Chapter 19: Human Evolution

Chapter 19: Human Evolution

Biology 102, Week 9

Genetic Concepts in Evolution

  • Genotype Frequencies:

    • Hardy-Weinberg Principle: This principle is a mathematical model used to calculate genotype frequencies in a population. It describes a theoretical condition where allele and genotype frequencies remain constant across generations in the absence of evolutionary influences.

      BB2+2Bb+bb2=1BB2+2Bb+bb2=1

    • Alternatively presented as:

      B+b=1B+b=1

    • What it does: It provides a baseline to determine if a population is evolving. If the observed genotype frequencies deviate from those predicted by the Hardy-Weinberg equations, it indicates that evolutionary forces (such as natural selection, mutation, gene flow, genetic drift, or non-random mating) are at play within that population.

Vertebrate Evolution and Diversity

Primate Diversity and Hominin Evolution

  • Evolution of Primates:

    • Evolved as small arboreal mammals by 65 million years ago.

    • Key Characteristics of Primates:

    • Limber joints,

    • Grasping hands and feet with flexible digits,

    • Short snout,

    • Forward-pointing eyes (enhances depth perception).

Phylogenetic Tree of Primates

  • Primate Groups:

    1. Prosimians: Lemurs, lorises, and bush babies

    2. Tarsiers

    3. Anthropoids: Monkeys and apes

  • Divergence of Anthropoids:

    • Began about 55 million years ago from other primates.

Primate Diversity

  • Apes:

    • Includes gibbons, orangutans, gorillas, chimpanzees, and humans.

    • Characteristics Compared to Other Primates:

    • Larger brains relative to body size

    • More flexible behavior

  • Physical Differences Between Apes and Monkeys:

    • 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.

Hominin Evolution

  • Overview of Hominins:

    • Comprises modern humans and their extinct relatives.

    • Field of Study:

    • Paleoanthropology - the study of human origins and evolution.

  • Notable Extinct Hominin Species:

    • Approximately 20 species identified, with some coexisting.

Timeline of Hominin Evolution

  • Key Milestones:

    1. 7 million years ago: Divergence of humans from chimpanzees.

    2. 4.4 million years ago:

    • Ardipithecus ramidus - small brain, evidence of bipedal walking, also lived in trees.

    1. 2.6 million years ago:

    • Australopithecus - could walk upright and used tools.

    1. 800,000 years ago:

    • Homo erectus - controlled fire, advanced tool use.

    1. 200,000–800,000 years ago:

    • Homo sapiens - characterized by a big brain and advanced cognitive skills.

Bipedalism in Hominins

  • Distinguishing Features of Humans Compared to Chimpanzees:

    1. Bipedalism (walk upright)

    2. Larger brain size.

  • Fossil Evidence for Bipedalism:

    • The location of the spinal cord exit in the skull provides clues to bipedalism, which evolved before larger brain sizes.

Evolutionary Evidence and Migration Hypotheses

  • Out-of-Africa Hypothesis:

    • Indicates that modern humans originated in Africa and later migrated to other regions.

  • Mitochondrial DNA (mtDNA) Studies:

    • mtDNA derived from mothers can track human ancestry and evolution.

    • Helps build an evolutionary tree known as "Mitochondrial Eve," linked to early migrations.

Genetic Similarities Among Humans

  • Genetic Consistency:

    • Humans share 99.9% of their DNA, indicating minimal genetic divergence.

    • Traits arise from the small genetic differences (0.1%), leading to diverse human characteristics (e.g., skin color, height).

    • The human genome consists of about 3 billion nucleotide base pairs.

Variation in Skin Color

  • Melanin and Skin Tone:

    • Skin tone is influenced by melanin levels, which are genetically determined.

    • Geography affects skin tone due to varying UV radiation levels.

  • Impact of UV Light on Nutrients:

    • Folate:

    • Degraded by UV light, essential for development (low folate correlates with birth defects).

    • Vitamin D:

    • Produced with UV exposure, crucial for immune system support.

Natural Selection and Skin Color

  • Environmental Selection:

    • Different skin tones are observed based on UV-light exposure, enabling adaptation through natural selection.

    • Light-skinned individuals thrive in low-UV environments, while dark-skinned individuals excel in high-UV settings.

  • Genetic Evidence Against Racial Differences:

    • Human populations exhibit gradual variations in skin color without distinct racial boundaries.

Summary of Key Concepts

  • Variation in skin color reflects adaptations to environmental factors, influenced by evolutionary trade-offs.

  • Humans descended from a common African ancestor, with both physical and genetic evidence supporting this theory.

  • The concept of "biologically distinct races" does not hold as all humans are members of a single species, Homo sapiens.