Week 9 Human Evolution and Population Ecology

Out-of-Africa hypothesis

Modern humans originated in Africa and later migrated to other continents

Mitochondrial DNA (mtDNA)

DNA inside mitochondria, inherited only from mothers; used to trace ancestry and migration

Mitochondrial Eve

All modern-day humans can trace a portion of their genetic ancestry back to a single woman.

Genetic similarity among humans

All people are 99.9 % genetically identical; biological races do not exist

Melanin

Pigment that darkens skin and absorbs UV light; protects folate but reduces vitamin D production

Skin tone and UV adaptation

Light skin evolved in low-UV areas to make vitamin D; dark skin evolved in high-UV areas to protect folate

Folate

Vitamin B needed for cell division and fetal development; destroyed by UV light unless protected by melanin. Destroyed by UV light in skin

Vitamin D

Produced in skin when exposed to UV; important for bones and immunity; low levels cause rickets and weak bones

High UV environments

Dark skin offers advantage by preventing folate breakdown and UV damage

Low UV environments

Light skin offers advantage by allowing more vitamin D production

Bipedalism

Walking upright on two legs; evolved before large brain size in hominins

Hominins

Modern humans and extinct human-like ancestors (20 Known species)

Ardipithecus ramidus

Lived 4.4 MYA; small brain; walked upright but also climbed trees

Australopithecus

Lived ~2.6 MYA; upright walker; used simple stone tools

Homo erectus

800 000 years ago; controlled fire; long-distance traveler

Homo sapiens

Appeared 200 000–300 000 years ago; large brain; advanced communication and culture

Tool milestone

Earliest tools 2.6 MYA; hand axes and spears show planning and teaching first invented by Australopithecus

Ecology

Study of how organisms interact with each other and their environment

Biotic factors

Living elements of ecosystems (plants, animals, microbes)

Abiotic factors

Non-living environmental conditions (light, water, soil, temperature)

Levels of organization

Organism → Population → Community → Ecosystem → Biosphere

Population distribution patterns

Clumped (groups), Uniform (Evenly spaced out), Random (no pattern)

Population growth rate

defined as the birth rate minus the death rate.

Exponential growth

Rapid increase without limits under ideal conditions

Logistic growth

Growth slows as population approaches carrying capacity (K)

Carrying capacity (K)

Maximum population size an environment can sustain long-term

Boom-and-bust cycle

Population overshoots carrying capacity, then crashes and recovers

Density-dependent factors

Effects increase with population size (e.g., disease, food scarcity)

Density-independent factors

Affect populations regardless of density (e.g., storms, fires)

Apes:

Lack a tail

Longer arms and shorter legs

Broader chest and less hair.

Monkeys

Possess tails for balance

Forelimbs are the same length as their hind limbs

More hair and less intelligence.