1: Origin of sex & multicellular life

Question 1

Describe the Snowball/slushball Earth hypothesis

Answer 1

(635-720 Ma)
→ Cryogenian
→ initially v controversial
→ whole earth frozen = mass extinction event
2 extensive glaciations:
Marinoan Ice age (635-650 Ma)
Sturtian Ice age (660-720 Ma)

Question 2

Describe the geology of the snowball earth hypothesis

Answer 2

• Boulder clays representing the glaciations are bounded below and above cap by carbonates
• Carbonates below = exceptionally high C-isotope values
  
  • Due to continental break up producing
    narrow seaways with enhanced C burial
  • These reduced atmospheric Co2 leading to
    glaciations
• BIFs occur within boulder clays - for first time in 1.8 billion years iron accumulates in oceans
• The cap carbonates = exceptionally low C-isotope values

Question 3

Around when did sexual reproduction originate

Answer 3

around 2 Ga
→ eukaryotes evolved before they were sexually reproducing

Question 4

Why did evolution speed up when sexually reproducing organisms emerged?

Answer 4

Because sexual reproduction creates increased variation due to mutation, independent assortment and crossing over
- Compared to asexual reproduction (only variation due to mutation)

Question 5

Sex is a huge fitness cost to organisms, why does it persist?

Answer 5

• Sex can combine advantageous mutations = drives evolution
• Sex can shed harmful mutations (most common type)
• In asexual pop. you cannot get rid of these deleterious mutations = evolutionary ratchet

Question 6

What are the 4 main theories on the origin of sex?

Answer 6

1) A method of adapting to (environmental) change by evolving faster
2) A method of fending off disease/parasites
3) A method of repairing genes
4) Historical accident

Question 7

Around when did multicellular life evolve?

Answer 7

1 billion YA

Question 8

What did multicellular life almost certainly evolve from?

Answer 8

Almost certainly evolved from colony formation e.g corals
→ allows for a division of labour