The History in Our Genes

The History in Our Genes

Students’ Learning Objectives

  • Explain the Multiregional and Out of Africa hypotheses on the origin of modern humans.
  • Discuss the evidence used to determine the origin of modern humans.
  • Describe the types of gene mutations and how they are influenced by selection and drift.
  • Explain the neutral theory of evolution.
  • Explain how the molecular clock works.
  • Important dates: Chapter 7 Homework & Video due Oct 19; Test 2 covering Chapters 4, 5, and 6 scheduled for Oct 21 & 22.

Evolution of Homo Sapiens

  • Timeline Overview:
    • Homo erectus emerged approximately 1 million years ago.
    • Two primary hypotheses regarding the origin of modern humans:
      • Multiregional Hypothesis.
      • Out of Africa Hypothesis.

Multiregional Hypothesis

  • Proposes that:
    • The origin of modern humans (Homo sapiens) is traced back to Africa.
    • Homo sapiens evolved gradually across the entire Old World (Africa, Europe, Asia) over the past 1 million years.
    • Continuous gene flow among regional populations contributed to the simultaneous evolution of modern humans worldwide.

Out of Africa Hypothesis (Replacement Theory)

  • Asserts that:
    • The origin of modern humans is also in Africa.
    • After the first out-migration from Africa approximately 1.8 million years ago, archaic humans diverged into several species that did not interbreed.
    • These archaic species were later replaced by anatomically modern humans (Homo sapiens) who migrated out approximately 100,000 years ago.

Evidence Supporting the Out of Africa Hypothesis (Fossil Evidence)

  • Fossils show:
    • The earliest fossils with recognizable modern anatomical features date back to 30,000 years in Africa.
    • The oldest fossils outside Africa, located in Israel, are dated to about 100,000 years.
    • Physical characteristics of fossils found in Eurasia are often derived from African populations, indicating a migratory connection, rather than separate evolutionary lineages.

Evidence Supporting the Out of Africa Hypothesis (DNA Evidence)

  • Analysis of human DNA reveals:
    • Most genetic diversity exists in African populations, indicated by longer branch lengths on evolutionary trees, which signifies more time for genetic mutations to accumulate.
    • Current Africans possess greater genetic diversity compared to other global populations.
    • The deepest phylogenetic branches—indicating ancient divergence—are primarily among African peoples.
    • It highlights that humans have been in Africa significantly longer than in other parts of the world.

Gene Mutations and Their Effects on Fitness

  • Gene mutations can have varying impacts:
    • Synonymous Mutation (Silent Mutation):
      • Definition: Substitution that does not alter the amino acid sequence of a protein.
      • Characteristics: Often selectively neutral, less prone to selection due to lack of effect on protein function.
    • Nonsynonymous Mutation:
      • Definition: Substitution that alters the amino acid sequence of a protein.
      • Characteristics: More likely to be subject to positive or negative selection since it can impact protein activity.
    • A Nucleotide Substitution occurs when a mutation arises in a gene and eventually becomes fixed, implying every individual in the population carries it.

Rate of Nucleotide Substitution

  • Defined as the speed at which a specific DNA base (either A, T, C, or G) is replaced by another over time in a population.
  • Significance:
    • Measures how quickly genetic changes become permanent (or fixed) in a species' DNA across generations.
    • For synonymous (neutral) mutations, the rate of change across generations remains constant.

The Neutral Theory of Molecular Evolution

  • Formulated by Motoo Kimura in 1968, stating:
    • Most molecular evolution at the DNA level is neutral and mostly results from genetic drift.
    • Variation in genomes largely originates from neutral mutations, which become fixed at a consistent rate across lineages.
    • It implies that the majority of molecular changes in DNA and proteins do not result from adaptive natural selection but rather from random fixation of neutral mutations.
    • Example: If a DNA changes at a rate of approximately one base per 25 million years, it can be expected to see two base changes in a span of 50 million years.

Applications of the Neutral Theory of Evolution

  • Forms the theoretical basis for:
    • Molecular clock: a method to estimate the time of genetic divergence between species.
    • Provides a null hypothesis for recognizing instances of natural selection in evolutionary studies.

Molecular Clock Concept

  • As species diverge, they acquire distinct sets of neutral mutations:
    • The duration since separation influences the number of different fixed mutations.
    • For example, if a mutation rate is determined to be 1 mutation per million years, and two species have accumulated 20 different neutral mutations, the time since divergence can be estimated.
    • Calculation Example:
      • If 1 mutation equals 1 million years, then for 20 mutations:
        20 ext{ mutations} imes 1 ext{ million years} = 20 ext{ million years}
      • Therefore, it takes 20 million years for 20 mutations to accumulate, providing a rough estimate for when species diverged.

Calibration of the Molecular Clock

  • Calibration is essential for effective use of molecular clocks:
    • Requires known evolutionary events or fossil records.
    • Absolute dates can be assigned to divergence events for more accurate mutation rates estimates.
    • Example: If fossil evidence indicates two species diverged 10 million years ago, and their genomes show 100 mutations, the mutation rate can be calculated as:
      rac{100 ext{ mutations}}{10 ext{ million years}} = 10 ext{ mutations per million years}

Summary on Molecular Clocks

  • Molecular clocks relate the genetic differences between species to their divergence times:
    • The extent of genetic divergence connects to how long the species have been separated and isolated genetically.
    • Utilizes mutation rates to estimate divergence time in a phylogenetic context.
    • The technique allows researchers to infer evolutionary timelines based on molecular changes.

Understanding Molecular Clocks

  • How Do Molecular Clocks Work?
    • They leverage rates of molecular change to deduce divergence time between lineages in a phylogenetic tree.
    • They assess the probability of observed genetic data given a specific evolutionary model and tree structure.
    • They assist in constructing phylogenetic trees by clustering taxa according to the similarities in their DNA or protein sequences.