lecture 12 - human evolution

Question 1

How can related pieces of DNA from different organisms be identified by DNA hybridisation?

Answer 1

1. Heat to separate strands
2. Combine single strands of DNA
3. Cool to allow renaturation of double-stranded DNA
   4.Determine degree of hybridisation
   Complete hybridisation: organisms identical
   Partial hybridisation: organisms related
   No hybridisation: organisms unrelated

Question 2

What is the molecular evidence that humans are more closely related to chimpanzees than any other primate?

Answer 2

the more mismatch the easier it is to break apart double stranded DNA
perfect complementarity: 95C denaturation
some mismatch: 93C denaturation
more mismatch: 91C denaturation
Human-Chimpanzee DNA molecules separated at temperature 1°C less than human-human DNA molecules
Calibrated against DNA hybridisation in other species this was estimated as a 1% difference in DNA sequences between chimpanzees and humans

Question 3

How do we know humans are closely related to chimpanzees and other large apes?

Answer 3

Morphological evidence places humans in the taxonomic order of primates, revealing that humans are most closely related to chimpanzee, bonobo, gorilla and orang-utan
Karyotypes are similar, but show a number of important differences (reduction of chromosome number from 48 to 46)
Comparison of human and chimpanzee genomes has identified 53 functional human genes that are completely or partially deleted in the chimpanzees
In total the the amount of lineage specific gains or losses of functional sequences in great apes is 3-7 Mb
One of the most common gains or losses appear to olfactory receptor genes, and genes related to immunity or male fertility
Molecular comparisons show that in two thirds of their DNA sequences humans and chimpanzees are more closely related to one another than either is to gorillas
Estimates of the time since chimpanzee and humans shared a common ancestor is about 5 to 7 Mya

Question 4

Describe the geography of human ancestry

Answer 4

Multiregional?
H. erectus left Africa 2Mya
evolved independently into H. sapiens around the world
not much evidence for this

Better theories:
Out of Africa?
H. sapiens arose from H. heidelbergensis and left Africa 100 to 200 kya
replaced H. erectus everywhere
Out of Africa again and again?
repeated migrations
some interbreeding with earlier migrants (Neanderthals and Denisovans)
restriction enzmyes can be used to map DNA from different sources

Question 5

What makes us human?

Answer 5

brain growth trajectory
brain size
descended larynx
eccrine sweat gland density
endurance running
earlier onset of labour and longer duration of labour
lacrimation (crying)
T cell function (enhanced immune function)
longer and more extended thumb

Question 6

Using FOXP2 as an example, describe the evidence that recent evolution of gene sequences that may contribute to the human phenotype

Answer 6

FOXP2 is a member of a large family of evolutionarily conserved genes that encode transcription factors that play an important role in development
FOXP2 is highly conserved in vertebrates
Individuals with mutations in FOXP2 have difficulty with speech and language
This is due to a verbal dyspraxia characterised by impaired learning, poor mobility of the lower face, deficits in language language reception or grammar
FOXP2 is known to regulate diverse regions of the brain - including those required for speech production
Changes in FOXP2 sequence occur rarely, but undergo positive selection in humans
There is only a single amino acid change in FOXP2 between mice and primates, although their most recent common ancestor lived about 75 million years ago
In the human lineage, two changes have occurred since the split with chimpanzees 6-7 million years ago.
When the two amino acid substitutions are inserted into the mouse Foxp2 gene the mice showed specific changes in dopamine levels, neuronal morphology, synaptic plasticity in specific neuron populations, and pup vocalisations
FOXP2 has also been implicated in vocal learning in birds
Suggests that the two amino acid substitutions affect specifically cortico-striatal circuits, which are involved in the learning of motor and cognitive skills
Could FOXP2 have contributed to the evolution of language acquisition?