Lesson 29

29

What Makes a Human?

What traits set humans apart from every other creature on Earth?

Human evolution involved changes in walking, hands, brains, language, and social life over millions of years. Fossils reveal a branching history in which several hominin species lived at the same time.

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Key Ideas

  • Bipedalism came first: Australopithecus was walking upright 3–4 million years ago with a brain still roughly chimpanzee-sized. Walking upright freed the hands, which may have helped lead to tool use, more calories, and brain growth.
  • The hominin brain grew from ~380 cc (chimpanzee-sized) to ~1,350 cc (modern human) over roughly 3 million years.
  • Language and dexterous hands together unlocked cultural accumulation, with each generation building on the knowledge of the one before, without having to rediscover everything from scratch.
  • Human evolution is a branching tree, not a ladder. Multiple hominin species coexisted. Neanderthals had larger brains than us, buried their dead, and interbred with Homo sapiens. People outside Africa carry up to about 2% Neanderthal DNA.
  • What makes a human is not any single trait but a combination: bipedalism, large brain, dexterous hands, and complex language, each building on and enabling the others in a feedback loop that produced an animal unlike any before it.

Vocabulary

  • Hominin: The group including modern humans and our extinct relatives and ancestors, such as Australopithecus and Homo erectus.
  • Bipedalism: Walking upright on two legs; a defining hominin feature that freed the hands for tool use and carrying.
  • Australopithecus: An early hominin genus (2–4 mya) in Africa; bipedal but with a smaller brain than modern humans.
  • Dexterity: Skill and precision in using the hands; human dexterity allows tool making, writing, and complex tasks.
  • Language: A system of communication using sounds, symbols, or gestures; complex, grammatical language is uniquely human.
  • Cranial Capacity: The volume of the skull, used as an estimate of brain size; human cranial capacity is much larger than other primates.
  • Tool Use: Using objects to accomplish tasks; humans create complex tools from scratch in a way no other animal does.

Discussion Questions

  1. Why did bipedalism come before large brains in human evolution?
    Sample Answer: Bipedalism freed the hands, which allowed tool use. Tool use brought more calories, and that extra energy supported larger brains.
  2. If you could keep only language or dexterous hands, which would you choose, and what becomes impossible with each loss?
    Sample Answer: Answers will vary. Think about everything you did today that required one or both: writing, cooking, teaching, building, communicating. Without language, knowledge could not pass across generations or allow large-scale cooperation. Without dexterous hands, tool making, fire tending, cooking, sewing, and writing become impossible.
  3. Neanderthals had larger brains than modern humans, buried their dead, and possibly made art and music. What does that tell us about how we should think about intelligence?
    Sample Answer: Brain size alone is not intelligence. Other factors, like organization, connectivity, and specific regions of the brain, might matter more.
  4. What does it mean that human evolution is a branching tree, not a ladder marching toward us?
    Sample Answer: Multiple hominin species coexisted. There was no direction toward modern humans, just populations adapting to conditions. Homo sapiens happened to survive, and others did not.

Digging Deeper

  1. Bipedalism and Dexterity Challenge: Mark a walking path on the floor. Place small objects at one end. Walk upright along the path while carrying objects in your hands. Notice how walking upright frees your hands to carry, manipulate, and transport. Now try crawling on all fours and carrying objects at the same time. How does bipedalism change what your hands can do? Write or draw your observations.
    Sample Answer: Answers will vary. Walking upright frees the hands completely, so they can carry and handle objects while you move. Crawling on all fours uses the hands for movement, so carrying is much harder.
  2. Brain Volume Timeline: Create a bar chart showing hominin brain volumes: chimpanzee (~380 cc), Australopithecus (~450 cc), Homo habilis (~600 cc), Homo erectus (~900 cc), Homo neanderthalensis (~1,500 cc), Homo sapiens (~1,350 cc). Notice: Neanderthals had larger brains than us. What does that tell you about the relationship between brain size and intelligence? What other factors matter?
    Sample Answer: Neanderthals had larger brains than modern humans, so brain size alone does not equal intelligence. Other factors, like organization, connectivity, and specific brain regions, might matter more.
  3. Neanderthal Coexistence: People outside Africa carry up to about 2% Neanderthal DNA from interbreeding tens of thousands of years ago. Research what we know about Neanderthal culture: tools, burial practices, possible art and music, use of medicinal plants. Does the evidence suggest they had symbolic thinking? What does their extinction tell us about the relationship between Homo sapiens and other human species?
    Sample Answer: Answers will vary. Neanderthals buried their dead, made tools and jewelry, used ochre, and possibly created art and music. They went extinct about 40,000 years ago, possibly due to competition with Homo sapiens, climate change, or both.