Human evolution Study Guide

📖 Core Concepts Anthropogeny – scientific study of human origins. Hominization – evolutionary process that created the hominin lineage (bipedalism, brain growth, reduced canine size). Branching Web Model – human evolution is a bush with multiple species, interbreeding, not a straight line. Out‑of‑Africa (Recent Single‑Origin) Model – Homo sapiens arose in Africa 300 kyr, then dispersed worldwide, largely replacing earlier hominins. Multiregional Model – proposes continuous gene flow among regional populations; less supported by modern genetics. Bipedalism – upright walking; first seen in early hominins (Sahelanthropus, Orrorin). Provides free hands, lower locomotor cost, better vision, heat dissipation. Encephalization – increase in brain size relative to body; from 400 cm³ (chimp‑size) to 1 330 cm³ in modern humans. Drives language, social cognition. Gene Flow / Admixture – modern non‑African humans carry 1–4 % Neanderthal DNA and up to 6 % Denisovan DNA; introgressed alleles affect immunity, metabolism, etc. Stone‑Tool Industries – Oldowan (≈2.6 Myr), Acheulean (≈700 kyr–300 kyr), Levallois (≈350 kyr), Upper Paleolithic (≈50 kyr–10 kyr). Tool complexity parallels brain and hand evolution. --- 📌 Must Remember DNA similarity: Human and chimpanzee single‑nucleotide polymorphisms are 98.4 % identical. Key split times (million years ago, Myr): Gibbons–great‑ape split: 18–12 Myr Orangutan split: ≈12 Myr African hominids from orangutans: ≈14 Myr Gorillas from hominins: 8–9 Myr Chimpanzee–human split: 4–7 Myr Fossil dates (approx.): Sahelanthropus – 7 Myr Ardipithecus – 5.5–4.4 Myr Australopithecus afarensis – 3.9–2.9 Myr Homo habilis – 2.4 Myr Homo erectus – 1.8 Myr–108 kyr H. sapiens – ≈300 kyr (first fossils) Brain volumes: Chimpanzee 400 cm³ H. habilis 600 cm³ H. erectus 800–1 100 cm³ Neanderthal 1 200–1 900 cm³ Modern human 1 330 cm³ Genetic admixture: Neanderthal DNA in all non‑Africans: ≈2 % Denisovan DNA in Melanesians: ≈4–6 % (up to 6 %); EPAS1 high‑altitude allele derived from Denisovans. Robust vs. gracile australopiths: Robust (Paranthropus) → large chewing muscles, thick enamel; Gracile (A. afarensis) → lighter jaws, smaller teeth. Fire & cooking: Associated with H. erectus; hypothesized driver of brain enlargement and gut reduction. --- 🔄 Key Processes Evolution of Bipedalism Energy‑efficiency of upright walking → selection for S‑shaped spine, shortened lumbar vertebrae, anterior foramen magnum, aligned big toe. Brain Enlargement (Encephalization) Dietary shift to high‑calorie meat & cooked foods → surplus energy → selection for larger cortex → rapid post‑natal growth. Out‑of‑Africa Dispersal H. erectus → 1.8–1.0 Myr: first exit to Asia/Europe. H. sapiens → 65–50 kyr (single wave) or multiple waves (coastal first, inland second). Genetic Admixture Interbreeding events: modern humans ↔ Neanderthals (≈50–40 kyr), ↔ Denisovans (Southeast Asia), plus unidentified archaic gene flow in East Asians. Cultural Transmission Social learning → cumulative culture → increasingly complex tools, symbolic behavior, language. --- 🔍 Key Comparisons Out‑of‑Africa vs. Multiregional OOA: Single African founder, mtDNA L3 lineage universal, high Neanderthal/Denisovan introgression but overall replacement. Multiregional: Continuous gene flow across continents, predicts more regional continuity in fossils. Robust vs. Gracile Australopiths Robust: Paranthropus – massive masticatory apparatus, thick enamel. Gracile: Australopithecus – lighter jaws, smaller teeth, more human‑like posture. H. erectus vs. H. habilis H. habilis: 2.4 Myr, larger brain than australopiths, simple Oldowan tools. H. erectus: 1.8 Myr–108 kyr, brain 850 cm³, fire use, Acheulean hand‑axes, long-distance dispersal. Neanderthal vs. Modern Human Brain Neanderthals: larger average volume (1 200–1 900 cm³) but different shape; low surface‑to‑volume ratio → cold‑adaptation. Modern humans: slightly smaller on average but more globular, supporting language‑related temporal lobe expansion. --- ⚠️ Common Misunderstandings Linear Evolution: Evolution was not a straight line; many side branches (e.g., H. floresiensis, H. naledi) co‑existed. “Hobbit” Species: H. floresiensis is debated; not universally accepted as a distinct species. Neanderthal DNA Percentage: Only non‑African populations have 2 % Neanderthal ancestry; Africans have much less. Mitochondrial Eve Date: Refers to the most recent common female ancestor of all living humans (200 kyr), not the only woman alive then. Brain Size = Intelligence: Absolute size matters less than cortical reorganization; Neanderthals had large brains but different functional architecture. --- 🧠 Mental Models / Intuition Branching Bush Model: Picture a tree with many off‑shoots; species appear, overlap, interbreed, and many go extinct. Energy‑Efficiency Funnel: Walking upright saves energy → frees calories for brain growth → larger brain → more sophisticated tools → further energy gains (meat, cooking). Mosaic Evolution: Traits (bipedalism, dentition, brain) evolve at different rates; early hominins show mixtures of primitive and derived features. Social Brain Scaling: Neocortex size ≈ log of maximum stable social group size; bigger brains support larger, more complex societies. --- 🚩 Exceptions & Edge Cases Undetected Archaic Ancestry: East Asian genomes contain admixture from an unknown hominin beyond Neanderthal/Denisovan. High‑Altitude EPAS1: Tibetan adaptation derived from Denisovan introgression, not a de‑novo mutation. Birth Canal Trade‑off: Human pelvis widened for bipedalism narrows the birth canal, driving neoteny (extended infant dependence) and the “grandmother hypothesis.” Robust Australopith Survival: Robust forms persisted longer in East Africa despite gracile competitors, indicating niche specialization. --- 📍 When to Use Which Model Choice (OOA vs. Multiregional): Use OOA when explaining the universal mtDNA L3 lineage and the 2 % Neanderthal/4–6 % Denisovan admixture patterns; invoke Multiregional only when emphasizing regional fossil continuity without genetic bottleneck evidence. Robust vs. Gracile Classification: Apply “robust” when dental and mandibular morphology show large molars, thick enamel, and sagittal crest; use “gracile” for lighter jaws and smaller dentition. Tool Industry Identification: < 3 Myr → Oldowan (simple cores, choppers). 700–300 kyr → Acheulean (hand‑axes). ≈350 kyr → Levallois (prepared‑core flakes). > 50 kyr → Upper Paleolithic (bladelets, bone tools). Genetic Interpretation: Cite Neanderthal DNA percentages only for non‑African populations; refer to Denisovan introgression for Melanesian or high‑altitude Tibetan contexts. --- 👀 Patterns to Recognize Temporal Overlap of Species: Multiple Homo species often coexist (e.g., H. erectus and H. heidelbergensis). Trait Mosaic in Early Hominins: Presence of both arboreal (grasping big toe) and bipedal (foramen magnum) features in the same specimen. Correlation of Brain Size Jumps with Technological Change: Brain enlargement spikes coincide with emergence of Acheulean tools and fire use. Geographic “Out‑of‑Africa” Wave Signs: Coastal sites in SE Asia dated earlier than inland Levantine sites suggest a first maritime dispersal. --- 🗂️ Exam Traps Distractor: “Humans share 99 % DNA with chimpanzees.” – Correct figure is 98.4 % for SNPs. Trap: “All modern humans have Neanderthal DNA.” – Only non‑African populations have 2 % Neanderthal ancestry; African groups have negligible amounts. Misleading Choice: “H. floresiensis proved that dwarfism evolved independently in Homo.” – Its status is controversial; could be a pathological modern human. Wrong Reasoning: “The presence of a chin proves a specimen is H. sapiens.” – Chins are unique to modern humans but fossil preservation may obscure this feature; other traits must be considered. Date Mix‑up: “Mitochondrial Eve lived 300 kyr ago.” – The outline gives ≈200 kyr. ---