quick demic replacement given the small size of other Homo groups, their high proposed deleterious load, the question of sterility, and the existing aDNA in modern H. sapiens (Brauer, 1992)

Leaky Replacement Hypothesis

Similar to RAOWH but relatively modern, based on genetic data obtained from both Neanderthal and Denisovans, coined by Paabo in 2011

Cognitive Revolution

The transition between the Middle and Upper Paleolithic period is marked by abundant archeological evidence that newly arrived modern humans were thinking and behaving differently than earlier Neanderthals; occurred around 50 kya; included new kinds of stone tools and cave paintings; this is the point that humans become 'behaviorally modern' (they had already been anatomically modern for about 200 kya)

Self-domestication hypothesis (Hare et al., 2012)

Selection against (reactive) aggression during the course of human evolution has facilitated the high levels of prosociality, tolerance, and cooperation that characterize human society

Hare, Wobber, and Wringham, 2012 (Hare et al., 2012)

Proposed the self-domestication hypothesis by comparing bonobos to chimpanzees, suggesting that bonobos' natural selection caused a 'domestication-like syndrome;' drew comparisons between bonobos and domesticated animals based on their morphology, physiology, and psychology (reduced canine teeth, juvenile cranium, increased tolerance and less aggression, altered hypothalamic-pituitary axis)

Belyaev's fox experiment

Attempted to domesticate wild foxes based on the idea that domestication is a selection for 'tameness'; thus there is a 2-part domestication syndrome:

1. Selected traits - reduction in aggression and fear response

2. Unselected traits - an unpredictable mixture - in his case, white patches, floppy ears, curly tails, juvenile features, etc.

Domestication syndrome

Suite of traits not selected for, but somehow linked to a reduction in aggression

Hare, 2017

Proposed that humans' self-domestication process was a result of selection for pro-sociality; unintended by-products included a lighter body, shorter face, juvenilization of skull and skeleton, 10-15% brain reduction as compared to H. sapien samples from 30kya

The cultural niche

Humans have evolved into social creatures such that we can engage in niche construction; our unique ability for social learning-accumulating and spreading knowledge-has allowed for culture to 'become' our environment (Boyd, Richerson and Henrich, 2011)

Wrangham, 2017

Argues that self-domestication occurred as aggression as selected against by the use of 'selective coalitionary proactive aggression,' aka capital punishment - although capital punishment is a form of violence itself, it is different because it is 'controlled'

2 routes to human self-domestication

1. Cultural niche - Social institutions and norms favor less aggressive individuals who follow rules, etc.

2. Selective coalitionary proactive aggression (capital punishment)

Seyfarth, Cheney and Marler, 1980

These monkeys produce specific responses to different predators: running into trees when a leopard is spotted, looking up for eagles, and looking down for snakes; this shows that these monkeys possess semantic meaning and demonstrate first-order intentionality; further research suggested that there is context specific usage of these calls, although the production of these calls is innate, limiting their scope and flexibility

Seyfarth et al., 1980

Showed that vervet monkeys have a 'rudimentary language' with semantic meaning; monkeys respond to calls about different predators (snakes, leopards, eagles) in different ways

Cheney and Seyfarth, 1985

Showed Frequency of alarm calls from dominant female vervets increased in the presence of their offspring but decreased in the presence of unrelated juveniles, suggesting that alarm calls are not merely automatic but based on social context

Crockford et al., 2012

Conducted a field experiment showing chimpanzee calling behavior is altered based on the awareness of other group members; chimpanzees were more likely to issue alarm calls about snakes in the presence of group members who were unaware of the danger; demonstrating second-order intentionality

Orders of intentionality

Zero order intentionality - caller has no intentions while vocalizing, vocalization is automatic;

First order intentionality - caller wants to change the behavior of others with their vocalization;

Second order intentionality - caller wants to change the knowledge of others with their vocalization

Oren et al., 2024

Employed machine learning technology to analyze marmoset vocalizations, discovering that marmosets use distinct vocal labels for individual group members (essentially names); marmosets respond to these labels directly, demonstrating a level of personal identification and social interaction

Halina et al., 2018

Observed that apes in captivity used pointing gestures mainly to request food; this use of pointing is primarily goal-oriented, reflecting an imperative communication rather than a declarative one

Tomasello, 2006

Suggested that apes do not understand human pointing gestures because they are declarative, not imperative; in several studies, apes are tested when a director points towards the location of their desired food, but the apes don't grasp this communication

Differences between primate and human communication

Humans have shared intentionality, theory of mind, and ostensive communication; while primates do communicate, their methods lack the depth, flexibility, and scope of human language

Shared intentionality

Humans are much more likely to share goals, coordinate behavior, prompt others, offer help, etc.

Theory of mind

Humans are able to understand that others have different goals, knowledge, desires and beliefs from oneself

Ostensive communication

Recognition of a signal as an attempt to communicate that allows listeners to make meaningful inferences (e.g. winking)

Badihi et al., 2023

Demonstrated socially-derived (cultural) variation amongst leaf-modifying behaviors in chimpanzees; researchers found that between two neighboring communities of East African chimpanzees, the Waibira group displayed multiple types of leaf-modifying behaviors, while the Sonso group exhibited very few

Primate dialects in communication

These are predominantly observed as vocal signals, but extant primates often display these in non-vocal activities such as leaf-modifying; by ruling out ecological differences, scientists conclude that differences in these behaviors are socially derived

Van Schaik and Pradhan, 2003

Developed a model that found a greater incidence of tool use, a culturally acquired skill, in more sociable populations of orangutans and chimpanzees; they indicate that sociable gregariousness is more likely to produce the maintenance of invented skills; supports the social intelligence hypothesis

Social intelligence hypothesis

As suggested by Dunbar, 1998; intelligence evolved as a response to challenges posed by social complexity

Dunbar, 1998

Found an association between neocortical size and group size in primates, used as evidence for social intelligence theory

Steele et al., 2012

Found that broca's area in primates is involved in motor control, revealing that the evolution of language in humans is likely a co-option of this area; there are many similarities between the use of tools and the use of linguistic syntax, seemingly suggesting how language evolved in people from primate tool-use skills

Holekamp and Benson-Amram, 2017

Suggested that outside of the primate order, there is no or limited association between animals that live in large groups and have large brains

Cultural intelligence hypothesis (Whiten & Van Schaik, 2007)

Evolution of high intelligence is linked to the acquisition and use of socially learned information, or 'culture;' the evolution of intelligence is supported by longer life history strategies and larger brains that can acquire, organize, store, and retrieve socially learned information

Street et al., 2017

Provided evidence supporting the cultural intelligence hypothesis, as they found an association between social learning, group size, brain volume, and longevity across primate species

Caveats from primate comparisons

Bias from human observation, effect from human observation, effect from captivity, difference between primates and LCA

Bias from human observation

In Badihi et al., 2023, researchers suggested that their own bias may have played a role in some of the results; as humans may be more likely to notice some leaf-modifying behaviors when the behaviors are also accompanied by other behaviors; also behaviors that solicit human interactions are more likely to be recorded

Effect from human observation

Close contact with humans, regardless of whether the chimpanzees are wild or in captivity, can affect results; as primates may act a particular way when they are observed

Effect from captivity

Many primates use tools in captivity that they don't use or even manufacture in the wild; being in captivity can affect subjects in all sorts of ways (example of chimpanzee audiogram)

Chimpanzee audiogram

Tests of captive chimpanzees' hearing suggest they have a W-shaped audiogram, rather than the usual chevron shape of most mammals; it has been suggested this is from the effect of captivity as it mimics the shape of noise-induced hearing loss in humans

Evolution

Change in the properties of groups of organisms over the course of generations (Futuyma, 2005)

Ontogeny

The development of a single individual, or a system within an individual, from the fertilized egg to their birth, maturation and death (Smith, 1960)

Culture

"The way we do things;" group-typical behavior patterns shared by members of a community that rely on socially learned and transmitted information (Laland & Hoppitt, 2003)

Childhood

Period of development with dependency as the key defining aspect; those in this period rely on parental investment of protection, but they have weaned; it includes rapid brain growth, immature dentition, and a small digestive system; ages 2-7 (Bogin & Smith, 1996)

Juvenile Period

Period of development with maturation of cognition, more self-sufficient as compared to childhood; 90% of the brain weight of adults; Eruption of 1st premolar; ages 7-11 (Bogin & Smith, 1996)

Adolescence

Period of development best defined by puberty (sexual maturity), growth spurts, and greater self sufficiency; ages 11-18 (Bogin & Smith, 1996)

Cooperative ontogenetic niche

A developmental period defined by having more 'carers' (e.g. allo-parenting)

Migliano et al., 2017

Study of Agta and BaYaka hunter-gatherer peoples suggested that hunter-gatherer social networks are organized to support the efficient transmission of information (e.g. cultural traits) as the proportion of non-kin connections increase during the developmental period

Bogin & Smith, 1996

Argued that, although social mammals have 3 basic stages of postnatal development (infancy, juvenile, adult), the pattern of human growth is best characterized by 5 stages (infancy, childhood, juvenile, adolescence, adult)

Adulthood

Final period of development, full reproductive maturity and complete autonomy (Bogin & Smith, 1996)

Life history

addresses how organisms allocate their limited time and energy to the various activities that comprise the life cycle, namely physical and cognitive growth, maintenance of bodily tissues, mating, and parenting (Roff, 2002; Stearns, 1992 in Ellis et al., 2022); often trade-offs between long-term survival and short-term reproduction

Human life history strategy

Wean earlier (around age 2.5); prolonged period of dependency; mature/reproduce later relatively; more children & shorter interbirth intervals; longer lifespan, menopause

Grandmother hypothesis

Evolutionary theory that explains why human females live longer than they can reproduce; suggests that maternal grandmothers are able to increase their fitness through non-reproductive means, like through caring and provisioning for child relatives, potentially allowing mothers to wean earlier, comes from HBE (Hawkes et al., 1998)

Why did childhood evolve?

Most hypotheses point to the energy (and therefore time) required for human learning and growth, particularly brain growth and maturation; essentially, childhood allows for larger brains

Kuzawa et al., 2014

PET and MRI data was analyzed for brain-body metabolic trade-off; during childhood, brain metabolic requirements take up 43% of daily energy requirement; brain growth appears to effectively inhibit body growth at certain points of development, e.g. body grows after brain grows

Complex Foraging Niche Hypothesis (Schuppli et al., 2016)

Suggests that the components of the human foraging niche, such as social egalitarianism, cooperation (extractive foraging, some cooperative hunting) and mobility, led to the evolution of a multilevel social structure with intergenerational transfers of information; also suggests that a long period of childhood because children's foraging proficiency increases more slowly for more difficult-to-extract resources, such as tubers and game, while increasing more rapidly for easier-to-extract resources, such as fruit and fish; thus childhood evolved as a way for humans to learn the complex foraging skills required for their niche

Volk & Atkinson, 2013

Argue that infancy and childhood are adaptations in their own right; children are not simply incomplete or underdeveloped adults, but rather adapted to meet the needs and demands of childhood

Why is childhood significant to human evolution?

Children learn adaptive physiological and behavioral strategies; there is great plasticity in learning strategies, selection does not just happen on adults; genetic and nongenetic factors guide and constrain development so that children can maximize their adult fitness

Developmental plasticity

The ability to modify cognition and behavior in response to environmental conditions

Walasek et al., 2022

Argues that childhood is a sensitive period (not critical period!), as plasticity in childhood shapes development, 'we have evolved to be adaptable;' but even adults can continue adapting

Gabriel and Lynch, 1992

Calculated that the evolution of reaction norms would be favored over broadly adapted genotypes if the between-generation variation in the environment is large compared to the within-generation variation

Tollrian, 1995

Study of the water flea Daphnia pulex demonstrated plasticity during development phase; exposure to particular chemicals caused the waterflea to develop neck teeth to protect against predatory phantom midge larvae

Gopnik, 2016

Argues that allo-parenting allows child to be exposed to a wide variety of information and models, thus improving the child's range of plastic developmental possibilities

Evolutionary developmental psychology

Rather than being adapted to the pleiocosetence, human minds are constantly adapting within lifespans; minds have innate cognitive mechanisms like attention, memory, and reasoning designed by natural selection to be sensitive to environmental conditions

Cumulative risk-research

More environmental risks to which children are exposed, the more compromised their development will be (Ellis et al., 2022)