16  Human Evolution

16.1 Learning Objectives

By the end of this chapter, you should be able to:

  1. Describe our place in the primate family tree and identify key distinguishing primate characteristics
  2. Trace the major events in hominin evolution from early australopithecines to Homo sapiens
  3. Explain the evidence for human evolution from paleontology, archaeology, and genetics
  4. Describe key adaptations in human evolution including bipedalism, brain expansion, and tool use
  5. Analyze debates about modern human origins (Out of Africa vs. multiregional models)
  6. Explain how genetic evidence informs our understanding of human evolution and migration
  7. Describe recent human evolution and ongoing evolutionary processes
  8. Evaluate common misconceptions about human evolution

16.2 Introduction

Human evolution represents the story of our own species—where we came from, how we became what we are, and what makes us unique among primates. This chapter traces our evolutionary journey from early primates to modern humans, examining the fossil evidence, genetic data, and archaeological remains that tell this remarkable story. Understanding human evolution not only satisfies our curiosity about our origins but also provides insights into human biology, behavior, and health. It reminds us that we are part of the natural world, shaped by the same evolutionary processes that have shaped all life on Earth.

16.3 Primate Origins and Characteristics

16.3.1 What Makes a Primate?

Primate characteristics (shared derived traits):

  1. Grasping hands and feet: Opposable thumbs/big toes
  2. Nails instead of claws: Tactile pads with fingerprints
  3. Forward-facing eyes: Stereoscopic vision, depth perception
  4. Large brains relative to body size
  5. Extended parental care: Long gestation, postnatal development
  6. Social complexity: Varied social systems

Evolutionary trends:

  • Reduction of snout
  • Expansion of visual cortex
  • Increased behavioral flexibility

16.3.2 Primate Phylogeny

Major groups:

  • Strepsirrhines: Lemurs, lorises (more primitive features)
  • Haplorhines: Tarsiers, monkeys, apes, humans

Anthropoids (monkeys and apes):

  • Platyrrhines (New World monkeys): Flat noses, prehensile tails
  • Catarrhines (Old World monkeys and apes): Downward nostrils
    • Cercopithecoids (Old World monkeys)
    • Hominoids (apes and humans)

Apes (Hominoidea):

  • Lesser apes: Gibbons, siamangs
  • Great apes: Orangutans, gorillas, chimpanzees, bonobos, humans

16.3.3 Our Closest Living Relatives

Genetic evidence:

  • Humans and chimpanzees: 98.8% identical at DNA level
  • Divergence time: ∼6-7 million years ago
  • Common ancestor: Not a chimpanzee, but a chimp-human ancestor

Comparative anatomy:

  • Similarities: Skeletal structure, brain organization, blood proteins
  • Differences: Bipedalism, brain size, language, culture

16.4 Early Hominin Evolution

16.4.1 Defining Hominins

Hominins: Humans and all ancestors after divergence from chimpanzee lineage

Key adaptations:

  1. Bipedalism: Walking on two legs
  2. Tool use (later)
  3. Brain expansion (later)
  4. Language and culture (much later)

16.4.2 The First Hominins

Sahelanthropus tchadensis (∼7 million years ago):

  • Discovery: Chad (2001)
  • Features: Small brain, ape-sized, but possibly bipedal
  • Significance: Possibly earliest hominin

Orrorin tugenensis (∼6 million years ago):

  • Discovery: Kenya (2000)
  • Evidence for bipedalism: Femur structure

Ardipithecus (∼5.8-4.4 million years ago):

  • Ardipithecus ramidus (“Ardi”):
    • Discovery: Ethiopia (1994)
    • Features: Both climbing and bipedal adaptations
    • Environment: Woodland, not savanna

16.4.3 Australopithecines

Time range: ∼4.2-1.9 million years ago

Key features:

  • Brain size: 400-500 cc (similar to chimps)
  • Body size: 1-1.5 m tall, 30-50 kg
  • Bipedal: But also arboreal adaptations
  • Diet: Mostly plant-based

Major species:

  1. Australopithecus anamensis (∼4.2-3.9 mya): Possible ancestor
  2. Australopithecus afarensis (∼3.9-2.9 mya): “Lucy”
    • Laetoli footprints (3.6 mya): Clear evidence of bipedalism
  3. Australopithecus africanus (∼3.0-2.1 mya): Taung Child
  4. “Robust” australopithecines (Paranthropus): Massive jaws, sagittal crest
    • P. boisei (2.3-1.2 mya): “Nutcracker Man”

16.4.4 Why Bipedalism?

Advantages (hypotheses):

  1. Energy efficiency: More efficient for long-distance travel
  2. Heat stress: Less surface area exposed to sun
  3. Carrying: Free hands for tools/food
  4. Seeing over grass: In savanna environment
  5. Display: Sexual selection

Evidence for arboreal heritage:

  • Curved fingers/toes
  • Shoulder joint orientation
  • Lower limb proportions

16.5 The Genus Homo

16.5.1 Early Homo

Homo habilis (∼2.4-1.4 million years ago):

  • Name means: “Handy man”
  • Brain size: 500-800 cc
  • Associated with: Oldowan stone tools
  • Diet: More meat than australopithecines

Homo erectus (∼1.9 million - 110,000 years ago):

  • Key developments:
    1. First out of Africa: ∼1.8 mya (Dmanisi, Georgia)
    2. Acheulean tools: Handaxes (symmetrical, bifacial)
    3. Larger body size: More human-like proportions
    4. Brain size: 750-1250 cc
    5. Possible use of fire: ∼1.0 mya (controversial)
  • Regional variants:
    • H. ergaster (Africa)
    • H. erectus (Asia)
    • Possibly H. antecessor (Europe)

16.5.2 Later Homo Species

Homo heidelbergensis (∼700,000-200,000 years ago):

  • Possible ancestor of both Neanderthals and modern humans
  • Brain size: ∼1200 cc
  • Tools: More advanced Acheulean, possibly spears
  • Evidence of hunting: Large game

Homo floresiensis (“Hobbit”, ∼100,000-50,000 years ago):

  • Discovery: Flores, Indonesia (2003)
  • Features: 1 m tall, 400 cc brain
  • Tools: Stone tools despite small brain
  • Origin: Possibly dwarfed H. erectus

Denisovans (discovered 2010):

  • Known from: DNA from finger bone (Siberia)
  • Relationships: Sister to Neanderthals
  • Legacy: Interbred with modern humans (Melanesians, Australians)

16.6 Neanderthals and Modern Humans

16.6.1 Neanderthals (Homo neanderthalensis)

Time range: ∼400,000-40,000 years ago

Distribution: Europe, western Asia

Physical characteristics:

  • Robust build: Cold adaptation
  • Large nose: Warming/humidifying air
  • Brain size: 1200-1750 cc (larger than modern humans)
  • Occipital bun: Bulge at back of skull

Culture and behavior:

  • Mousterian tools: More refined than Acheulean
  • Use of fire: Definite evidence
  • Burial of dead: With grave goods (possible ritual)
  • Possible art: Engravings, pigments
  • Language capability: Similar hyoid bone, FOXP2 gene

Extinction: ∼40,000 years ago

  • Causes: Climate change, competition with modern humans, interbreeding

16.6.2 Anatomically Modern Humans (Homo sapiens)

Earliest fossils: ∼300,000 years ago (Jebel Irhoud, Morocco)

Key features:

  • High, rounded skull
  • Small face tucked under braincase
  • Prominent chin
  • Gracile skeleton

Behavioral modernity (∼50,000 years ago):

  • Complex tools: Blade technology
  • Art: Cave paintings, figurines
  • Personal adornment: Beads, pigments
  • Musical instruments: Flutes
  • Long-distance trade

16.6.3 Modern Human Origins Debate

Out of Africa (Replacement) Model:

  • Modern humans evolved in Africa ∼300,000 years ago
  • Migrated out ∼60,000 years ago
  • Replaced other Homo species with little interbreeding
  • Evidence: Genetic, fossil dating

Multiregional Model:

  • Modern humans evolved simultaneously in multiple regions
  • Gene flow between populations maintained species unity
  • Evidence: Regional continuity in fossils

Current consensus: Mostly Out of Africa with some interbreeding

16.7 Genetic Evidence for Human Evolution

16.7.1 Nuclear DNA Comparisons

Human-chimp divergence: ∼6-7 million years

Genetic differences: ∼1.2% (35 million base pair differences)

Accelerated evolution in humans: Genes related to brain, speech, immune system

FOXP2 gene: Language and speech

  • Human-specific changes: Two amino acid differences from chimp
  • Neanderthals: Shared human version
  • Implication: Language capability predates human-Neanderthal split

16.7.2 Mitochondrial DNA and “Mitochondrial Eve”

Mitochondrial Eve: Most recent common ancestor via maternal line

  • Lived: ∼150,000-200,000 years ago in Africa
  • Not the only woman alive at that time
  • Evidence: All living humans share mtDNA from this ancestor

Y-chromosomal Adam: Most recent common ancestor via paternal line

  • Lived: ∼200,000-300,000 years ago
  • Different from mtEve (not same individual)

16.7.3 Ancient DNA Revolution

Sequencing ancient DNA from fossils

Key findings:

  1. Neanderthal genome: Sequenced 2010
  2. Denisovan genome: 2010
  3. Interbreeding: Non-Africans have 1-4% Neanderthal DNA
  4. Melanesians: Additional 4-6% Denisovan DNA
  5. Multiple interbreeding events

Migration patterns: Traced through genetic markers

16.7.4 Recent Human Evolution

Lactase persistence: Digestion of milk into adulthood

  • Mutation: Different in Europeans and Africans
  • Selection: Strong with dairy farming

High-altitude adaptation (Tibetans):

  • EPAS1 gene: From Denisovans?
  • Mechanism: Different hemoglobin response

Skin pigmentation:

  • Variation: Related to UV radiation
  • Genes: Multiple, different in different populations

Disease resistance: Genes shaped by pathogen exposure

16.8 Human Migration and Dispersal

16.8.1 Out of Africa

First dispersal (Homo erectus): ∼1.8 million years ago

Second dispersal (modern humans): ∼60,000 years ago

Routes:

  1. Southern route: Along coast to Southeast Asia, Australia (∼50,000 years ago)
  2. Northern route: Through Middle East to Europe, Central Asia
  3. Bering land bridge: To Americas (∼15,000-20,000 years ago)

16.8.2 Peopling of the World

Australia: ∼50,000 years ago (oldest human remains outside Africa)

Europe: ∼45,000 years ago

Americas: Debate over timing

  • Clovis first: ∼13,000 years ago
  • Pre-Clovis sites: Possibly 15,000-20,000 years ago
  • Routes: Coastal vs. inland ice-free corridor

Pacific islands: Last major region settled

  • Polynesia: ∼3,000 years ago
  • New Zealand: ∼700 years ago
  • Madagascar: ∼1,500 years ago

16.8.3 Effects of Migration

Genetic drift: Founder effects in new populations

Adaptation: To new environments (climate, diet, pathogens)

Cultural evolution: Development of new technologies, languages

16.9 Human Uniqueness and Evolution of Culture

16.9.1 What Makes Humans Unique?

Biological differences:

  • Brain size and organization: Prefrontal cortex expansion
  • Bipedalism: Free hands
  • Vocal tract: Complex speech
  • Life history: Extended childhood, menopause

Behavioral/cultural differences:

  • Complex language: Syntax, recursion
  • Cumulative culture: Improvement over generations
  • Symbolic thought: Art, religion, mathematics
  • Technology: Beyond basic tools

16.9.2 Evolution of Human Characteristics

Bipedalism: Earliest major adaptation (∼6-7 mya)

Brain expansion: Gradual increase, then rapid in Homo

  • H. habilis: 500-800 cc
  • H. erectus: 750-1250 cc
  • H. sapiens: 1200-1700 cc

Tool use:

  • Oldowan: Simple flakes (∼2.6 mya)
  • Acheulean: Handaxes (∼1.7 mya)
  • Mousterian: Prepared cores (∼300,000 ya)
  • Upper Paleolithic: Blades, specialized tools (∼50,000 ya)

Language: Timing debated

  • Anatomical evidence: Hyoid bone (∼500,000 ya?)
  • Genetic evidence: FOXP2 (before human-Neanderthal split)
  • Archaeological evidence: Symbolic artifacts (∼100,000-50,000 ya)

16.9.3 Gene-Culture Coevolution

Examples:

  • Lactase persistence: With dairying
  • Alcohol metabolism: With fermentation
  • Amylase gene copies: With starch consumption
  • Disease resistance: With agriculture, population density

Cultural evolution: Can be faster than genetic evolution

16.10 Recent and Ongoing Human Evolution

16.10.1 Evolution in Historical Times

Evidence for recent selection:

  • Lactase persistence: Strongest known signal in Europeans
  • Skin pigmentation: Multiple selective events
  • High-altitude adaptation: Tibetan, Andean, Ethiopian
  • Infectious disease resistance: Malaria, plague, smallpox

Methods for detection:

  • Extended haplotype homozygosity: Long stretches of identical DNA
  • Population differentiation: Fst outliers
  • Ancient DNA comparison: Changes over time

16.10.2 Are Humans Still Evolving?

Arguments for:

  • Selection still operates (e.g., age at first birth)
  • Mutation still occurs
  • Some traits changing (e.g., age at menarche, height)
  • New selective pressures (e.g., diet, modern diseases)

Arguments against:

  • Medical care reduces selection
  • Birth control affects reproductive patterns
  • Globalization increases gene flow
  • Cultural evolution may dominate

Probably yes, but differently than in past

16.10.3 Future Human Evolution

Possible directions:

  • Resistance: To new diseases, environmental toxins
  • Adaptation: To changing climate, new diets
  • Cognitive traits: If correlated with reproductive success
  • Artificial selection: Through genetic technologies

Uncertainties:

  • Environmental changes
  • Technological developments
  • Social and cultural factors

16.11 Misconceptions and Controversies

16.11.1 Common Misconceptions

  1. “Humans evolved from chimpanzees”: No, we share a common ancestor
  2. “Evolution is progressive”: No direction or goal
  3. “Missing links”: All fossils are transitional
  4. “Evolution is just a theory”: Theory in scientific sense
  5. “If we evolved from monkeys, why are there still monkeys?”: Branches, not linear progression

16.11.2 Race and Human Variation

Genetic variation:

  • Within populations: ∼85% of total human genetic variation
  • Between populations: ∼15%
  • Between continents: ∼10%

Race as biological concept:

  • No clear genetic boundaries
  • Continuous variation, not discrete categories
  • More variation within than between “races”
  • Social construction with biological consequences

16.11.3 Evolutionary Psychology

Study of: Psychological adaptations

Controversies:

  • Just-so stories: Unfalsifiable explanations
  • Genetic determinism: Ignoring culture, plasticity
  • Adaptationism: Assuming all traits are adaptations

Valid insights: Some universal human behaviors may have evolutionary basis

16.11.4 Creationism and Intelligent Design

Scientific response:

  • Evolution is well-supported by multiple lines of evidence
  • No scientific controversy about fact of evolution
  • Intelligent design not testable science

Teaching evolution:

  • Important for scientific literacy
  • Does not conflict with religious belief for many
  • Understanding evolution essential for biology, medicine

16.12 Chapter Summary

16.12.1 Key Concepts

  1. Primate heritage: We share characteristics with other primates
  2. Hominin evolution: From bipedal apes to modern humans over ∼7 million years
  3. Key adaptations: Bipedalism, tool use, brain expansion, language
  4. Multiple Homo species: Coexistence and interbreeding
  5. Modern human origins: Mostly Out of Africa with interbreeding
  6. Genetic evidence: DNA confirms and extends fossil record
  7. Human migrations: Peopling of the world over last 60,000 years
  8. Recent evolution: Ongoing but changed by culture and technology
  9. Human uniqueness: Complex culture, language, cumulative technology
  10. Misconceptions: Need to correct common misunderstandings

16.12.2 Timeline of Human Evolution

Time Event Significance
∼7 mya Hominin-chimp split Beginning of human lineage
∼4.4 mya Ardipithecus Early bipedalism, woodland habitat
∼3.6 mya Laetoli footprints Clear evidence of bipedalism
∼2.6 mya First stone tools Beginning of technology
∼1.8 mya H. erectus out of Africa First intercontinental migration
∼300 kya First H. sapiens Anatomically modern humans
∼60 kya Modern humans out of Africa Global dispersal begins
∼40 kya Neanderthal extinction End of other Homo species in Eurasia
∼15 kya Peopling of Americas Last major continents settled

16.12.3 Major Hominin Species

Species Time Range Key Features
Sahelanthropus ∼7 mya Possible earliest hominin
Ardipithecus 5.8-4.4 mya Mixed climbing/bipedal
A. afarensis 3.9-2.9 mya “Lucy”, definite bipedalism
H. habilis 2.4-1.4 mya First stone tools
H. erectus 1.9 mya-110 kya First out of Africa, larger brain
H. heidelbergensis 700-200 kya Possible ancestor of Neanderthals and humans
H. neanderthalensis 400-40 kya Cold-adapted, complex culture
H. sapiens 300 kya-present Anatomically and behaviorally modern

16.12.4 Genetic Evidence Summary

Type What It Shows Key Findings
Nuclear DNA Overall relatedness 98.8% identical to chimp
mtDNA Maternal lineage “Mitochondrial Eve” in Africa
Y chromosome Paternal lineage “Y-chromosomal Adam”
Ancient DNA Past populations Neanderthal, Denisovan interbreeding
Selection scans Recent evolution Lactase persistence, high-altitude adaptation

16.12.5 Human Migration Routes

  1. Africa → Middle East (∼60,000 years ago)
  2. Middle East → Asia, Europe (∼50,000 years ago)
  3. Coastal route to Australia (∼50,000 years ago)
  4. Central Asia → Americas (∼20,000-15,000 years ago)
  5. Taiwan → Pacific (∼3,000 years ago)

16.13 Review Questions

16.13.1 Level 1: Recall and Understanding

  1. What are the key characteristics that distinguish primates from other mammals?
  2. List the major groups in the human evolutionary lineage from earliest to latest.
  3. What evidence supports the Out of Africa model of modern human origins?
  4. How does genetic evidence complement fossil evidence for human evolution?
  5. What are three characteristics that make humans unique among primates?

16.13.2 Level 2: Application and Analysis

  1. Why is bipedalism considered a key adaptation in human evolution?
  2. How do Neanderthals differ from modern humans anatomically and behaviorally?
  3. What does it mean that non-Africans have 1-4% Neanderthal DNA?
  4. How has human evolution been influenced by cultural developments like agriculture?
  5. Why is the concept of biological race problematic from a genetic perspective?

16.13.3 Level 3: Synthesis and Evaluation

  1. Evaluate the statement: “Human evolution stopped when culture developed.”
  2. How has the discovery of ancient DNA changed our understanding of human evolution?
  3. Why is understanding human evolution important for medicine and public health?
  4. Design a research project to investigate a question about recent human evolution.

16.14 Key Terms

  • Hominin: Humans and all species more closely related to humans than to chimpanzees
  • Bipedalism: Walking on two legs
  • Australopithecus: Genus of early hominins that lived 4.2-1.9 million years ago
  • Homo habilis: Early species of Homo known for stone tool use
  • Homo erectus: First hominin to spread out of Africa
  • Neanderthals: Extinct species of Homo that lived in Europe and western Asia
  • Out of Africa hypothesis: Theory that modern humans originated in Africa and migrated elsewhere
  • Mitochondrial Eve: Most recent common ancestor of all living humans via maternal line
  • Denisovans: Extinct hominin group known from DNA evidence
  • FOXP2 gene: Gene involved in speech and language development
  • Oldowan tools: Earliest stone tool industry
  • Acheulean tools: Stone tool industry characterized by handaxes
  • Behavioral modernity: Set of traits including complex tools, art, and symbolic thought

16.15 Further Reading

16.15.1 Books

  1. Stringer, C. (2012). Lone Survivors: How We Came to Be the Only Humans on Earth. Times Books.
  2. Reich, D. (2018). Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past. Pantheon.
  3. Dawkins, R. (2004). The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution. Houghton Mifflin.

16.15.2 Scientific Articles

  1. White, T. D., et al. (2009). Ardipithecus ramidus and the paleobiology of early hominids. Science, 326(5949), 64-86.
  2. Green, R. E., et al. (2010). A draft sequence of the Neandertal genome. Science, 328(5979), 710-722.
  3. Sankararaman, S., et al. (2014). The genomic landscape of Neanderthal ancestry in present-day humans. Nature, 507(7492), 354-357.

16.15.3 Online Resources

  1. Smithsonian Human Origins Program: https://humanorigins.si.edu
  2. Becoming Human: http://www.becominghuman.org
  3. The Genographic Project: https://genographic.nationalgeographic.com

16.16 Quantitative Problems

  1. Genetic Divergence: Humans and chimpanzees diverged ∼7 million years ago. Genetic divergence is ∼1.2%.
    1. What is the average rate of genetic divergence per million years?
    2. If mutation rate is constant, when did humans and Neanderthals diverge (∼0.1% difference)?
    3. What are the limitations of this calculation?
  2. Population Genetics: Lactase persistence allele frequency in northern Europeans is ∼0.9. In the ancestral population (before dairy farming), it was ∼0.01.
    1. If selection coefficient s=0.1, how many generations to reach current frequency?
    2. If generation time is 25 years, how many years?
    3. Does this match archaeological evidence for dairying (∼8,000 years ago)?
  3. Fossil Dating: A hominin fossil is found between two volcanic layers. Lower layer: 2.1 ± 0.1 million years (K-Ar dating) Upper layer: 1.9 ± 0.1 million years
    1. What is the age range of the fossil?
    2. If similar fossils are known from 2.0-1.8 mya elsewhere, is this consistent?
    3. How would additional dating methods improve precision?

16.17 Case Study: The Hobbit - Homo floresiensis

Background: Discovered in 2003 on Flores Island, Indonesia, this 1-meter tall hominin with a 400 cc brain lived ∼100,000-50,000 years ago.

Questions:

  1. What are the possible explanations for H. floresiensis (dwarfed H. erectus, separate lineage, diseased modern human)?
  2. What evidence supports each hypothesis?
  3. How does this discovery challenge traditional views of brain size and intelligence?
  4. What does the coexistence with modern humans (until ∼50,000 years ago) suggest about human dispersal?

Data for analysis:

  • Height: ∼1 m
  • Brain size: ∼400 cc (chimp-sized)
  • Tools: Stone tools associated
  • Other fauna: Dwarf elephants, giant rats (island rule)
  • Dating: 100,000-50,000 years ago
  • Controversy: Some argue it’s a modern human with pathology

End of Part IV: Evolution & Diversity

Next Part: Part V: Organismal & Systems Biology