Week 9 Human Evolution and Population Ecology

Oldest fossils of modern humans

Found in Ethiopia (Africa) about 160,000–195,000 years ago; evidence for Out-of-Africa hypothesis

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

Zygote mitochondria source

All mitochondria in a zygote come from the egg (mother), none from the sperm

Mitochondrial Eve

Woman in Africa

mtDNA mutation rate

Mutates at a steady rate, allowing scientists to estimate divergence times and build ancestry trees

Genetic similarity among humans

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

Natural selection for skin color

Different UV levels favored alleles for darker or lighter skin to balance vitamin D and folate needs

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

Skin tone and relatedness

People with similar skin color are not necessarily closely related; skin color reflects environment, not ancestry

Folate

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

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

Blood type and skin tone

No connection; blood type distribution does not follow skin-color patterns

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

Diet milestone

Shift from plant to meat and cooked foods gave more energy for brain growth

Tool milestone

Earliest tools 2.6 MYA; hand axes and spears show planning and teaching

Communication milestone

By ~500 000 years ago speech structures developed; Homo sapiens used symbols and art

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)

Biotic–abiotic interaction

Living things depend on and modify their physical environment

Levels of organization

Organism → Population → Community → Ecosystem → Biosphere

Isle Royale

Island in Lake Superior used for long-term wolf-and-moose ecology study (over 60 years)

How wolves and moose arrived

Moose swam to island; wolves crossed on an ice bridge

Population size estimation

Wolves counted by aerial surveys; moose counted by ground and bone records

Population distribution patterns

Clumped (groups), Uniform (territorial), Random (no pattern)

Population growth rate

Determined by births, deaths, immigration, and emigration

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

Predator–prey relationship

Wolves control moose numbers; moose affect tree growth on Isle Royale

Abiotic factors impact

Climate and weather events alter population sizes

Biotic factors impact

Competition, disease, and predation affect population sizes

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)

Human intervention in ecology

Science can inform management decisions but cannot decide ethical actions