# Human Evolution and Hominization

History I · Prehistory · https://tryals.app/learn/history-i/human-evolution-and-hominization

## The Feet Came First

**Bipedalism precedes encephalization by millions of years**: we walked upright for roughly 3 million years with an ape-sized brain before skulls inflated. **Lucy** (_Australopithecus afarensis_, ~3.2 Ma) walked upright with a ~400 cc brain. At **Laetoli** (3.6 Ma), footprints show a human arch and stride, no knuckles.

| Stage | Roughly | Brain |
| --- | --- | --- |
| Bipedalism | 4+ Ma | ~400 cc |
| Stone tools | 3.3–2.6 Ma | ~500–600 cc |
| Expansion | ~2 Ma onward | 600 → 1,350 cc |

**Pitfall**: Evolution does not optimize. Walking first wrecked backs, knees, and made childbirth dangerous.

## Brains and Bushes

A modern brain uses **20% of resting energy**. This was funded by **meat and marrow**, **fire and cooking** (which shrank guts), and **groups** (the social brain). Wide pelvises walk efficiently, but narrow pelvises fit birth canals: human babies are born **early and helpless**.

The march-of-progress is a lie. Hominin history is a **bush**: multiple human species coexisted for millions of years. _Homo sapiens_ arose ~300,000 years ago. Non-Africans carry **1–4% Neanderthal** DNA. 

$$\text{one surviving species} = \text{recent} + \text{temporary} + \text{abnormal}$$

## Practice questions

6 of this lesson's 10 practice questions, with answers. The full set is in the app.

### 1. Lucy walked upright with a brain of about 400 cc, roughly a chimpanzee’s. Why does that ordering matter for how hominisation is explained?

A. Because bipedality came millions of years before big brains
B. Because it shows brain size is unrelated to intelligence
C. Because it means bipedality and large brains evolved simultaneously
D. Because it proves Lucy was not a hominin

**Answer:** A. Because bipedality came millions of years before big brains

**Why:** Sequence constrains causation. The "brain first" story was intuitive and is simply excluded by the dates, which is why explanations now begin with locomotion, diet and habitat.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/lucy-walked-upright-with-a-brain-of-about-400-cc-roughly-a

### 2. What plausibly paid the energy bill for a brain that burns ~20% of resting metabolism?

A. Meat and marrow, dense calories unlocked by stone tools
B. Social complexity driving an arms race in mind-reading
C. Cooking, which predigests food and lets the gut shrink
D. A dramatic reduction in overall body activity

**Answer:** A. Meat and marrow, dense calories unlocked by stone tools; B. Social complexity driving an arms race in mind-reading; C. Cooking, which predigests food and lets the gut shrink

**Why:** The brain is 2% of body weight and eats a fifth of resting energy. Marrow, fire, and the social arms race are the leading candidates, and note our guts are small for a primate our size, exactly as the cooking hypothesis predicts.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/what-plausibly-paid-the-energy-bill-for-a-brain-that-burns-20-of

### 3. Brains are metabolically expensive and ours consumes about a fifth of our energy at rest. What made that bill payable?

A. A reduction in muscle mass, which freed energy for the brain
B. Longer sleep, which lowered total energy expenditure
C. Group living, which reduced each individual's foraging costs
D. A higher-quality diet and external processing

**Answer:** D. A higher-quality diet and external processing

**Why:** Aiello and Wheeler's expensive-tissue hypothesis pairs the growing brain with a shrinking gut, and cooking is what makes a smaller gut viable, energy released rather than found. It also reframes fire as a metabolic technology rather than merely a comfort.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/brains-are-metabolically-expensive-and-ours-consumes-about-a-fifth-of

### 4. Order the milestones of hominization.

**Answer:**

1. Bipedalism established on an ape-sized brain
2. Laetoli footprints pressed into volcanic ash
3. First flaked stone tools appear
4. Brain expansion accelerates
5. Homo sapiens appears in Africa
6. Sapiens becomes the only surviving human species

**Why:** Walking, then tools, then brains, then us, and the last line only becomes true within the last ~50,000 years, which is a rounding error on this scale.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/order-the-milestones-of-hominization

### 5. A human brain is about 2% of body weight. **What share of resting energy does it burn?**

**Answer:** 20 (within ±4)

**Why:** ~20%, ten times its share of body mass. No animal gets that discount free, which is why the story of the brain is really a story about calories: marrow, fire, and a shrunken gut.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/a-human-brain-is-about-2-of-body-weight-what-share-of-resting

### 6. Complete the statement about our species’ situation.

**Answer:** Homo sapiens appears in Africa by roughly **300,000** years ago; most people outside Africa carry a **1–4%** Neanderthal inheritance; and being the only living human species is a **recent** anomaly.

**Why:** Three facts that together dismantle human exceptionalism: we are older than people think, we are partly them, and our solitude is brand new.

Page: https://tryals.app/practice/history-i/human-evolution-and-hominization/complete-the-statement-about-our-species-situation