Lecture 9 Flashcards
What was the first evidence of AMH
In lake mungo a fossil was found estimated to be from 40KYA years ago (Australia)
Some archaeological evidence of humans from even earlier (50KYA)
During the last Ice age, the continent known as Sahul (Australia and Pape ne Guinea was very close to Sunda so people could cross without sailing)
“An aboriginal Australian genome reveals separate human dispersals into Asia”
They didn’t have ancient DNA but they had DNA from aboriginal Australian from the early 20th Century (earliest they could get)
They ruled out any European ancestry of the sample and they wanted to test the idea that aboriginal Australians represented a dispersal from Africa before the one that lead to European and Asian populations
What were the two models
Model 1: single dispersal from Africa settled Europe, Asia and Australia
Model 2: One dispersal from Africa settled Australia (two separate dispersal events, some gene flow between the population that led to ABR and the population that was in Asia
How did they test this dispersal event?
Used Patterson’s D (similar to ABBA BABA) shows a better fit to a multiple dispersal model than a single dispersal model
They compared genomes between an African sample and a European sample and a newly generated Australian genome (different predictions depending on which of these two ideas is correct)
They found that the Aboriginal Australian had more allele sharing with Asian populations than European populations so rejected single dispersal model in favour of the multiple dispersal model
What does more recent work suggest
That the 2 dispersal model was wrong
They studied 83 aboriginals and 25 Papua New guinean genomes
300 genomes of 142 diverse understudied global populations
Subsequent papers showed that the idea is no longer correct, they both used a combination of modern data and took advantage of the denisovan genome
Mallick et al paper
The results are at odds with an inference of substantial early dispersal ancestry in a previous analysis of an Australian genome - the study was a less complete model that did not include Denisovan admixture into Australo-Melanesians
First paper failed to take account Denisovans contributed to Australian ancestry and some Asian ancestry (so not surprising that aboriginal Australian and Asian populations shared genetic data)
Malaspinas et al. paper
When considering only modern human genomes, we find evidence for two waves OoA, with a dispersal of Australo-Papuans 14,000 years before Eurasians
When taking into account Neanderthal and Denisovan introgression into modern humans, the SFS analysis supports a single origin for the OoA populations marked by a bottleneck - 72 KYA
What were the Australian conclusions
- Australia and Papua New Guinea were populated a surprisingly long time ago
- Australia & PNG were probably first settled by the population that also went on to found Eurasian populations i.e. one initial Out Of Africa event, rather than a separate earlier one.
- Note that all Non-African groups show evidence of Neanderthal admixture
The archaeological record for America
First people likely to have crossed from Siberia during the last ice age (Beringian Land Bridge)
Therefore, from 18KYA onwards (recent)
First evidence we can find of humans dates 15-18kya
The last glacial period ended 15kya, there was an ice sheet between modern day eastern Russia and modern day north west America
What was the earliest fossil in America
Luzia in Brazil (discovered in 1975)
Other important specimens include Kennewick Man, Washington state (discovered 1996)
‘Clovis’ culture from about 13.5KYA. Named after site where constructed points (arrow heads)were discovered.
“Reconstructing Native American population history”
- First big genetic study from Reich group
- 52 Native American, 17 Siberian populations
- Merged with other global datasets e.g. HapMap3
This used contemporary samples from native American groups rather than ancient DNA
Some Native American popns cluster with Siberian popns (green); others unique to Americas (blue)
What did the structure analyses show (american study)
They did a structure type analyses to see how these different groups were related to one another
Two rows: top row, no masking, takes the samples as they are and ignores the fact there is some European ancestry in modern day populations
Bottom row: possible European introgressed regions (post 1492) more relevent, attemps to remove the traces of European ancestry
Found clear distinct clusters. e.g. Subsaharan Africa, Western European populations and various North and South American populations and Siberian populations
Populations from across North and South America are often part of the same cluster
They tried to model how many episodes of gene flow into North America require to explain the patterns of contemporary populations once you’ve removed that trace of European introgression modelled whether there was 1, 2 or 3 founding populations
How many events where there
A minimum of 2 events required to explain the data in those populations - they do this for different combinations of populations. The P values are always really low, can always rule out that there’s just one population, can sometimes rule out that there’s just 2 populations
What they argue is that there’s a minimum of 3 events which have led to the native American populations that were there before the Europeans arrived
“Upper Palaeolithic Siberian genome reveals dual ancestry of Native Americans” paper
- Willerslev group main rivals to Reich group
- Sequenced 24KYA AMH from Mal’ta in Siberia. Oldest AMH genome
- Compared it to contemporary Native American genomes.
- Test similar to Patterson’s D (ABBA BABA) shows that Mal’ta sample genetically closer to all American populations than to contemporary East Asian populations
- Suggestion is strong contribution of this ancient Siberian population to modern American populations
“The genome of a late Pleistocene human from a Clovis burial site in western Montana”
- Sequenced Clovis individual from ~12.5 KYA
- This was the first American AMH genome to be generated
- Genetically similar to the Mal’ta Siberian genome- further evidence that the siberia population was one of the source populations to native america
- Similar to contemporary Native Americans
