Human evolution

How can we distinguish traits?

Primates have evolved functional traits which respond to different environmental pressures e.g. limbs adapting to their habitat, teeth responding to their diet

Some traits may have evolved because they’re adaptive, while other may have evolved in association with adaptive traits (not because they were useful themselves)

Isometric scaling = two traits increase in direct proportion (this is what we expect, a trait follows body size)
Allometric scaling = two traits increase at different rates (this suggests a special adaptation or constraint because it doesn’t passively follow body size) e.g. primates have larger relative brain sizes compared to their body sizes than other mammals - this is positive allometry and it likely evolved as an adaptation for enhanced cognition

What is the social brain hypothesis?

Species with larger social groups tend to have a larger neocortex
(Kudo & Dunbar, 2001) provide evidence which directly supports the social brain hypothesis and proposes that the evolution of large brains in primates was driven by the cognitive demands of managing complex social relationships
Limitation = this pattern holds for primates and birds, but not for octopi (solitary but highly intelligent), which suggests different evolutionary paths to cognition

Gorillas are larger than chimpanzees, have smaller relative brains, and lower basal metabolic rates (allometric scaling - larger animals need less energy per unit of weight)
Large animals can rely on lots of low quality food, while small animals need less higher-quality foods

This shows how energetic constraints can shape brain evolution

What is the Giarmon-Bell hypothesis?

Small animals have higher metabolic rates and must eat higher energy foods to survive

As animals become larger, they become more metabolically efficient and need less energy per unit of body weight (larger animals can survive on lower-quality food, and if they have access to higher quality food, they can use the excess energy to support a larger brain

What are some implications on life history?

Larger brains require longer developmental periods, so offspring are born altricial (helpless) and require extended parental care - this promotes group living and cooperative caregiving

Larger mammals produce less offspring than smaller mammals, they have longer life expectancies, and slower life histories (longer gestational periods, slower rates of postnatal development, etc.)

Generations can overlap which allows for social learning

What other adaptations do primates have?

Primates show a greater reliance on vision - larger eyes, smaller snouts, reduced olfactory lobes in the brain, increased colour vision and depth perception

Arboreal life hypothesis = vision evolved to help primates gauge distances between branches to leap from one to the other to find food and escape predators

Visual predation hypothesis = depth perception and precise vision evolved to locate and catch small prey

These hypotheses are not mutually exclusive (enhanced vision provides dual advantages of living in trees and hunting prey)

They have five digits, an opposable thumb, and a partially opposable big toe to allow for precise grip (they can also use their tail as another limb)

They have nails instead of claws and tactile pads with dense sensory nerves to enhance coordination and tactile sense

They have a clavicele which allows reposition of the upper body and enhanced arm mobility for a greater range of motion

These are key adaptations to arboreal life, and for the further involvement of tool use and fine motor skills

What is the co-evolution principle?

This suggests that reciprocal evolutionary changes between pairs of species or groups can occur as they interact with one another (species evolve in response to each other)

Angiosperm co-evolution hypothesis = early primates evolved key traits for the purpose of eating and living from fruits (excellent colour vision and fingers without claws), while angiosperms evolved fruits and flowers that were easier to disperse

Provides an alternative, diet-based hypothesis to the previous hypotheses (social brain hypothesis, arboreal life hypothesis, visual predation hypothesis)

Where can we place humans within the lineage?

Humans are part of the hominid family (great apes) and the hominin family (the human lineage post splitting from chimpanzees)
Hominins are considered the ancestors of humans
Fossils show a gradual change from ape-like features to human features

The single species hypothesis = only one hominin could exist at a given time due to competitive niche exclusion (any two species that occupy the same niche will compete until one of them becomes extinct)
Homohabilis and Paranthropus boisei coexisted (which challenges this idea), this suggests early hominin diversity
Homo habilis were more advanced than Paranthropus boisei (they had more developed cognitive and motor capabilities)

We can use molecular clocks to show that the human lineage is nested within the great apes (they estimate when species diverge by calculating the tick rate - the rate of genetic mutations over time)

What are the differences between humans and great apes?

Humans have larger brains with a football shaped brain case and a much smaller face

Apes have large, sexually dimorphic canines with a C/p3 honing complex (these could be used for sexual selection or social signalling)

Bipedalism is a key hominin trait (shorter and broader pelvis, valgus knee, a foramen magnus is positioned centrally under the skull)

What are the earliest hominins?

Sahelanthropus tchadensis

• Small brain and a forward foramen magnum

Orrorin tugenensis

• Partial femur which hints at bipedalism

• Large canines with a reduced C/p3 complex

• Significant climbing abilities

, Ardipithecus

• Ardipithecus kadabba

  • Residual honing ande female ape-like C/p3 honing complex

  • Single toe bone (could indicate bipedalism)

• Ardipithecus ramidus

  • Small brain (hominin-like) and flat face (ape-like)

  • Sexual dimorphism in canines

  • Hominin-like pelvis

Early hominins were adapted to both trees and ground living

What are the Australopiths?

A group of early hominins which belong to the genus Australopithecus and Kenyanthropus

They are small-bodied and show weak sexual dimorphism in canine size (this suggests multi male, multi female groups or amalgamous pairs)

They have a valgus knee, and a human-like pelvis, a curved lumbar lordosis (similar to humans, to maintain centre of gravity)

They have a U-shaped hyoid bone (similar to humans) which indicates the possibility of speech

Australopiths were bipedal with many human-like adaptations, but retained some ape-like traits

What are Paranthropus?

Australopithecus split into Paranthropus and humans

Paranthropus is the sister clade to the Homo genus (it is not an ancestor, but is a close distinct cousin) which went extinct for unkown reasons

They lived during the Cenozoic (cooler and more variable climates with increased C4 biomass)

Paranthropus includes Paranthropus robustus and Paranthropus boisei (they share anatomical features)

They have large jaws, faces, and teeth (adapted for heavy chewing - large temporalis muscle and rotary chewing)

Paranthropus boiseis ate mainly C4 grasses (no other primates do this), and this has been gathered from isotope evidence
There are other hard-shelled foods which also would leave a C4 signal
(Ungar & Sponheimer, 2011) showed that these animals would have different toothwear based on their foods - hard, brittle food = crushing movement (complex, pitted teeth) and soft, tough food = shearing movement (simple, striated teeth)

What are early Homo?

Early homo are exclusively African species (Homo habilis, Homo rudolfensis, Homo erectus)

Expensive tissue hypothesis = in order to grow a larger brain, more energy is needed - smaller guts and bipedalism will free up energy to evolve larger brains to support more complex behaviours

Recognising early Homo in fossils

• Larger brain and less prognathic face

• Larger body size with adaptations for bipedalism

• Use of stone tools and evidence of carnivory

Homo erectus (the last to appear of the early Homo) originated in Africa but quickly moved to Eurasia, and has a different frontal lobe topology to earlier species

What are Neanderthals?

They are closely related but distinct hominin species to humans

They interbred with humans through introgression

Their reasons for extinction are unkown

What can be said about Homo sapien evolution?

Multiregional continuity = modern humans evolved from archaic hominin populations dispersed across Africa and Eurasia

Out of Africa = all modern humans share a single origin in Africa, and then they spread out across the world (this is the supported model)

• Earliest fossils, genetic diversity, and lineages track back to Africa

• Late emergence of art and symbolism in Africa

What is health and disease?

Health = a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity

Disease = an abnormality of structure or function

Bodies evolve for reproductive success, not necessarily optimal health

What is the epidemiological transition framework?

It describes the shift in disease patterns within a population over time

Phase 1 - pestilence and famine (high mortality and low population growth)

• Associated with the neolithic revolution (the development of food production)

Phase 2 - receding pandemics (disease control begins and population begins rising)

• Increase in crowding, poverty, poor sanitation, pollution (lack of exposure to sunlight)

• To improve this, we can use antibiotics, aseptic techniques, and vaccines

Phase 3 - degenerative and man-made diseases (lifestyle-related illnesses)

• Dietary excess, inactivity, smoking, alcohol consumption

• Using disinfectants is training our immune system to not be able to fight diseases as effectively

Phase 4 - delayed degenerative diseases and emerging infections (chronic disease treatment improves, but new infections emerge)

• We can treat non-commincable diseases, but emerging infections are rising again

Overgeneralisation - it assumes a universal, linear progression across all societies

How else can we represent disease spread?

R0 is the number of people you would expect to infect as an individual, if R0 > 1, we will see an exponential growth of infection
Density will affect how many people you interact with, and therefore can impact the spread of disease

SIR model = we can divide the population into groups of susceptible, infected, and recovered

Zoonotic diseases refer to diseases that can be acquired through close contact with animals (especially mammals e.g. bats)

What is the neolithic revolution?

A shift to agriculture, which included sedentism, population growth, domestication of plants and animals

This led to a decline in health - decline in stature, increased arthiritis and dental issues, malnutrition from carbohydrate-heavy diets, and higher diseases risks due to poor sanitation and close association with animals

What are the different types of pathogens?

Heirloom pathogens = ancient pathogens that co-evolved with humans e.g. TB

Souvenir pathogens = pathogens that were picked up during human migration to Eurasia and into the Americas e.g. plague

(Zeber & Pablo, 2021) showed that ancient DNA reveals some Neanderthal alleles increase or decrease susceptibility to Covid-19

What is the obstetric dilemma?

Obstructed labour (when the foetus doesn’t progress through the birth canal) can be caused by malpresentation (poor positioning), cephalopelvic disproportion (pelvis is too small), macrosomia (baby is too large)

The obstetrical dilemma = the evolutionary trade-off in humans between having a pelvis narrow enough for efficient bipedal walking and wide enough to allow the passage of a large-brained baby during childbirth

Maternal obesity and short stature increase these risks

Cultural factors can contribute to the dilemma e.g. the use of the supine birth position (giving birth in a vertical position can allow the pelvis to open up naturally)

We can solve this dilemma by improving female childhood nutrition and maternal growth to enhance pelvic dimensions

What can be said about larger mammals?

They reproduce later, they have a longer maximum lifespan, they have a lower annual fertility rate (they don’t reproduce every year, or when they do reproduce, they produce less offspring)

Primates live longer than expected for their body size

How can we describe human development?

Human growth = childhood, juvenility, adolescence

Brain growth finishes earlier than body growth

Humans stay energetically dependent for much longer than other primates (they rely on the mother for much longer)

The extended juvenile period supports learning and brain development

What can be said about energetics and reproduction?

Birth stacking = lots of offspring are dependent on the mother at the same time (this saves energy)

Life history = the strategy that the organism is using

Humans will reproduce slowly but invest heavily (K selection)
R selection = selection for growth and reproduction, these organisms reproduce very fast
K selection = selection for survival, these organisms won’t reproduce as quickly but they will live a long time
It’s difficult to be R selected because these organisms need to invest a lot of energy into their bodies

How do organisms meet the energetic costs of their demanding life histories?

Organisms don’t just increase their total energy expenditure, instead they shift to a high-investment, high-return foraging niche (hunter-gatherer niche)

Human foragers produce more energy than they consume (they spend less time foraging and get more energy in return)

Females show energy deficits due to childcare (social systems and cooperation can balance these energy deficits)

Reproductive strategy
Humans evolved to live longer and reproduce over time
Grandmother hypothesis = grandmothers will help increase their indirect fitness by helping raise their grandchildren
Selectve shadow = harmful genes acting reproduction aren’t selected against e.e. genes for Alzheimer’s

Energetic interdependence = the positive correlation between the youngest child’s age and the productivity of the mother

Humans form pair-bonds (long-term sexual/reproductive partnerships)
This leads to high levels of paternal care, reduced testosterone levels in men (reduced mating, more caring), paternal certainty (allows offspring to recognise their fathers and the father is reassured that the offspring is his so will invest into it more), the production of full siblings (which will increase kin selection)

How can we visually represent the evolution of humans?