Lecture 1: The first 3 billion years

Question 1

The recipe for life on earth today:

Answer 1

microbial metabolism =

Photosynthesis + respiration

Question 2

timeline for life on earth =

Answer 2

3. 5 bill = prokaryotic life
4. 5 bill = photosynthesis
5. 5 bill = eukaryotes
6. 5 bill = multicellularity

Question 3

before photosynthesis oxygen

Answer 3

lacking free oxygen, locked into H2O, Co2 etc

Question 4

The Archean atmosphere before free O2

Answer 4

“faint young sun paradox”

• 3 bn years ago sun was 30% less bright than today
• if atmosphere was the same as today H2O would be frozen
• but evidence of water (life)

Question 5

solution to faint young sun paradox

Answer 5

• extreme greenhouse effect
• CO2 and CH4 (methane comes from microbial ecosystem) accumulation
• FIORELLA AND SHELDON 2017

Question 6

evolution of oxygenic photosynthesis evolved

Answer 6

just once in cyanobacteria

• all life depends on it
• WATER + CO2 + LIGHT ENERGY —> GLUCOSE + O2 + H2O

Question 7

endosymbiosis =

Answer 7

gave rise to all eukaryotic algae and plants (chloroplasts)

• primary and secondary endosymbioses
• ARCHIBALD 2015

Question 8

how do we know when photosynthesis evolved?

Answer 8

Free oxygen level increase!
Sulphur isotope geochemistry
- Mass-dependent fractionation VS mass-independent fractionation (MIF)

– MIF rare, confined to gas phase reactions

Question 9

MIF signal in past 4 billion years

Answer 9

MIF extremely variable up to 2.5 billion years ago, settles down

• probably formation of stratospheric ozone layer
• oxygen accumulated to a level for stratospheric layer to form ozone

Question 10

stratospheric ozone layer =

Answer 10

shields the earths lower atmosphere from UV radiation

• ozone protects earth
• high (where aeroplanes fly)

Question 11

ozone & oceans

Answer 11

Ozone saved Earth’s oceans from their fate on Mars

- As UV breaks down H2O to O2 & H2

Question 12

Methane greenhouse & oxygen

Answer 12

oxygen destroyed Earth’s methane greenhouse

CH4 + 2O2 –> CO2 + 2H2O

• Pulse of O2 into atmosphere broke down greenhouse = so not hot enough = snowball earth

Question 13

Snowball earth

Answer 13

• at least 3 snowball earths

- runaway ice-albedo-temperature feedback

Question 14

runaway ice-albedo-temperature feedback

Answer 14

• lower temperatures
• more ice
• higher albedo (ice covers dark surface, reflect light)
• heat not absorbed
• positive feedback loop

Question 15

how did snowball earth thaw?

Answer 15

• Volcanic activity (geochemical carbon cycle)

- CO2 accumulated in the atmosphere till enough to warm = CO2 greenhouse effect

Question 16

oxygen enables ____ respiration

Answer 16

aerobic

• first evolved in bacteria
• generate more ATP, 31:2 mol ATP
• more efficient
• more energy left over for growth etc
• O2 = greater mobility
• vascular oxygen delivery system

Question 17

______ helped maintain an early oxygen-free atmosphere

Answer 17

methanogenesis

Question 18

oxygenic photosynthesis evoled

Answer 18

once,