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.