Fossils and Human Evolution – Week 2 Comprehensive Notes

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Weekly Learning Outcomes (Week 2 – Fossils & Human Evolution)

• Explain how fossils provide evidence of past life and how they corroborate phylogenetic evidence of evolution.
• Describe three independent lines of evidence for human–great-ape relatedness:
  • Anatomical
  • Genetic
  • Fossil
• Demonstrate why the human lineage originated in Africa.
• Identify distinct morphological features across the hominin lineage.
• Interpret genetic diversity to explain the recent global colonisation by modern humans.
• Discuss potentially unique human traits: culture, language, consciousness.

Workshop-Specific Outcomes

• Re-emphasise all weekly outcomes with practical, hands-on skull analysis & discussion.
  • “Touch on” items: human uniqueness & recent colonisation.

Human Evolution – Framing the Story

• Evolution is a continuous chain “from bacteria to you”.
• Great apes are our closest extant relatives.
• Human traits reflect adaptations to changing environments & behaviours (e.g., bipedalism, enlarged brains).

Activity 1 – Key Skull Characteristics of Hominins

• Students examine 10 cranial characters on real or replica skulls.
• Scored as Small/Medium/Large (or present/absent, etc.).
• Traits assessed:
  1. Projection of face (prognathism)
  2. Position of foramen magnum (brain opening)
  3. Sagittal crest size
  4. Overall braincase size (proxy for brain volume)
  5. Degree of vaulting (cranial height)
  6. Zygomatic arch size (cheek-bone flare)
  7. Brow-ridge size
  8. Canine-tooth size
  9. Presence/absence of a chin
  10. Size of posterior teeth (molars/premolars)
• Worksheets A, B, C allocate specific species pairs for comparison.

Worksheet Allocation

• Worksheet A ⟶ Ardipithecus ramidus & Homo erectus.
• Worksheet B ⟶ Australopithecus afarensis & Homo neanderthalensis.
• Worksheet C ⟶ Ardipithecus ramidus & Paranthropus boisei.

Species Profiles / Skull Cheat-Sheet

(All ages approximate; MYA = million years ago)

Pan troglodytes (Common Chimpanzee)

• Evolutionary context:
  • Last common ancestor with humans ≈ 7\,\text{MYA}.
  • Central African distribution.
• Lifestyle: quadrupedal; both arboreal & terrestrial.
• Skull character template (chimp = reference base):
  • Strong prognathism, rear foramen magnum, large sagittal crest variable, small braincase, low vault, medium zygomatics, large brow ridges, large canines, no chin, small back teeth.

Ardipithecus ramidus (“Ardi”)

• Age: 4.4\,\text{MYA} (Early Pliocene).
• Geography: Eastern Africa.
• Locomotion: habitual biped with grasping foot (mosaic arboreal + terrestrial adaptation).
• Skull:
  • Strong prognathism, foramen magnum more anterior than chimp, small braincase, low vault, medium zygomatics, large back teeth, canines reduced, chin absent.

Australopithecus afarensis (“Lucy”)

• Age: 3.5\,\text{MYA}.
• Eastern Africa; fully bipedal but retained curved fingers for climbing.
• Skull:
  • Strong prognathism, foramen magnum at skull base, small sagittal crest, small braincase, low vault, medium zygomatics, brow ridges medium, chin absent, medium back teeth.

Homo erectus (“Upright Man”)

• Age span: 1\,\text{MYA} (first appearance ≈ 1.9\,\text{MYA}; persisted for over 1 Myr).
• Range: Africa → Asia (first hominin outside Africa).
• Morphology: long legs for endurance walking/running; significantly expanded brain.
• Skull:
  • Mild prognathism, basal foramen magnum, sagittal keel rather than crest, medium braincase, low vault, small zygomatics, brow ridges pronounced, chin absent, medium back teeth.

Homo neanderthalensis (Neanderthal)

• Age: 200{,}000 years to 40{,}000 years ago.
• Range: Europe & western/central Asia.
• Features: very large brain (comparable or larger than H. sapiens), stocky body, symbolic behaviour (burials, pigments).
• Skull:
  • Mid-facial prognathism, basal foramen magnum, no sagittal crest, large braincase, medium-high vault, robust brow ridges, chin absent (retromolar gap), large nasal aperture.

Homo sapiens (Modern Human)

• First appearance: \ge 300{,}000 years ago (Jebel Irhoud, Morocco).
• Global distribution; only surviving hominin.
• Skull key points:
  • Weak prognathism (flat face), basal foramen magnum, no sagittal crest, large/high-vault braincase, small zygomatics, reduced brow ridges, small canines, definite chin, small molars.

Paranthropus boisei (“Nutcracker Man”)

• Age: 2\,\text{MYA}.
• Eastern Africa.
• Specialised for heavy chewing: massive molars, flared zygomatics, sagittal crest for huge temporalis muscles.
• Skull:
  • Prognathism moderate, basal foramen magnum, very large sagittal crest, small-medium braincase, low vault, extremely large zygomatics, brow ridges variable, chin absent, enormous molars (“megadontia”).

Comparative Trends Across Hominin Evolution

• Face projection: strong → weak.
• Foramen magnum: moves from posterior (quadrupedal) to centrally-based (bipedal).
• Brain size: small (~350\,\text{cc}) in early hominins to large (~1350\,\text{cc}) in H. sapiens.
• Sagittal crest: present in robust australopiths, absent in later Homo.
• Brow ridges: large in archaic Homo, reduced in H. sapiens.
• Teeth: canines shrink; molars enlarge then reduce again in modern humans.
• Chin: unique derived feature of H. sapiens.

Activity 2 – Building a Phylogenetic Tree

1. Position each species on a cladogram based on shared-derived skull traits.
2. Map trait changes (e.g., chin appearance, increase in braincase vaulting). Example notation:
   • “P(↑)” = chin gained on the branch leading to Homo sapiens.
   • “Ta(↑)” = taller brain vault.
   • “Brow S(↓)” = brow-ridge size reduction.
3. Annotate temporal axis in \text{MYO} (millions of years ago) to show first/last appearance dates.

Graphical reference (slide 19): Time scale 0–6 MYO; icons marking character transitions.

Activity 3 – What Makes Humans Unique?

• Utilised results from a pre-learning poll (n = 28) ranking putative unique traits.
• Partial list (highest votes not specified on slide, order appears arbitrary):
  • Bipedalism, Brow ridges, Consciousness, Culture, Curiosity & playfulness, Currency, Fingernails, Exchange of services, Face projection, Foramen magnum position, FOXP2 gene, Lack of body hair, Language, Long-distance travel, Muscular neck/shoulder attachments, Metabolically expensive brains, Large brain-to-body ratio, Opposable thumb, Social living, Tool use, Use of fire, Use of seasoning.
• Class discussion prompt:
  • “Can you think of another animal that shares this characteristic?”
  • Aim: tease apart truly unique vs convergent traits (e.g., tool use in crows/chimps, culture in whales, language proxies in songbirds, bipedalism in birds, currency behaviours in capuchin monkeys, etc.).

End-Pleistocene Bottleneck & Survival of Homo sapiens

• By the end of the last Ice Age, only one hominin species—H.\,sapiens—remained.
• Open questions:
  • Why did other contemporaries (Neanderthals, Denisovans, H. erectus relicts) go extinct?
  • How did modern humans out-compete or absorb them? Hypotheses: climate resilience, broader diet, social networks, advanced cognition, symbolic culture, or pathogen transfer.

"Missing Links" & Ongoing Evolution

• Fossil record is inherently incomplete; new discoveries constantly refine the tree.
• Debate persists over how many species vs. regional variants exist within Homo.
• Genetic & phenotypic data show humans are still evolving (e.g., lactase persistence, high-altitude adaptations, disease resistances).

The Human Revolution (Media Reference)

• Mention of documentary “The Human Revolution” now on ABC iView (Science Frontiers series).
• Implies broader public engagement with themes of human origins & genetics.

Key Take-Home Messages / Conclusions

• Human anatomical & behavioural traits are adaptations shaped by environmental and social pressures.
• Fossils, combined with phylogenetic analysis, provide robust, independent evidence for evolutionary history.
• Africa is the cradle of hominin evolution; multiple dispersal events led to global spread.
• Although only one hominin species survives today, the broader evolutionary narrative is complex and dynamic.
• Much remains to be learned; every new fossil or genome can reshape the story.

Essential Terminology

• Fossil: mineralised remains or impressions of past organisms.
• Phylogeny: evolutionary relationships diagrammed as a tree.
• Prognathism: forward projection of the facial skeleton.
• Foramen magnum: aperture where spinal cord exits the skull.
• Sagittal crest/keel: bony ridge along midline for muscle attachment.
• Zygomatic arch: cheekbone bridge housing masseter muscle.
• MYA/MYO: million years ago (chronological unit).
• Derived vs. Ancestral trait: evolved change vs. retained primitive condition.

Numerical / Statistical References & Equations

• Last common ancestor human–chimp: \sim 7\,\text{MYA}.
• Ardipithecus ramidus: 4.4\,\text{MYA}.
• Australopithecus afarensis: 3.5\,\text{MYA}.
• Paranthropus boisei: 2\,\text{MYA}.
• Homo erectus: first appearance \sim 1.9\,\text{MYA}; slide reference 1\,\text{MYA} (specimen discussed).
• Homo neanderthalensis: 0.2\,\text{MYA} (200 kyr) to 40\,\text{kyr}.
• Homo sapiens: \ge 0.3\,\text{MYA} (300 kyr) to present.
• Cranial capacity trend (illustrative):
  \text{Chimp brain volume} \approx 350\;\text{cc} \quad\rightarrow\quad \text{H. sapiens brain volume} \approx 1350\;\text{cc}
• Poll participation: n = 28\text{\,students}.

Ethical, Philosophical & Practical Implications

• Privacy: recording policies safeguard student confidentiality.
• Copyright: respecting intellectual property in educational materials.
• Evolution & Identity: understanding shared ancestry fosters perspective on human uniqueness and commonality with other life.
• Conservation: insights into great-ape relatedness underscore ethical responsibilities toward extant primates.
• Ongoing human evolution indicates adaptability but also raises questions about future selective pressures (technology, climate change).