Phylogeny & Classification
Where Do We Fit?🌳 Humans Are Apes — Not Descended from Apes
A critical IEB distinction: humans did NOT evolve from modern chimpanzees. Humans and chimpanzees share a common ancestor that lived approximately 6–7 million years ago. Both lineages evolved separately from that shared ancestor. Humans belong to the Order Primates, Family Hominidae (the great apes), and our lineage — the hominins — is distinguished by bipedalism, reduced canine teeth, and progressive brain enlargement over millions of years.
Taxonomy
Human Classification
Know the full hierarchy from Kingdom to Species — and what distinguishes each level for humans.
| Taxonomic Level | Human Classification | Key Shared Feature |
|---|---|---|
| Kingdom | Animalia | Multicellular, heterotrophic, no cell wall |
| Phylum | Chordata | Notochord, dorsal hollow nerve cord, pharyngeal slits (at some stage) |
| Class | Mammalia | Hair, mammary glands, endothermic, 4-chambered heart |
| Order | Primates | Forward-facing eyes, grasping hands, large brain relative to body size, nails not claws, reduced snout |
| Family | Hominidae | Great apes — no tail, large body, complex social behaviour, great intelligence. Includes gorillas, chimpanzees, orangutans, bonobos, and humans. |
| Genus | Homo | Fully bipedal, large brain (>600 cc), reduced face and jaw, tool use |
| Species | Homo sapiens | Brain ~1400 cc, chin, high forehead, fully modern skull, language and symbolic thought |
Key Features
Primate Characteristics — Why They Matter for Human Evolution
Our primate heritage gave us the building blocks for intelligence, tool use, and social complexity.
🧠 Primate Features Relevant to Human Evolution
- Forward-facing eyes — binocular stereoscopic vision; excellent depth perception; critical for tool manipulation and judging distances
- Grasping hands — opposable thumbs; precision grip; critical for tool making and use
- Large brains — relative to body size; associated with complex social behaviour, learning, and problem solving
- Reduced olfaction, enhanced vision — colour vision in three wavelengths; diurnal lifestyle
- Slow reproduction, extended parental care — longer period of learning and brain development; small litter size
🧬 Molecular Evidence of Common Ancestry
- Human DNA is ~98.7% identical to chimpanzee DNA — the closest living relative
- Gorilla DNA is ~98.3% identical to humans
- These comparisons use DNA hybridisation and sequencing techniques
- Cytochrome c (a protein involved in cellular respiration) is identical in humans and chimpanzees — used as molecular evidence of common ancestry
- Shared non-functional DNA sequences (pseudogenes, endogenous retroviruses) at the same locations in human and chimp genomes — impossible to explain except through common ancestry
📌 IEB Key Distinction — Hominin vs Hominid
Hominid = Family Hominidae = all great apes including humans (gorillas, chimps, orangutans, bonobos, humans). Hominin = the human lineage specifically — bipedal primates more closely related to humans than to chimpanzees; includes Australopithecus, Homo habilis, Homo erectus, Homo sapiens, and others. All hominins are hominids, but not all hominids are hominins.
The Hominin Timeline
6 Million Years of Change⏳ A Branching Bush, Not a Ladder
Human evolution was NOT a straight line from ape to modern human. It was a branching bush with many species coexisting, competing, and going extinct. For the IEB, you need to know the key species in the human lineage in chronological order, their approximate dates, their key physical features, and the general trend toward bipedalism, reduced jaw, and increased brain size over time.
~6–4 Million Years Ago
Earliest Hominins (Sahelanthropus, Ardipithecus)
The split from the chimpanzee lineage. Sahelanthropus tchadensis (~7 mya, Chad) has a relatively flat face and small canines suggesting partial upright posture. Ardipithecus ramidus (~4.4 mya, Ethiopia) walked upright but could also climb trees. These species are not well known in the IEB curriculum but set the scene for Australopithecus.
~4–2 Million Years Ago
Australopithecus (Southern Ape)
Key IEB species. Multiple species including A. afarensis (Lucy, Ethiopia, ~3.2 mya) and A. africanus (Taung Child, South Africa, ~2.5 mya). Fully bipedal but with ape-like features: small brain (400–500 cc — similar to chimpanzee), large brow ridges, prognathous (protruding) face, large cheek teeth for tough plant food, no significant tool use. The pelvis and leg bones confirm bipedalism. They are the link between earlier apes and the genus Homo.
~2.4–1.5 Million Years Ago
Homo habilis (Handy Man)
Key IEB species — first of genus Homo. Discovered at Olduvai Gorge, Tanzania (Leakey, 1960). Brain ~650 cc — significantly larger than Australopithecus. Smaller teeth and jaw than australopithecines. Named "Handy Man" because associated with Oldowan stone tools — simple flaked pebble tools, the earliest known stone tool technology (~2.6 mya). Still had a relatively flat forehead and protruding face. Considered by many the first stone tool maker, though this is debated.
~1.9 Million – 300 000 Years Ago
Homo erectus (Upright Man)
Key IEB species. First hominin to leave Africa — spread to Asia and Europe. Homo ergaster is the African form; H. erectus the Asian form (IEB uses both names — know either). Brain 850–1100 cc. Fully modern body proportions and long limbs suited for long-distance walking and running. Made Acheulean hand axes — more advanced bifacial tools. First evidence of use of fire (~1 mya). Taller than australopithecines, reduced brow ridges compared to them, more rounded skull. Lived alongside early Homo sapiens in some periods — the two species overlapped.
~600 000 – 200 000 Years Ago
Homo heidelbergensis (Heidelberg Man)
Transitional species between H. erectus and both H. neanderthalensis and H. sapiens. Brain ~1200–1300 cc. Large brow ridges but more rounded skull than H. erectus. Made more refined tools. First evidence of hunting large game. Found in Africa, Europe, and Asia. Gave rise to Homo neanderthalensis in Europe and Homo sapiens in Africa.
~400 000 – 40 000 Years Ago
Homo neanderthalensis (Neanderthals)
Lived in Europe and western Asia. Brain ~1400–1600 cc — as large or larger than modern humans! Cold-adapted: stocky build, large nose (warms/humidifies cold air), heavy brow ridges, no chin. Made Mousterian tools. Buried their dead — evidence of symbolic thought. Interbred with modern Homo sapiens: 1–4% of non-African human DNA is Neanderthal. Extinct by ~40 000 years ago — possibly outcompeted by H. sapiens.
~300 000 Years Ago – Present
Homo sapiens (Wise Man)
Modern humans. Originated in Africa (~300 000 years ago — Jebel Irhoud, Morocco). Brain ~1400 cc but with a distinctly rounded, high-domed skull. High forehead, small brow ridges, flat face, protruding chin (unique to H. sapiens), small teeth. Behavioural modernity by ~70 000 years ago: art, music, symbolic objects, complex language, long-distance trade. Spread out of Africa in multiple waves (~70 000–50 000 years ago) and colonised every continent. The "Out of Africa" model is supported by fossil and genetic evidence; mitochondrial DNA traces all modern humans to a common African ancestor.
Skull Features & Trends
Reading the Fossil Record💀 What Skulls Tell Us
The IEB requires you to identify and compare hominid skull features — from fossil photographs or diagrams. You need to know which features changed over time, in which direction, and what those changes mean for behaviour and lifestyle. The main trends are: increasing brain size, decreasing jaw and tooth size, reducing brow ridges, changing skull shape from flat to rounded, and the appearance of a chin.
🦴
Australopithecus
africanus / afarensis
4–2 MYA
🔨
Homo habilis
Handy Man
2.4–1.5 MYA
🚶
Homo erectus
Upright Man
1.9–0.3 MYA
🧠
Homo sapiens
Modern Human
0.3 MYA–Now
IEB Core Comparison
Skull Features Compared Across Species
Know ALL these features and the direction of change from Australopithecus to Homo sapiens — this is high-frequency IEB content.
| Feature | Australopithecus | H. habilis | H. erectus | H. sapiens |
|---|---|---|---|---|
| Brain size (cranial capacity) | 400–500 cc | ~650 cc | 850–1100 cc | ~1400 cc |
| Skull shape | Low, flat skull; sagittal crest (in some) for jaw muscle attachment | Slightly more rounded than Australopithecus | Low, elongated skull with thick bones | High, rounded, domed skull with thin bones |
| Forehead | Very sloping, receding | Slightly less sloping | Low, sloping | High, vertical, prominent |
| Brow ridges (supraorbital torus) | Heavy, prominent brow ridge | Pronounced but less than Australopithecus | Heavy, continuous brow ridge (most prominent in genus Homo) | Minimal — small or absent |
| Face / prognathism | Very prognathous — face projects forward significantly | Prognathous but less than Australopithecus | Moderately prognathous | Flat, orthognathous — face sits below braincase |
| Jaw and teeth | Large jaw; large molars and premolars for grinding tough plant food; reduced canines | Smaller than Australopithecus | Smaller, thinner jaw | Small, parabolic (rounded) jaw; small teeth |
| Chin | Absent — receding chin | Absent | Absent | Present — protruding bony chin. Unique to H. sapiens |
| Occipital region (back of skull) | Flat; occipital torus (ridge) | Less pronounced ridge | Occipital torus present | Rounded; no occipital torus |
| Foramen magnum position | More forward than apes — confirming bipedalism | Forward | Forward | Directly beneath skull — fully bipedal |
| Tool use | No stone tools | Oldowan tools — simple pebble choppers | Acheulean tools — bifacial hand axes; fire | Complex tools, art, language, symbolic thought |
📌 Trends to State in an IEB Essay
Over time from Australopithecus to Homo sapiens: (1) Brain size INCREASED (400 cc → 1400 cc). (2) Skull became more ROUNDED and high-domed. (3) Brow ridges became SMALLER/less prominent. (4) Face became FLATTER (less prognathous). (5) Jaw became SMALLER with smaller teeth. (6) A CHIN appeared (unique to H. sapiens). (7) Forehead became more VERTICAL. These trends are all linked to increasing brain development and dietary change (cooking, less need for heavy jaw muscles).
⚠️ Exam Watch — Foramen Magnum Position
The foramen magnum is the hole at the base of the skull through which the spinal cord passes. In quadrupeds (four-legged animals) it points backward. In bipeds it points downward (directly below the skull), balancing the head on top of the spine without neck muscles pulling backward. In Australopithecus the foramen magnum is positioned forward and downward — confirming bipedalism even though they had small, ape-like brains. This is critical evidence: brain size does NOT correlate with bipedalism onset.
Defining Feature
Bipedalism — Evidence & Advantages
Bipedalism evolved BEFORE large brains — this is a key IEB point. Know the skeletal evidence and the adaptive advantages.
🦴 Skeletal Evidence for Bipedalism
- Foramen magnum — positioned centrally below skull (not at back like in quadrupeds)
- Pelvis — bowl-shaped (broad, short ilium) to support abdominal organs in upright position; vs long, narrow ape pelvis
- Femur — angled inward (valgus angle) so feet are under centre of gravity; apes have vertical femur
- Knee joint — adapted for full extension and weight-bearing in upright posture
- Foot — arched foot with non-opposable big toe aligned with other toes; apes have grasping feet
- Vertebral column — S-shaped curve (lumbar lordosis) absorbs shock and balances head over pelvis; apes have C-shaped
- Laetoli footprints (Tanzania, 3.6 mya) — fossilised footprints of A. afarensis showing human-like bipedal gait with arched foot and aligned big toe
✅ Advantages of Bipedalism
- Frees the hands — for carrying food, infants, and eventually making and using tools
- Energy efficient — bipedal walking is more energy-efficient than knuckle-walking over long distances
- Thermoregulation — upright posture reduces surface area exposed to midday sun; head is further from hot ground; allows longer foraging in open savanna heat
- Elevated head — better view over tall grass for predator detection
- Long-distance running (persistence hunting) — Achilles tendon, arched foot, gluteus maximus allow endurance running unique to humans
Evidence for Human Evolution
Multiple Lines of Proof🔬 Convergent Evidence from Many Fields
The evidence for human evolution does not come from a single source — it converges from palaeontology (fossil record), comparative anatomy, molecular biology, and archaeology. The IEB requires you to know what each type of evidence is, give examples, and explain what conclusions can be drawn. Crucially, you should be able to explain why multiple independent lines of evidence strengthen scientific confidence in the theory of evolution.
Type 1
Fossil Evidence
Fossils provide direct physical evidence of past organisms. Dating methods establish when they lived.
🪨 What Fossils Tell Us
- Fossils are the preserved remains or traces of past organisms — bones, teeth, shells, casts, footprints, impressions
- Hard tissues (bone, teeth) are most commonly preserved — soft tissue rarely fossilises
- Skull morphology shows changes in brain size, jaw structure, brow ridges, and face shape over time
- Post-cranial skeleton (pelvis, femur, foot) shows transition to bipedalism
- The fossil record shows a progression of increasing complexity and human-like features over time — though with many branches, dead ends, and gaps
⏱️ Dating Fossil Specimens
- Radiometric dating — uses radioactive decay of isotopes (e.g. potassium-40 → argon-40 with a half-life of 1.25 billion years; carbon-14 for material up to ~50 000 years old). Gives absolute age in years.
- Stratigraphy — relative dating using rock layers (strata). Deeper layers are older. Fossils found in the same stratum are approximately the same age. Does NOT give an absolute age.
- Palaeomagnetism — uses reversals in Earth's magnetic field recorded in rocks to date strata
- Biostratigraphy — using known ages of other fossil species found in the same layer to date specimens
📌 Key IEB Fossil Sites
Olduvai Gorge, Tanzania — where the Leakeys discovered Homo habilis (1960) and Oldowan tools. Afar Depression, Ethiopia — Lucy (A. afarensis, 3.2 mya). Laetoli, Tanzania — hominid footprints 3.6 mya preserved in volcanic ash. Cradle of Humankind, South Africa — Sterkfontein, Swartkrans, Drimolen; A. africanus, A. sediba, H. naledi (see SA Fossils tab). Jebel Irhoud, Morocco — oldest H. sapiens fossils at ~315 000 years.
Type 2
Comparative Anatomy
Homologous structures reveal shared ancestry. Vestigial organs are evolutionary remnants. Know the examples.
🦴 Homologous Structures
- Structures that share the same basic anatomical plan (same bones, same developmental origin) even if they have different functions — evidence of common ancestry
- The pentadactyl limb: human arm, horse leg, whale flipper, bat wing, dog leg — all have the same sequence of bones (humerus, radius/ulna, carpals, metacarpals, phalanges) despite very different functions
- Primate forelimbs are homologous — reflecting shared evolutionary origin of the vertebrate limb
- Distinguished from analogous structures (same function, different origin — e.g. bird wing and insect wing): analogous structures indicate convergent evolution, NOT common ancestry
🌫️ Vestigial Structures in Humans
- Structures that have reduced function compared to related species — evidence that we evolved from ancestors that used them
- Coccyx (tailbone) — remnant of tail; 3–5 fused vertebrae; serves as muscle attachment but no tail function
- Plica semilunaris — small fold of tissue in corner of eye; remnant of the nictitating membrane (third eyelid) present in many vertebrates
- Erector pili muscles — cause "goosebumps"; in ancestors raised body hair for insulation or threat display — no significant function in modern hairless humans
- Wisdom teeth (third molars) — functional in ancestors with large jaws and tough diets; now frequently impacted due to reduced jaw size
- Palmaris longus muscle — forearm muscle for gripping and swinging in trees; absent in ~10–15% of modern humans with no functional loss
Type 3
Molecular Evidence
DNA and protein comparisons quantify the degree of relatedness between species — and can date divergence events.
| Technique | What It Compares | What It Shows |
|---|---|---|
| DNA sequencing | Nucleotide sequences of the same gene across species | The more similar the sequence, the more closely related. Humans share 98.7% DNA with chimps, 98.3% with gorillas — confirming the phylogenetic tree built from anatomy. |
| DNA hybridisation | How tightly DNA strands from two species bind together | More similar sequences bind more tightly (higher melting point). Used to estimate degree of relatedness before sequencing was routine. |
| Protein comparison | Amino acid sequences of conserved proteins (e.g. cytochrome c, haemoglobin) | Differences in amino acid sequence reflect accumulated mutations since divergence. Cytochrome c is identical in humans and chimps — extremely conserved. |
| Mitochondrial DNA (mtDNA) | Maternally inherited DNA with high mutation rate | Used to trace female lineages. All modern human mtDNA traces back to a common African ancestor ("Mitochondrial Eve") ~150 000–200 000 years ago — supports Out of Africa model. |
| Y-chromosome analysis | Paternally inherited DNA | All modern human Y chromosomes trace to a common African ancestor ("Y-chromosomal Adam") — also supports African origin of modern humans. |
South African Fossil Heritage
Cradle of Humankind🇿🇦 South Africa — The Birthplace of Human Palaeontology
South Africa holds some of the most important fossil hominin sites in the world. The Cradle of Humankind (northwest of Johannesburg) is a UNESCO World Heritage Site containing a complex of cave systems — Sterkfontein, Swartkrans, Kromdraai, Drimolen, Rising Star — that have yielded hundreds of hominin fossils spanning 3+ million years. South Africa's contribution to understanding human origins is unmatched. For the IEB, knowing the key discoveries, who found them, and what they revealed is essential.
1924 Discovery
The Taung Child — Australopithecus africanus
The discovery that changed everything. Raymond Dart proved Africa — not Asia or Europe — was the cradle of humankind.
📍 Discovery Details
- Discoverer: Raymond Dart, University of the Witwatersrand, 1924
- Site: Taung limestone quarry, Northern Cape, South Africa
- Specimen: Skull of a child aged approximately 3–4 years; includes endocast (cast of brain cavity)
- Age: ~2.5–2.8 million years old
- Named: Australopithecus africanus — "Southern Ape of Africa"
🔬 Why It Was Significant
- The foramen magnum was positioned forward and downward — confirming bipedalism even with a brain the size of a chimpanzee's (~440 cc)
- Smaller canine teeth than apes — indicating reduced aggression function
- Rounded orbits and some humanlike dental features
- Dart proposed it was a human ancestor — but was initially ridiculed by the European scientific establishment who believed the "Piltdown Man" (later revealed as a hoax) was the true ancestor
- Dart was ultimately vindicated — the Taung Child remains one of the most important fossil discoveries in history
Sterkfontein Caves
Mrs Ples, Little Foot & More
Sterkfontein has yielded more Australopithecus fossils than any other site in the world — including the most complete Australopithecus skeleton ever found.
| Specimen | Discoverer / Year | Significance |
|---|---|---|
| "Mrs Ples" (Sts 5) | Robert Broom, 1947 | Nearly complete A. africanus skull. One of the most famous hominin fossils. Initially thought to be female (hence the nickname), more recent analysis suggests possibly male. Confirms Dart's earlier findings on a much more complete specimen. |
| "Little Foot" (StW 573) | Ron Clarke, 1994–ongoing (still being excavated) | Most complete Australopithecus skeleton ever found (~90% complete). ~3.67 million years old. Shows mosaic of ape and human features — could both walk upright and climb trees. Took decades to excavate from solid rock. |
| Stw 53 | Hughes & Tobias, 1976 | Partial skull possibly representing early Homo or transitional form; debated classification. Shows Sterkfontein contains multiple hominin species over time. |
2013–2017 Discovery
Homo naledi — The Star of Lesedi
A remarkable new hominin species discovered in SA's Rising Star cave — with puzzling features that challenge our understanding of human evolution.
📍 Discovery
- Site: Dinaledi Chamber ("Chamber of Stars"), Rising Star cave, Cradle of Humankind
- Discoverer: Lee Berger (Wits University) and team, 2013; announced 2015
- Specimens: Over 1 550 bone fragments representing at least 15 individuals — one of the largest hominin fossil collections ever found in Africa
- Age: Surprising — 236 000–335 000 years old, meaning H. naledi lived at the same time as early H. sapiens
🔬 Unique Features — A Mosaic Species
- Small brain — ~465–560 cc, similar to Australopithecus despite being Homo
- Hands — human-like curved fingers suitable for tool use but also for climbing
- Feet — nearly identical to modern human feet — fully bipedal
- Teeth — small, simple teeth more like H. sapiens than australopithecines
- Possible deliberate burial — location deep in cave with no evidence of predation or water transport suggests intentional body disposal — raises questions about cognitive complexity
📌 Also Know: Australopithecus sediba
Discovered at Malapa, Cradle of Humankind, 2008 by Lee Berger (his son Matthew found the first bone). Approximately 1.977 million years old. Mosaic of Australopithecus and Homo features — some researchers consider it transitional between A. africanus and Homo. Has very human-like hands suited for precision grip. Important for understanding the transition to genus Homo.
🎯 Human Evolution Assessment
Eight IEB-style questions on human evolution.
Question 1 of 8
A student claims: "Humans evolved from chimpanzees." Correct this statement and explain the actual evolutionary relationship between humans and chimpanzees.
Question 2 of 8
The Taung Child was discovered in 1924 by Raymond Dart. State THREE pieces of evidence from the Taung Child skull that Dart used to classify it as a hominin rather than an ape.
Question 3 of 8
List the hominin species in the correct chronological sequence from oldest to most recent, and state the general trend in brain size across this sequence.
Question 4 of 8
Explain why the position of the foramen magnum is important evidence for determining whether a fossil hominin was bipedal.
Question 5 of 8
Give TWO examples of vestigial structures in humans and explain what each suggests about human evolutionary history.
Question 6 of 8
Homo naledi was discovered in the Rising Star cave, South Africa. State THREE features of Homo naledi that make it scientifically significant or puzzling.
Question 7 of 8
What is the difference between homologous and analogous structures? Give an example of each and explain what each type of structure tells us about evolution.
Question 8 of 8
A skull is found with the following features: very heavy continuous brow ridge, low sloping forehead, no chin, brain capacity of ~900 cc, and large flat face. To which species does it most likely belong, and what tool technology would you expect to be associated with it?