Lecture 21 - Genetics of human evolution Flashcards
When and where were the first fossils found outside of Africa?
100,000 years ago in China
How old are the oldest definite H.sapien fossils?
60,000 years old
What is the out of Africa hypothesis?
H. sapiens originated in Africa and then moved to other parts of the world
Why do some paleontologists disagree with the out of Africa hypothesis?
They believe there are similarities between the features of H. sapien and earlier hominins
For example the relatively prominent brow ridges shared by modern europeans and neandethals
What is the multiregional process?
H. sapiens emerged in parallel in different parts of the old world, evolving directly from the older Homo species that occupied those regions before them
This is possible if there is a certain amount of interbreeding between populations from different parts of the world
What are the implications of the out of Africa hypothesis?
The difference between modern human populations are relatively recent variations that evolved mostly during the last 50,000 years
What are the implications of the multiregional model?
Modern human populations are much less related, and originate in H. erectus rather than H. sapiens
MRCA
Most common recent ancestor
All extant living alleles are descended from a single ancestral allele present in one individual
Not the only male or female alive at that time
All the alleles derived from everyone else alive at that time have now died out
Why can each different part of the genome have a different MRCA
Due to recombination
Single locus studies have been useful but only tell part of the study
Why are mitochondria often studied in evolution?
Mitochondria do not recombine
Phylogenetics
Reconstruct shape (topology) of relationships
Estimate divergence times calibrated by molecular clocks
Usually at species level but can look at loci within species
One or more sequences per species
Coalescence
Simulate past population sizes and test models of gene flow
Estimate divergence and changes to population size measured in the number of generations
Usually within species or between diverging sub-sepcies
Requires representative sample of alleles
How long ago was the MRCA according to out of Africa?
0.1-0.2 Myr
How long ago was the MRCA according to the multiregional model?
1.8 Myr
What were the results of the study looking into mitochondria to find the MRCA?
Found the most MRCA was a woman (mitochondiral eve)
She lived in Africa, and one of these segments being made up solely of African mitochondrial DNAs
Accodring to the molecular clock she lived 100,000-200,000 years ago which agrees with the out of Africa hypothesis
What were problems with mitochondrial eve?
Many of the Africans used in the study came from San Francisco with their genetic origins not certain
The computers used to analyse data took shortcuts that might have led to an incorrect tree. The computed builds a tree with the first few sequences that are entered. It then tests this with later sequences. The final tree depends on the order in which sequences are entered into the computer. The study can also give tress with much older MRCA
Why is mitochondrial eve correct?
Since the first study there have been many studies examining mitochondrial DNA in modern humans.
All of these later studies confirm relatively recent date for mitochondrial eve
Recent estimates for mtDNA MRCA from 95,000 to 179,000
There have also been studies on the Y chromosome which follow the male lineage and the paternal MRCA lived in Africa between 192,000 and 307,000
What does the out of Africa hypothesis suggest with our relationship with Neanderthals?
There is no continuity between Neanderthals and modern Europeans
What does the multiregional model suggest our relationship is with Neanderthals?
There is a continuity between Neanderthals and modern Europeans
Ancient DNA
DNA from a dead organism
Sources of ancient DNA
BOnes and teeth up to 100,000 years in cold, dry conditions
Frozen mammoth tissue up to 50,000 years
Naturally mummified material up to 10,000 years
Coprolites up to 20,000 years
Dried plant remains up to 5000 years
What DNA was extracted from a Neanderthal?
377 bp sequence obtained from the Neanderthal Mitochondrial genome
What does the Neanderthal mitochondrial DNA tell us?
Found that the Neanderthal sequence is in a branch on its own connection to the root of the tree but not linked directly to any of the modern human sequences
How long did Humans and Neanderthals live in the same region together?
They lived in the same regions for 5000 years
