Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.7 Humans Are Mammals That Have a Large Brain and Bipedal Locomotion
Homo sapiens is the only surviving member of a highly branched evolutionary tree of hominins. While closely related to chimpanzees, humans evolved a unique set of traits—including bipedalism, symbolic thought, and tool use—over a complex and nonlinear evolutionary path that spans over 6 million years.
Derived Characters of Humans
- Upright posture and bipedal locomotion
- Larger brain (~1,300 cm³); supports language, tool use, abstract thinking
- Reduced jawbones, shorter digestive tract, smaller teeth
- ~99% genomic similarity to chimpanzees; small regulatory differences lead to major traits
- Traits evolved gradually across multiple hominin species
Early Hominins
- Hominins: extinct species more closely related to humans than chimpanzees
- First appeared ~6.5 million years ago (Sahelanthropus tchadensis)
- Traits:
- Smaller canines, flatter faces
- Upright posture (foramen magnum under skull)
- Brain size ~300–450 cm³
Australopiths: A Key Group of Early Hominins
- Lived ~4–2 million years ago
- Australopithecus afarensis ("Lucy"):
- Walked upright, had long arms for climbing
- Brain size similar to chimpanzees
- Laetoli footprints prove upright walking
- "Gracile" vs. "Robust" forms:
- Gracile (e.g., A. afarensis): lighter jaws, soft diet
- Robust (e.g., Paranthropus boisei): strong jaws, large teeth, tough diet
Bipedalism and Tool Use
- Bipedalism may have evolved for energy-efficient walking
- All early hominins show some upright traits but retained arboreal features
- Tool use predates brain expansion:
- 2.5-million-year-old bones with cut marks
- Possibly used by Australopithecus garhi
Genus Homo
- Homo habilis (~2.4–1.6 MYA): used stone tools, brain ~550–750 cm³
- Homo ergaster (~1.9–1.5 MYA):
- Larger brain, smaller teeth, more slender body
- Fully bipedal, may have used fire and complex tools
- Reduced sexual dimorphism suggests pair bonding
- Homo erectus:
- First to migrate out of Africa
- Survived until ~70,000 years ago
Neanderthals and Interbreeding
- Homo neanderthalensis: Europe/Asia; extinct ~28,000–40,000 years ago
- Large brain, used tools, buried dead, possible symbolic thought
- Genetic evidence of interbreeding with H. sapiens and Denisovans
- Modern humans carry some Neanderthal and Denisovan DNA
Homo sapiens
- Originated in Africa ~200,000–300,000 years ago
- Oldest fossils (~315,000 years) found in Morocco
- Spread from Africa ~180,000 years ago
- Reached Europe, Asia, Australia, and New World (~15,000 years ago)
- Created symbolic art, jewelry, and tools (~75,000–30,000 years ago)
Other Members of Homo
- Homo naledi:
- Discovered in South Africa
- Small brain, modern hands/feet
- Lived ~300,000 years ago
- Homo floresiensis ("Hobbit"):
- Lived in Indonesia ~100,000–60,000 years ago
- Small body/brain, possibly from H. erectus
- Likely separate evolutionary lineage
In a Nutshell
Humans evolved through a complex, branching tree of hominin ancestors. Bipedalism, tool use, and brain expansion evolved in stages. Multiple hominin species coexisted, and some interbred with Homo sapiens. Our species, with its unique cognitive abilities and cultural innovations, is the only surviving hominin—but not the only one to leave a mark on human evolution.