Lecture Notes – Hominin Evolution: Bipedalism, Brain Expansion & Language

Anatomical Foundations of Bipedalism

• S-shaped vertebral column
  • Shifts the thorax directly over the pelvis → maintains center of gravity when upright.
  • Distinct from the gentle C-curve seen in quadrupedal great apes.

• Foramen magnum position
  • Located centrally under the skull in hominins; posterior in quadrupeds.
  • Allows head to balance on the spine with minimal muscular effort.

• Pelvis
  • Broad, bowl-shaped ilium shortens the distance between hip and sacrum → stabilizes abdominal organs and anchors large gluteal muscles for single-leg support.
  • Contrast: narrow, tall pelvis of chimpanzees that favors climbing.

• Femur (bicondylar angle / valgus knee)
  • Long femoral shaft slants medially, angling knees beneath the pelvis.
  • Keeps body mass over a single support foot during stride.

• Lower limb & foot
  • Enlarged heel (calcaneus) and double longitudinal/​transverse arches act as shock absorbers and energy springs.
  • Hallux (big toe) is aligned, non-grasping, critical for push-off and balance.
  • Early, more arboreal ancestors show a divergent, grasping hallux—a key comparative trait discussed for Monday.

Functional & Ecological Advantages of Bipedalism

• Feeding versatility
  • Permits efficient terrestrial foraging while retaining some tree-climbing ability in transitional forms.
  • Two-limb locomotion lowers daily caloric expenditure compared with quadrupedal knuckle-walking.

• Energetics
  • Crawling on elbows/knees (military “low-crawl”) metaphor highlights high energy cost of non-bipedal gaits.
  • Bipedal stride stores elastic energy in Achilles tendon/arches → ~75% cheaper per unit distance vs. chimp quadrupedalism.

• Thermoregulation
  • Vertical posture exposes less surface area to direct solar radiation in open, hot savanna environments.
  • Higher body elevation receives greater convective airflow → dissipates heat.

• Freeing of the hands
  • Enables carrying of food, infants, tools, and later agricultural implements.
  • Foundation for later cultural behaviors (tool craft, agriculture, art).

• Predator avoidance & ranging
  • Elevated vantage improves long-distance vision; energy savings increase daily foraging radius.

Brain Expansion in Hominins

• Disproportionate brain size
  • Graph shows body mass (x) vs. brain mass (y); expected allometric line (blue) predicts average Mammalia values.
  • Modern humans lie well above line, indicating encephalization quotient (EQ) > 1.
  • Examples: ostrich far below line, lion ≈ expected, male gorilla ≈ expected.

• Growth trajectory
  • Human brains grow rapidly for ≈ 20 yrs; chimp brains reach ≈ 90 % adult volume by 1 yr.
  • Prolonged juvenile period correlates with longer dependency and parental care.

• Possible adaptive payoffs
  • Enhanced memory, planning, abstract thought.
  • Complex tool manufacture & cooperative hunting.
  • Language and symbolic communication.
  • Sophisticated social structures and cultural transmission (knowledge across generations).
  • Early agriculture (intentional hybridization of wild plants) uniquely H. sapiens.

• Energetic & life-history costs
  • Brain tissue consumes ~20 % of adult basal metabolism; demands continuous \text{glucose} and \text{O}_2.
  • Long altricial phase → parents (and alloparents) invest heavily in provisioning.

Genetics of Language – FOXP2

• FOXP2 (“language gene”)
  • Transcription factor present in many vertebrates (birds, mice, primates).
  • Humans differ from chimpanzees by two amino-acid substitutions → profound phenotypic effect.
  • Regulates neural circuits underlying vocal learning and fine orofacial control.
  • Supports evolution of the hyoid bone position, allowing articulate speech.
  • Possible similar sequence in Neanderthals ⇒ debate over speech capacity.

Neanderthals vs. Homo sapiens

• Lived contemporaneously with modern humans; archaeological evidence for hearths, burying dead, shared caregiving.

• Persistent myth of Neanderthals as “dull brutes” is refuted by tool assemblages (Mousterian), pigment use, and complex social life.

• Extinction hypotheses (to be covered Monday): competition, climate pulses, interbreeding/assimilation.

Comparative Primate Review (context from previous lectures)

• Adaptive radiation post-Cretaceous gave rise to early mammals ⇢ primates.

• Divergence sequence: Prosimians → New World monkeys (e.g., squirrel monkey, golden-lion tamarin) → Old World monkeys (e.g., baboons) → Lesser apes → Great apes (chimpanzees, bonobos, gorillas, humans).

• Hybrid-DNA uncoiling experiment showed ~1 % nucleotide divergence between humans & chimps.

Population Genetics Refresher (Hardy–Weinberg)

• Equations
  • Allele frequencies: p + q = 1
  • Genotype frequencies: p^2 + 2pq + q^2 = 1

• Worked example (mice)
  • 1 000 individuals; phenotype counts 400 BB, 400 Bb, 200 bb.
  • Genotype freqs: \frac{400}{1000}=0.4\;(BB), \frac{400}{1000}=0.4\;(Bb), \frac{200}{1000}=0.2\;(bb).
  • Allele freqs: p=0.6\;(B), q=0.4\;(b).
  • Illustrates method to test for equilibrium.

Ethical, Philosophical & Practical Implications

• Bipedalism paved way for tool dependence, altering ecosystems (agriculture, resource extraction).

• Enlarged brain enabled moral reasoning, cumulative culture—but also warfare and environmental impact.

• Understanding hominin diversity (Neanderthals, Denisovans, “Hobbit” H. floresiensis, “Ardi,” “Lucy”) reframes human uniqueness and promotes respect for extinct relatives.

At-Home Exploration (prep for Monday)

• Research: “Which SPECIFIC human behaviors likely emerged because of our enlarged, complex brain?”
  • Consider art, religion, complex trade, governance, predictive modeling, long-term planning, altruism, etc.

• Skim forthcoming posted article (Teams) for additional evidence.

Check-Your-Knowledge Guide (exam cues)

• Early mammal radiation & primate characteristics.

• Distinguishing traits of New vs. Old World monkeys, great apes, lesser apes.

• Hybrid-DNA method for human–chimp similarity.

• Hominin diagnostic traits: bipedal skeleton ↔ freed hands.

• Adaptive benefits/costs of bipedalism & brain size.

• Hardy–Weinberg problem solving & conditions for equilibrium.

Quick mnemonic for bipedal evolution: "HIP HEAT HANDS"
H = Higher vantage & Heat regulation
I = Inward femur angle
P = Pelvis bowl
H = Hands freed
E = Energy efficiency
A = Arches in foot
T = Toe (aligned hallux)