Genetics and Human Evolution: Large-Scale Data Revision Notes for HSC SSCE Biology

Genetics and Human Evolution: Large-Scale Data

Introduction to anthropological genetics

Anthropological genetics is an exciting new field of science that brings together several different approaches to understand where humans came from and how we evolved. This field combines:

DNA analysis (studying genetic material)
Historical records and evidence
Archaeological findings (studying ancient remains and artifacts)
Linguistic evidence (studying languages)

All of these sources help scientists determine the pathways humans took as they spread across the world.

The main goal of anthropological genetics is to explain why human populations are so diverse today. This diversity exists because of several evolutionary forces:

Mutation: Random changes in DNA that create new genetic variations
Natural selection: When certain traits help individuals survive and reproduce better than others
Genetic drift: Random changes in how common certain genes are in a population
Gene flow: The movement of genes between populations through migration and interbreeding

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Scientists measure genetic diversity in two main ways:

The number of different alleles (gene variants) in a population
How common each allele is (allele frequency)

Understanding these measurements is essential for interpreting genetic data about human populations.

Why study genetics instead of just fossils?

Traditionally, scientists studied human evolution by comparing fossil remains, such as ancient skulls. While fossils are valuable, they have limitations:

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Key limitations of fossil evidence:

Fossils are incomplete (we only find small fragments of past populations)
Interpreting fossils can be subjective
Fossils can't tell us about genetic relationships between populations

Studying the human genome (all of our genetic material) has dramatically changed our understanding of human evolution. Genetic data can reveal relationships between populations that fossils alone cannot show.

Human migration theories

Scientists have debated two main theories about how modern humans spread across the world from Africa.

Multiregional hypothesis (MRE)

The Multiregional hypothesis relies mostly on fossil evidence. It suggests that:

All modern human populations can be traced back to when Homo erectus first left Africa about $2$ million years ago
These early humans spread to different parts of the world
There was genetic exchange between neighbouring populations (gene flow)
Modern humans slowly evolved in multiple regions simultaneously
If this were true, different regions would have unique ancient alleles

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The Multiregional hypothesis predicts that we would find ancient alleles unique to different geographic regions, as human populations would have evolved independently in multiple locations over millions of years.

Replacement hypothesis (Out of Africa or Eve hypothesis)

The Replacement hypothesis, also called the Out of Africa or Eve hypothesis, proposes a different story:

Archaic Homo sapiens left Africa initially
A second, more important migration happened about $100,000$ years ago
These modern humans of African origin spread across the world
They replaced archaic human groups by interbreeding with them and out-competing them for resources
Modern humans outside Africa all descend from this relatively recent African population

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The key difference between these theories is the timeframe: the Multiregional hypothesis traces modern humans back $2$ million years to Homo erectus, while the Replacement hypothesis suggests a much more recent origin of just $100,000$ years ago.

Genetic evidence supporting the Out of Africa theory

Modern genetic studies have provided strong evidence for which theory is correct. Researchers have sequenced over 18,000 complete human mitochondrial DNA (mtDNA) genomes from people worldwide.

Why use mitochondrial DNA?

Mitochondrial DNA is ideal for studying human evolution because:

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Advantages of mtDNA for evolutionary studies:

It is inherited only through the maternal line (from mother to child)
It is not mixed through recombination like nuclear DNA
Each human cell contains $1016$ molecules of mtDNA
All mtDNA molecules within a person are usually identical
The mean rate of mtDNA divergence (change) is $2-4\%$ per million years
This provides a "molecular clock" to estimate when populations split apart

What are haplogroups?

When studying global populations, scientists grouped people according to specific mutations in their mtDNA. People who share the same mutations must be descendants of a common ancestor. These groups are called haplogroups.

By comparing mtDNA sequences, scientists created phylogenetic trees (family trees showing evolutionary relationships) from the mtDNA haplogroups.

Key findings from mtDNA studies

The research revealed several important patterns:

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Major Findings from Global mtDNA Analysis:

Finding 1: African populations show the most genetic diversity

Among modern humans, African populations (L haplogroups) have much more variation in their mtDNA sequences than any other population
This indicates Africa is the origin of modern humans

Finding 2: Non-African populations show less diversity

The mtDNA of Europeans, Asians, and Indigenous peoples of Australia, the Americas, and Pacific islands represent just a small subset of total human mtDNA diversity (M and N haplogroups)
This suggests more recent population origins outside Africa

Finding 3: All non-African groups trace back to Africa

Only the M and N haplogroups are found in indigenous populations outside Africa
Both M and N are closely related to the African L3 group
This demonstrates a clear African ancestry for all human populations

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This evidence strongly supports the Replacement (Out of Africa) hypothesis. If the Multiregional hypothesis were correct, modern populations would contain ancient alleles scattered across different regions of the world. Instead, all genetic diversity outside Africa represents just a branch of African genetic diversity.

Timeline of human migration

Using the molecular clock (the rate at which mtDNA changes over time), scientists estimate:

Diverse populations of modern humans evolved over $200,000$ years in Africa
The haplogroups that migrated out of Africa diverged about 70,000 years ago
These migrating groups were located in north-east Africa, nearest to the Middle East
This location makes sense as a starting point for migration into other continents

Out of Africa: Meeting other human species

When our ancestors first migrated out of Africa, they were not alone. At least two other species of humans were already living in Eurasia (Europe and Asia):

Neanderthals (Homo neanderthalensis)
Denisovans

Neanderthals

Neanderthal remains have been found in:

Western, central, eastern, and Mediterranean Europe
South-west, central, and northern Asia
As far east as the Altai Mountains in Siberia

Denisovans

Denisovans were a subspecies of humans, genetically different from both Neanderthals and modern humans. A $41,000$ -year-old finger bone found in a cave in the Altai Mountains in Siberia provided the first evidence of this group. The caves had been inhabited by both Neanderthals and Denisovans, possibly even at overlapping times.

Interbreeding with archaic humans

As modern humans migrated through Eurasia, they encountered and interbred with these other human species. This interbreeding introduced small amounts of Neanderthal and Denisovan DNA into the modern human genome.

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Key findings about archaic DNA in modern populations:

Most Europeans and Asians have about 2% Neanderthal DNA
Indigenous Australians have about 5% combined archaic DNA (both Neanderthal and Denisovan)
Eurasians and people from North and South America have about 2.5% Neanderthal DNA
Sub-Saharan Africans do not have Neanderthal DNA because their ancestors never left Africa and therefore never encountered Neanderthals

Interesting example: Genetic analysis of Ötzi the iceman, Europe's oldest mummy (who lived around $3400$ BCE), shows an even higher percentage of Neanderthal DNA than most modern Europeans.

Investigation 6.4: Using genetic evidence to map human migration

This investigation uses real data from a worldwide survey of mtDNA to determine the route our ancestors took when migrating out of Africa.

The data

Researchers collected mtDNA from $147$ people from five geographic regions:

Sub-Saharan Africa ( $n = 20$ )
Asian populations from China, Vietnam, Laos, the Philippines, Indonesia, and Tonga ( $n = 34$ )
Australian Aborigines ( $n = 21$ )
Caucasians from Europe, North Africa, and the Middle East ( $n = 46$ )
Aboriginal New Guineans ( $n = 26$ )

The table below shows the mtDNA sequence divergence (difference) within and between these five populations:

Population	African	Asian	Australian	Caucasian	New Guinean
African	$0.47\%$	-	-	-	-
Asian	$0.45\%$	$0.35\%$	-	-	-
Australian	$0.40\%$	$0.31\%$	$0.25\%$	-	-
Caucasian	$0.40\%$	$0.31\%$	$0.27\%$	$0.23\%$	-
New Guinean	$0.42\%$	$0.34\%$	$0.29\%$	$0.29\%$	$0.25\%$

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Understanding the table:

The diagonal shows divergence within each population
Other cells show divergence between different populations
Higher divergence percentages indicate more genetic differences

Key observations from the data

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Data Analysis and Interpretation:

Observation 1: African populations show the highest divergence within the population ( $0.47\%$ )

This indicates that African populations are the oldest
More time = more mutations = more divergence

Observation 2: Caucasian populations show the least divergence within the population ( $0.23\%$ )

This suggests this population is relatively young
Less time = fewer mutations = less divergence

Observation 3: Divergence patterns support migration from Africa

All non-African populations show less internal diversity than African populations
This pattern is exactly what we would expect if humans originated in Africa and then spread to other continents

What does this tell us about sub-Saharan Africans and Neanderthal DNA?

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Sub-Saharan Africans do not have any Neanderthal DNA in their genome because:

Their ancestors remained in Africa
Neanderthals lived only in Eurasia
Without contact, there could be no interbreeding
Only populations that migrated out of Africa and into Eurasia encountered and interbred with Neanderthals

Remember!

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Key Points to Remember:

Anthropological genetics combines DNA analysis with historical, archaeological, and linguistic evidence to understand human evolution and diversity
The Replacement (Out of Africa) hypothesis is strongly supported by genetic evidence, showing that modern humans evolved in Africa and migrated outward about $70,000$ years ago
Mitochondrial DNA is ideal for tracing human evolution because it is inherited only through the maternal line and provides a molecular clock for dating evolutionary events
Haplogroups are groups of people who share specific mtDNA mutations, indicating descent from a common ancestor. African populations have the most haplogroup diversity (L groups), while non-African populations have only M and N haplogroups
Modern humans interbred with Neanderthals and Denisovans, leaving about $2\%$ Neanderthal DNA in most Europeans and Asians, and up to $5\%$ archaic DNA in Indigenous Australians
Greater genetic diversity in African populations indicates they are older, supporting the theory that humans originated in Africa before spreading to other continents

Genetics and Human Evolution: Large-Scale Data (HSC SSCE Biology): Revision Notes