Human Evolution II

Question 1

Why is Africa so important to understanding human evolution?(3)

Answer 1

• Homo sapiens originated in Africa
  o Understanding the patterns of genetic diversity in Africa is essential to understanding the origins and evolution of our species
• Comparisons with African populations allow us to understand the evolution that has occurred in populations that left Africa
• Best chance to understand the genetic basis of human traits

Question 2

Why is there so much genetic diversity within Africa? (4)

Answer 2

o Old lineages – multiple lineages have had time to diverge for up to 170,000 years
o Large population size – Population sizes have remained relatively larger than non-African population (= less drift)
o Adaptation: Differences in ecology, technology and culture have selected for different adaptations
o Admixture – Reverse migration by non-Africans, and population interbreeding has led to admixture

Question 3

Why do non-African populations show such little genetic diversity?

Answer 3

o Only a subset of Africans migrated – founding population derived from eastern African descent
o Population bottleneck – The migration out of Africa consisted of approximately 1500 people (probably biased towards men)
o Young age compared to African populations – Geographically separate, but relatively young age of all non-African lineages (50,000-60,000 years old)

Question 4

Why has the rate of evolution of our genes changed in the last 8,000-10,000 years? (3)

Answer 4

o Increased population size = more beneficial mutations
o Increased population size = increased efficiency of selection (decreased drift)
o Our ecology has changed, leading to novel selection

Question 5

Describe lactose tolerance.

Answer 5

o In virtually all human babies, Lactase Pholorizin Hydrolase (encoded by the LCT gene) is active in your small intestine, breaking down milk into sugar, which are absorbed into the bloodstream

Question 6

Describe the convergent evolution of lactose tolerance.

Answer 6

o Europe/ME/N. Africa:
o One SNP is a cis-promoter region of LCT controls whether or not Europeans, Middle East, North and West Africans can metabolize milk
o Shows signature of a recent selective sweep (decreased genetic variation around the SNP) causing increase in frequency of the SNP
o Originated at about the time of cattle farming in middle-east, 8,000-10,000 years ago
o East Africa:
o East Africans are lactose tolerant as adults lack the European SNP
o Tishkoff et al. 2007 found 3 more SNPs that independently arose in the last 7,000 years, coinciding with origin of pastoralist existence

Question 7

Describe the biological mechanisms of pigmentation.

Answer 7

o Melanocyte cells produce different melanic pigments: eumelanin makes darker pigments. Pheomelanin makes lighter pigments
o Increased pigmentation protects individuals from damaging UV-B
o UV-B also makes vitamin D3, necessary for bone formation

Question 8

What are the five things we were told to remember in class?

Answer 8

1. Evolution is a fact. It has shaped the diversity of life on Earth and it happens everyday.
2. All life is related, with common ancestors and descendants forming a giant tree of life depicting our evolutionary history.
3. 5 mechanisms can cause a population to evolve (mutation, selection, drift, dispersal, non-random mating)
4. Evolution reflects past events, it never reflects future events and therefore evolution is not progressive
5. Nothing makes sense in biology. Even our own species.