Human Ecology 4 - Evolution of Homo Sapiens

What fundamental principles govern human ecology?

Humans, like all organisms, are subject to the laws of physics and abiotic and biotic selection pressures. As mammals, they are sensitive to factors such as temperature and UV radiation.

Where did most human evolution occur?

n Africa, particularly within tropical and subtropical environments over several hundred thousand years during periods of varying global climate.

When did Homo sapiens begin to leave Africa?

From at least 70,000 years ago (70 KYA), human populations began colonizing different environments around the world.

What is required for adaptation to occur?

Adaptation begins only after a random mutation arises that changes the phenotype in a way that improves survival or reproduction in response to environmental pressures.

What are acclimation and acclimatization, and what is plasticity?

Humans can respond to environmental pressures through acclimation (short-term) and acclimatization (medium-term). Together, these reversible changes are forms of phenotypic plasticity.

Give examples of recent human genetic adaptations (within the last 10,000 years).

• Sickle-cell trait (~7,500 years ago) and thalassemia — responses to malaria.

• Lactose tolerance (~6,500 years ago) — adaptation to post-weaning milk consumption.

How has cultural evolution influenced adaptation?

The evolution of social learning and communication enabled cultural adaptation, allowing humans to modify their environments rather than only adapting biologically.

What major environmental and behavioural changes occurred in the last 10–15 KYA?

• Warmer, less variable climate after the last Ice Age.

• Foragers settled, leading to food production through horticulture and agriculture.

• Humans began to alter environments to increase carrying capacity.

• Continued biological and cultural evolution with humans “outsourcing” energy to abiotic sources (e.g., fossil fuels).

What is “net primary productivity” (NPP) and how do humans affect it?

• NPP is the total products of photosynthesis available in ecosystems.

• Humans now appropriate a large share of NPP through farming, industry, and resource use — a trend called HANPP (Human Appropriation of Net Primary Productivity).

Describe early Homo sapiens (200–15 KYA).

• Large brains relative to body size.

• Omnivorous diet — eating across trophic levels.

• Small migratory bands of related individuals.

• Developed complex tools, problem-solving, and fire control.

What characterized forager (hunter-gatherer) societies?

• Depended on extracting food (fruits, tubers, meat, honey).

• Small bands (20–50 related individuals).

• Nomadic lifestyle — moved when resources declined.

• Accumulated only what could be carried.

What changes occurred 15–20 KYA with horticulture and agriculture?

• Humans became less migratory and began settling for months or years.

• Encouraged edible plants over inedible ones.

• Increased population density and organized conflict between groups.

• Agriculture (12–15 KYA): domestication of plants and animals on multiple continents.

• Resulted in less diverse diets, higher fertility, and higher mortality.

How did horticulture and domestication evolve independently?

• Multiple centres arose independently between 9–12 KYA and 6–7 KYA.

• Each region domesticated local plants and animals (e.g., grains in the Middle East, maize in the Americas).

• Multiple centres arose independently between 9–12 KYA and 6–7 KYA.

• Each region domesticated local plants and animals (e.g., grains in the Middle East, maize in the Americas).

• Cooking reduces the cost of digestion and increases nutrient availability.

• Facilitated consumption of fibrous, starchy foods, more than meat.

• Likely contributed to evolution of larger brains (Wrangham, 2009, Catching Fire).

What were the effects of agriculture on diet and health?

• Shift to simple carbohydrates and cereals.

• Increased fertility but also increased mortality due to disease and malnutrition.

How did early humans act as selection pressures on plants?

• By foraging and replanting, humans selected for:

  • Larger, faster-growing plants.

  • Sweeter fruits, larger grains, and fewer toxins or spines.

• This represents artificial selection, which increased carrying capacity.

Why can’t humans truly eat “Paleo” today?

• Modern crops have undergone thousands of years of artificial selection — larger, sweeter, with reduced natural protections.

• Caloric yield per work unit is now much greater than for ancient foraged foods.

What are the key variables in population change?

• N: population size

• b: birth rate

• d: death rate

• r: natural rate of increase (r = b – d)

What is the example of population growth rate (r = 0.011)?

• Population grows by just over 1% per year.

• Example:

  • Year 2000 → 6 billion

  • Year 2001 → 6 × (1.011)

  • Year 2002 → 6 × (1.011)², etc.

How has global population changed over time?

• CE 1000: ~275 million

• 2000: ~6 billion

• 2019: ~7.8 billion

• Rate of increase decreased from 0.014 (1960s) to 0.011.

What are demographic transitions?

• As societies industrialize, mortality and fertility decline, altering population structure.

• Populations move from youth-heavy pyramids to stable or aging structures.

What was the impact of the Irish Potato Famine (1845–50)?

• Introduction of potatoes increased carrying capacity (K).

• Famine (potato blight) led to 1 million deaths and mass emigration.

• Demonstrates how ecological and social events affect fertility and mortality.

What is HANPP (Human Appropriation of Net Primary Productivity)?

• Increasing human use of global energy from photosynthesis.

• 1810: Humans used ~13% of global NPP.

• 2005: Increased to ~25%.

• Total food output rises, but food per person remains constant.

How has human behaviour addressed ecological limits in the past?

Behavioural Solutions” include:

1. Migration to new areas.

2. Cultivation and domestication.

3. Expansion of cultivation area.

4. Artificial selection for productivity.

5. Substitution with fossil fuels.

6. Fertilizer use.

7. Reduction of competitors (pests).

8. Genetic modification of plants.

Each step increases productivity but becomes less efficient and more resource-intensive.

What genetic evidence shows humans are still evolving?

• Digestive enzymes:

  • AMY1 (amylase) — higher copy number in agricultural populations.

• Skin pigmentation:

  • OCA2, MYO5A, DTNBP1, TYRP1 under selection in Europeans.

• Carbohydrate metabolism:

  • ADH (alcohol dehydrogenase) in East Asians.

  • MAN2A1 (mannose metabolism) in Yoruba.

  • SI (sucrose) in East Asians.

  • LCT (lactose) in Europeans.

• Gut microbiome adaptations:

  • Japanese populations have bacteria enabling porphyrin degradation from seaweed diets.

  • Influenced by birth method, diet, and household size.

How do genetic and cultural adaptations differ?

Feature	Genetic Adaptation	Cultural Adaptation
Mechanism	DNA mutation and selection	Social learning and behaviour
Transmission	Vertical (parent → child)	Vertical & horizontal (peers, teachers)
Speed	Slow	Rapid
Direction	Random, irreversible	Goal-directed, reversible
Copying errors	Limited	Frequent

What are the main conclusions of Human Ecology 4?

• Homo sapiens evolved in the tropics — early physiological and behavioural traits are species-wide.

• Later, population-specific adaptations arose through founder effects and natural selection.

• Major change: settlement → agriculture.

• Natural selection lacks foresight — traits adaptive in the past may be detrimental today.

• Cultural adaptation depends on genetic preadaptation (e.g., brain capacity for social learning).

• Both genetic and cultural evolution continue today.