Lecture 22 - Human Migration Flashcards
Why is mitochondrial DNA used to study human evolution?
As it is the most rapidly evolving part of the human genome which is needly in order to distinguish closely related populations
Mitochondria do not recombine so analysis is easier
What is the problem with studying mitochondrial DNA?
It is just a single locus, equivalent to a single gene in the nuclear genome
It describes only female prehistory
It does not evolve in an entirely neutral fashion. It is subject to some natural selective pressures, possible adaptation to cold environments, or just purifying selection (the gradual removal of deleterious mutations from the genome)
What is the most variable part of the genome?
The D-loop on mtDNA
What are the two segments of the D-loop?
Hypervariable region1 (positions 16001-16568)
Hypervariable region2(001-574)
Haplogroups
Groups based on the sequences of hypervariable regions, the various mitochondrial genomes exist in the human population can be divided into around 100 groups
What are haplogroups made up of?
Haplotypes
Haplotypes share substitutions that define the haplogroup
How can evolutionary relationships be worked out in mtDNA studies?
Defining substitutions between haplogroups
What is an empty node?
A haplogroup that is unknown in the modern human population
It might exist in very few individuals whose DNA has not yet been sequenced
Or might have existed in the past but become extinct
Coalescence
Simulate past population sizes and test models of gene flow
Estimate divergences and changes to population size measured in number of generations
Usually within populations or between two populations with recent migration
Infer rate of coalescence of alleles
Requires representative sample of alleles
Coalescence time
The age of the haplogroup
Estimated from simulations using diversity of the haplotypes that make up the haplogroup
The greater the diversity among haplotypes, the more ancient the coalescence time
Coalescence times are estimated in generations because each generation is an opportunity for two allelic copies to coalesce (merge)
Founder analysis
Bottleneck events can reduce diversity and dramatically change the proportions of some alleles
This uses haplogroup diversities to estimate the time of a population split, such as occurs when a group of people leave their parent population and migrate to another location
If bottleneck persists, this can lead to further loss of diversity through genetic drift
When and where were the first modern humans found outside of Africa?
Skhul and Qafzeh caves in Levant around 90,000 years ago
Why were the fossils found in Skhul and Qafzeh caves in Levant probably not the first real migrants out of Africa?
The caves were reoccupied by Neanderthals at a later date
Modern humans were not seen there again for some time
What is the likely explanation for the Skhul and Qafzeh fossils?
Part of a failed attempt at colonization
An extension of the African population into Asia during the warm period
What is the likely initial migration out of Africa?
Humans migrated out of Africa from Ethiopia directly to Asia across the entrance to the red sea enabled by the sea level being 70 metres lower than it is today
This was around 50,000-70,000 years ago. Around 74,000 years ago there was a massive eruption of a super volcano on Lake Toba in Sumatra around 74,000 years ago. This caused climactic changes that might have led to the movement of humans out of Africa in search of more productive hunting and plant getehring areas
Only a subsample of the African population (1000 -2500 people)
What is the pattern of haplotype groups found in Africa today?
All haplotype groups are clustered together in the mtDNA network
What African haplotype group are all haplotype groups found outside of Africa linked to?
Haplogroup L3
What are and where were the immediate non-African descendants of haplogroup L3? What does this suggest?
M and N both found mainly in Asia suggesting that the initial migration was into Asia
Where are the modern Africans whose L3 haplotypes are most similar to M and N located?
Ethiopia suggests that is where the migration started
When did haplogroup M and N originate and what does this suggest?
50000-700000 years ago suggesting that the migration outside of Africa started around that time
Where did the first migrants outside of Africa go?
They went along the coast to SE Asia
They kept going until they reached Australia
When did humans reach Australia?
Archaeologists believe 50,000-60,000 years ago
What haplotypes are found on the Andaman Islands?
M haplotype that coalesce at about 60,000 years ago
Suggesting they arrived there around that time
What was the rate of migration along the southern Asia coast?
0.7-4km per year with side migration into the interior of the continent
How many haplotype groups predominate in modern european human populations?
11 and all originate in the near east
When did the haplotype groups enter Europe?
At different times
The oldest is U which appeared 50,000 years ago
Coincides with the period when the first modern humans moved into the continent as the ice withdrew to the north at the end of the last major glaciatoin
What haplotypes led the later migrations into Europe?
The haplotype groups J and T1 that arrived about the same time as agriculture spread from the near east 8500-7000 years ago
Went by two routes
Bering Strait
Seperates Siberia from Alaska
The obvious point of entry for humans into the new world
Quite shallow and if the sea level dropped by 50m it would be possible to walk from one continent to the other
The sea was shallow enough 60,000 and 11,000 years ago
But for a large part of this time it would have been impassable because of the build up of ice in the areas that are Alaska and northwest Canada
The glacier-free parts of Beringia would have been near-arctic but may have served as refugia
What happened 14,000-12,000 years ago regarding the new world?
Beringian land bridge was open at a time when the climate was warming and the glaciers were receding
During this time there was an ice free corridor leading from Beringia to central North America
The implication is that the first modern humans might have reached the Americas about 13,000 years ago
Haplogroups in the new world?
97% of Native Americans have one of four mitochondrial DNA haplogroups- A, B, C and D
These are common in East Asia suggesting that this is the source of colonizing populations indicating people went through the Bering Strait
The remaining 3% of native American mtDNAs belong to haplogroup X2.
It is very rare in Asia but found at low frequencies in some European populations. Implies some people from Europe colonized America. However Sub-groupX2a only in North America and dates to 15-17kya
When was the coalescence of the haplogroups A, B, C, D and X? What does this suggest?
15,000 to 17,000 years ago
Possible evidence that the haplogroups entered the new world together
But this was earlier than when the route through the ice was open
What is the Clovis-first hypothesis?
The first paloeindians were the clovis people
They made large spearheads called clovis points
First clovis points date to about 13,000 years ago so if they were the first Americans humans did not cross the Bering Strait until about that time
Why do people question the Clovis-first hypothesis?
There may be evidence of human occupation at sites that date to earlier than 13,000 years ago
But skeletons are rarely found, so evidence that humans were present is indirect
Such things as the possible remains of hearth where a fire was tended and food was cooked
When was there a megafauna extinction in North America?
12kya
The disappearance of 30+ genera of large mammals from the Americas
Humans may have been hunting in North America as much as 15kya
Coprolites in America?
Coprolites dating over 14,000 years ago in Paisley Cave in Orgeon contained human mitochondrial DNA
Pushing back first migration into the Americas 15,000 years ago or earlier when the ice corridor was closed
What is the updated view of how the first humans reached north America?
They followed the Pacific Coastal routh along the south of Beringia and then down the east of North and South America
Journey is thought to be possible anytime after 15,000 years ago
