Theme 5C

Question 1

How old are the Earth and its life?

Answer 1

Earth: 4.5bil
Unicellular prokaryotes: 3.5bil
Multicellular life: 2.1bil
Complex multicellular animals: 650mil

Question 2

What was early Earth’s atmosphere?

Answer 2

• originally little oxygen - reducing atmosphere
• oxidation prevented by removal of oxygen and other oxidizing gases or vapours (water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2) as major components)
• the input of energy (lightning) would transform these into organic compounds in ‘primordial soup’

Question 3

Archaean Period

Answer 3

• From 4bil to 510million years ago
• Prokaryotes, Eukaryotes, Multicellularity
• Cyanobacteria photosynthesized for energy, converted early reducing atmosphere into an oxidizing one
• Stimulated biodiversity and led to near-extinction of oxygen-intolerant organisms

Question 4

Great Oxygenation Event

Answer 4

• Free oxygen accumulates in the atmosphere about 2.5billion years ago, with a huge increase 850 million years ago

Question 5

What is the evidence of the great oxygenation event?

Answer 5

• Banded Iron Formations abundant between 2.5 and 1.8 billion years ago, decline after
• Major changes in the number of rock types formed after this event: hydrated and oxidized minerals

Question 6

Why the long gap before O2 atmosphere?

Answer 6

• Probably a long period of anoxygenic photosynthesis
• Free oxygen reacted with ocean chemistry (not immediately released into the atmosphere) –
• Banded Iron Formations – oxygen was reacting with iron and sulfur to form these rocks, common between 2.5 and 1.8 billion years ago

Question 7

What is endosymbiosis?

Answer 7

• The origin of key eukaryotic organelles resulting from the symbiosis between separate single-celled organisms
• mitochondria and plastids were free-living bacteria taken inside another cell

Question 8

Evidence of Endosymbiosis?

Answer 8

• Organelles are bound by membranes (just like prokaryotes)
• Organelles have their own DNA separate from DNA in the nucleus
• Mitochondrial DNA sequences similar to bacteria/ chloroplast DNA sequences similar to those of some cyanobacteria
• Reproduction: mitochondria replicate by pinching

Question 9

What is the Paleozoic period?

Answer 9

Cambrian explosion; invasion of land; the appearance of gymnosperms; major groups of tetrapods

Question 10

What was the Cambrian Explosion?

Answer 10

• Rapid appearance of many groups of organisms about 530 million years ago
• Preceded by appearance of small shell parts
• Unusually high number of sites with soft-body preservation (e.g. Burgess Shale)
• Includes evidence of arthropods, echinoderms, and a large number of extinct forms
• Features of many modern groups appear: heads, mouths, eyes, legs

Question 11

Why was it an explosion of life?

Answer 11

• Genetic diversity likely present
• Increasing O2 levels from eukaryotic algae (allowing for higher metabolic rates, larger body size, etc.)
• Evolution of grazing, reduction of algal mats
• Shift in ocean chemistry favouring production of calcium carbonate

Question 12

Extinction and Mass Extinctions

Answer 12

• Normal background rate of extinction varies but usually less than or equal to the rate of speciation over time
• Mass extinction: rate of extinction greatly exceeds the rate of speciation (usually defined as more than 75% of known species in a geologically short interval
• Mass extinctions are periodic

Question 13

Why are mass extinctions significant to evolution?

Answer 13

• Niches are cleared/ecological opportunities available

- Leaves ‘dead clades walking’ (low diversity remnants of once diverse lineages)