The major milestones in human evolutionary history — dates and defining features
~4.4 mya: Ardipithecus ramidus — bipedal, forested Ethiopia. ~3.2 mya: Australopithecus afarensis "Lucy" — fully bipedal, small brain (~450cc). ~2.5 mya: Homo habilis — first stone tools (Oldowan), ~600cc brain. ~1.8 mya: H. erectus — first to leave Africa, fire use, Acheulean tools, ~900cc. ~300,000 ya: H. sapiens — modern anatomy, ~1,350cc brain. Encephalization quotient: brain size relative to body.
4.4 mya
Ardipithecus — bipedal, still arboreal
3.2 mya
Australopithecus (Lucy) — bipedal, small brain
2.5 mya
H. habilis — first stone tools, ~600cc
1.8 mya
H. erectus — leaves Africa, fire, ~900cc
300k ya
H. sapiens — modern anatomy, ~1350cc
Bipedalism Before Big Brains
Bipedalism came FIRST (~4 mya). Encephalization came LATER (~2 mya). Walking upright ≠ big brain.
Bipedalism Before Encephalization
The evolutionary sequence that overturned a century of assumptions
Australopithecines: fully bipedal (Laetoli footprints, 3.6 mya) but had chimp-sized brains (~450cc). Bipedalism advantages: frees hands for tools, efficient long-distance travel, thermoregulation in savanna heat. Costs: slower running, difficult childbirth (obstetric dilemma). Encephalization: brain tripled in size from H. habilis to H. sapiens — driven by diet (meat, cooking), social complexity, tool use.
Primate Characteristics
Primate traits: grasping hands, forward-facing eyes, large brain, long infant dependency, complex social life.
Primate Defining Traits
Five key characteristics shared by all primates — including humans
Grasping hands/feet: nails not claws, opposable digits, tactile pads. Binocular vision: forward-facing eyes → depth perception → important for arboreal life. Relatively large brain. Extended infant dependency: slow life history = more learning time. Complex social groups. Primates: prosimians (lemurs, lorises, tarsiers), New World monkeys, Old World monkeys, apes (gibbons, orangutans, gorillas, chimps, humans). Humans share 98.7% DNA with chimpanzees.
Out of Africa Model
Out of Africa: modern H. sapiens evolved in Africa (~300k ya), dispersed globally ~60–70k ya, replacing other hominins.
Recent African Origin Model
The fossil and genetic evidence that modern humans originated in Africa
Fossil evidence: oldest H. sapiens at Jebel Irhoud, Morocco (~300,000 ya) and Omo Kibish, Ethiopia. Genetic evidence: mitochondrial DNA "Eve" ~150–200k ya; Y-chromosome "Adam" ~300k ya — all trace to Africa. Multiregional hypothesis (rejected): parallel evolution in multiple regions. Replacement vs assimilation: some interbreeding with Neanderthals and Denisovans occurred outside Africa. Bottleneck: genetic diversity decreases with distance from Africa.
Neanderthals
Neanderthal DNA: 1–4% of non-African human genomes. Pääbo sequenced genome (2010 Nobel). "We are part Neanderthal."
Homo neanderthalensis
Modern humans and Neanderthals interbred — we carry their DNA today
Neanderthals: Europe and western Asia, ~400,000–40,000 ya. Adaptations: stocky build for cold, large nasal passages, supraorbital torus. Brain size: equal to or larger than H. sapiens (~1,500cc). Mousterian tools. Evidence of culture: ochre use, feather ornaments, burial of dead (Shanidar). Svante Pääbo: ancient DNA revolution — Neanderthal genome sequenced 2010 (Nobel Prize 2022). Non-African humans carry 1–4% Neanderthal DNA.
Natural Selection Mechanisms
Evolution forces: SMAG — Selection, Mutation, genetic drift (random), And Gene flow. Selection is the only directional force.
Four Evolutionary Forces
The four mechanisms that change allele frequencies in populations over time
Natural selection: differential reproductive success based on heritable traits — the only directional force. Mutation: ultimate source of new variation — random, mostly neutral or harmful. Genetic drift: random allele frequency changes — strongest in small populations. Founder effect: small founding group → limited diversity (Amish, island populations). Gene flow: movement of alleles between populations — reduces differentiation. All four operate simultaneously in human populations.
Selection
Differential reproduction — directional
Mutation
New variation — random, mostly neutral
Drift
Random changes — strongest in small populations
Gene flow
Allele movement between populations
Human Biological Variation
"Race" is a social construct — not a biological reality. Human genetic variation is clinal, not clustered into discrete races.
Human Biological Variation
Why biological anthropologists reject the concept of biological races in humans
All humans share ~99.9% of DNA. More genetic variation within "racial" groups than between them (Lewontin, 1972). Skin color: adaptation to UV radiation — continuous (clinal) variation, not discrete categories. Sickle cell: follows malaria distribution, not "race." AAA Statement on Race (1998): race is a cultural/political/social phenomenon, not biological. Health disparities attributed to "race" actually reflect racism (stress, access, environment).
Primatology and Social Behavior
Primate social structures: solitary, pair-bonded, one-male groups, multi-male/multi-female. Reflect ecology, not fixed biology.
Primate Social Organization
The diversity of primate social systems — and what they tell us about human evolution
Jane Goodall (Gombe): chimps — tool use, warfare, hunting, social politics. Dian Fossey: mountain gorillas — one-male (silverback) groups, stable families. Bonobos (de Waal): female-dominated, use sex to reduce conflict — feminist primatology. Grooming: social bonding function beyond hygiene. Dunbar's number (~150): neocortex size predicts social group size. Chimpanzees and bonobos: our closest living relatives, ~98.7% shared DNA.
Paleoanthropology Methods
Paleoanthropology: finds fossils, dates them, interprets anatomy + behavior. "Where, When, Who, What did they do?"
Paleoanthropology Methods
How scientists reconstruct the lives of ancient human ancestors from bones and stones
Survey and excavation: Rift Valley (East Africa), Dmanisi (Georgia), Atapuerca (Spain). Dating: K-Ar, Ar-Ar for volcanic layers; C-14 for recent hominins; U-series for teeth and bones. Morphological analysis: cladistics — shared derived characters define taxa. Paleoneurology: endocasts reveal brain organization. Ancient DNA (aDNA): revolutionized understanding — Denisovans discovered only from a finger bone's DNA. Isotope analysis: diet and migration from tooth enamel.
Human Life History
Human life history is uniquely slow: long childhood, menopause, grandmothering. "K-selected" — few offspring, heavy investment.
Human Life History Theory
Humans have the slowest life history of any primate — and for good evolutionary reasons
Life history: timing of growth, reproduction, and death. Humans: slow growth, prolonged juvenile period (learning), late first reproduction, menopause (unique among primates), post-reproductive lifespan. Grandmother hypothesis (Hawkes): post-menopausal grandmothers provision grandchildren → increased survival → selected for long lifespan. Trade-offs: large brain requires long development. Alloparenting: cooperative child-rearing enables human brain size and life history.
Forensic Anthropology
Forensic anthropology: identify unknown skeletal remains for law enforcement and human rights investigations.
Forensic Anthropology
Applying skeletal biology to legal and humanitarian investigations
Biological profile: estimate age, sex, stature, ancestry from skeletal remains. Age: epiphyseal union (young adults), pubic symphysis morphology, dental wear (older adults). Sex: pelvis (most reliable — obstetric adaptation), skull robustness. Stature: long bone length regression formulas. Trauma analysis: distinguishes perimortem (at death) from postmortem damage. Mass grave investigations: ICMP (International Commission on Missing Persons), WGEID. Clyde Snow pioneered human rights forensic work in Argentina (dirty war victims).
When and why humans began behaving in distinctly modern ways
Anatomically modern humans: ~300,000 ya. Behaviorally modern: evidence clusters ~70,000–40,000 ya. Markers: cave art (Lascaux, Chauvet ~36k ya), personal ornaments (shell beads ~100k ya in Africa), ochre use, long-distance trade networks, hafted composite tools, symbolic burial. Blombos Cave (South Africa): ochre engravings ~75k ya — possible earlier behavioral modernity. "Revolution" vs gradual emergence debate ongoing.