Lecture 21 - Genetics of human evolution

Question 1

When and where were the first fossils found outside of Africa?

Answer 1

100,000 years ago in China

Question 2

How old are the oldest definite H.sapien fossils?

Answer 2

60,000 years old

Question 3

What is the out of Africa hypothesis?

Answer 3

H. sapiens originated in Africa and then moved to other parts of the world

Question 4

Why do some paleontologists disagree with the out of Africa hypothesis?

Answer 4

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

Question 5

What is the multiregional process?

Answer 5

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

Question 6

What are the implications of the out of Africa hypothesis?

Answer 6

The difference between modern human populations are relatively recent variations that evolved mostly during the last 50,000 years

Question 7

What are the implications of the multiregional model?

Answer 7

Modern human populations are much less related, and originate in H. erectus rather than H. sapiens

Question 8

MRCA

Answer 8

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

Question 9

Why can each different part of the genome have a different MRCA

Answer 9

Due to recombination

Single locus studies have been useful but only tell part of the study

Question 10

Why are mitochondria often studied in evolution?

Answer 10

Mitochondria do not recombine

Question 11

Phylogenetics

Answer 11

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

Question 12

Coalescence

Answer 12

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

Question 13

How long ago was the MRCA according to out of Africa?

Answer 13

0.1-0.2 Myr

Question 14

How long ago was the MRCA according to the multiregional model?

Answer 14

1.8 Myr

Question 15

What were the results of the study looking into mitochondria to find the MRCA?

Answer 15

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

Question 16

What were problems with mitochondrial eve?

Answer 16

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

Question 17

Why is mitochondrial eve correct?

Answer 17

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

Question 18

What does the out of Africa hypothesis suggest with our relationship with Neanderthals?

Answer 18

There is no continuity between Neanderthals and modern Europeans

Question 19

What does the multiregional model suggest our relationship is with Neanderthals?

Answer 19

There is a continuity between Neanderthals and modern Europeans

Question 20

Ancient DNA

Answer 20

DNA from a dead organism

Question 21

Sources of ancient DNA

Answer 21

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

Question 22

What DNA was extracted from a Neanderthal?

Answer 22

377 bp sequence obtained from the Neanderthal Mitochondrial genome

Question 23

What does the Neanderthal mitochondrial DNA tell us?

Answer 23

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

Question 24

How long did Humans and Neanderthals live in the same region together?

Answer 24

They lived in the same regions for 5000 years

Question 25

What was found in 1998 in Abrigo do Lagar Velho in Portugal?

Answer 25

In 1998 a skeleton of a four year old child was found at Abrigo do Lagar Velho in Portugal which was claimed to represent a Neanderthal/ modern human hybrid. Has modern human features but very robust

However the burial has been dated to 24,500 years ago, about 15,000 years after the last definite evidence for Neanderthals in Europe, so its status as a hybrid seems doubtful

Question 26

What did mitochondrial DNA suggest about humans and Neanderthals’ interbreeding?

Answer 26

Suggests that there is no female contribution to the modern human genome

This does not exclude the possibility of interbreeding involving some male Neanderthals

Question 27

what do genome comparisons suggest about human and Neanderthal interbreeding?

Answer 27

European and other populations share alleles at some loci with Neanderthals. These are much rarer in Africans, especially south in Sahara

This suggests a small amount of interbreeding between Neanderthals and H. sapiens in Europe

If there had been no interbreeding then modern Europeans and Africans should be indistinguishable when compared to Neanderthals

Found it was mainly male Neanderthal to female H. sapiens

Question 28

What are Denisovans?

Answer 28

New type of hominin that are related to Neanderthals but found in Asia rather than Europe

Question 29

How were Denisovans discovered?

Answer 29

DNA was initially obtained from a single-finger bone

The mitochondrial DNA sequence is significantly different from the Neanderthals

There was some interbreeding between Denisovans and Asian H. sapiens

Also found a tooth that was found to be more similar to Homo habiliis and Australopithecines

Question 30

Jawbone found in Xiahe cave at on the Tibetan plateau

Answer 30

Jaw bone that has recently been attributed to Denisovans

No ancient DNA was recovered

Jaw robust and molars bery similar to denisova cave

Uraniusm Thorium dating of calcium carbonate attached to jaw suggests 160kya

Question 31

Why is the out-of-Africa model incorrect?

Answer 31

There was some gene flow from Neanderthals and Denisovans into H.sapiens

Question 32

Why is the multiregional model wrong?

Answer 32

Even more wrong

The ancestor of Neanderthals and Denisovasn also originated in Africa

And left Africa up to 800,000 years ago

Question 33

When is the most likely common ancestor of modern humans and neanderthals?

Answer 33

400-800 thousand years ago

Question 34

Admixture of Homo species

Answer 34

Genome sequences show admixture with Homo sapiens

The complete Neanderthal and Denisovans genome show extent of admixture

Question 35

Denny

Answer 35

Fingerbone was found and used mass spectrometry to analyze them

Found out it came from a female

Mother was a Neanderthal

Father was a Denisovan

Was heterozygote for almost every genus for Neanderthal and denisovan which shows they were first generation

Lived 90,000 years ago

Part of the genome only was homozygous for Neanderthal which was inherited from one of the ancestors from several generations back

Tell us not only a first generation hybrid but one of the Denisovan ancestors was also a hybrid