Lecture 4 - Origin of prokaryotes & eukaryotes

Question 1

How are fossil stromatolites (rocks) evidence for prokaryotes life?

Answer 1

found in rural Australia - Sharks bay - outside of rock covered in bacteria which sticks to sediment. The bacteria migrates to the outside of rock to keep photosynthesising

• only over in hyper saline (high salt) - no fish in these area - as they would eat bacteria
• aerobic bacteria on outside of rock - anaerobic bacteria onside

Question 2

What is a stromatolite?

Answer 2

colony of bacteria

Question 3

How is fossil microorganisms evidence for prokaryote life?

Answer 3

• identical to modern bacteria (have been preserved)
• silt stones contain grains of sand, as well as skeletons of bacteria. If sand is dissolved, the bacteria pops out (these are called acritarchs)

Question 4

How can carbonaceous matter can be identified chemically as the product of ancient life?

Answer 4

rubisco prefers a light carbon isotope. Rubisco preferentially fixes carbon 12, as a result, high levels of carbon 12 found in fossils is a sign of life

Question 5

When did RNA turn into prokaryotes?

Answer 5

3.5 billion years ago

Question 6

Have prokaryotes changed a lot?

Answer 6

• prokaryotes have remained consistent
• prokaryotes remained relatively unchanged to the present day - they dominated the planet for some 2 billion years

Question 7

How did oxygen content in oceans increase?

Answer 7

3.5 billion years ago, prokaryotes advanced into the ocean. This consisted of complex & diverse communities of stromatolite form ping communities on the floor of shallow seas & free living prokaryotes floating around in the oceans.

• both communities include Cyanobacteria that produce oxygen as a biproduct of their photosynthetic metabolism
• a lot of oxygen is left as a biproduct, as a result

Question 8

Explain how early oxygen sinks?

Answer 8

1. Volcanic gases that compromised the early atmosphere readily combine (H –> H2O)
2. Dissolved iron that scavenged O2 to form BIF (banded iron formations) - this is because the oceans release rust, as oxygen reacts with iron causing rust and falling to the bottom of the oceans.
3. Microorganisms carrying out aerobic respiration (O2 + organic matter = H2O + CO2 + cellular energy). Some early organisms were no doubt facultative, respiring aerobically when O2 was available, but switching back to aerobic fermentation when it was in short supply.

Question 9

What is evidence for banded iron formations?

Answer 9

banded iron formations were present till around 2 billion years ago - this suggests that the oxygen were used up

Question 10

What is evidence for pyritic conglomerate (pebbles)?

Answer 10

also disappeared around 2 billion years ago - when they react with oxygen they disintegrate

Question 11

What is evidence for red beds?

Answer 11

red beds require oxygen to form and they formed around 2 billion years ago

Question 12

What event happened in the atmosphere around 2 billion years ago?

Answer 12

The atmosphere can then be said to go from an anaerobic atmosphere to an aerobic atmosphere around 2 billion years ago.

Question 13

What coincided with the change to an aerobic atmosphere 2 billion years ago?

Answer 13

the Huronian Ice Age caused by the elimination of atmospheric methane during the GOE

Question 14

What are the 2 considerations when thinking about the origin of eukaryotes?

Answer 14

• when were conditions suitable for the origin of eukaryotes
• fossil evidence for eukaryotes

Question 15

When could eukaryotes develop?

Answer 15

when the world became aerobic - as oxygen is toxic & poisonous to most eukaryotes
- 2 billion years ago - some acritarchs are large enough to suggest that they were eukaryotes
- there is fossil evidence of eukaryotes dating back 2 billion years

Question 16

What are the 3 theories of how eukaryotes originated?

Answer 16

Theory 1 - sybiosis - engulfment of bacteria
Theory 2 - elaboration of cell membrane
Theory 3 - multiple symbiotic events

Question 17

What eukaryotes has the symbiosis theory been used to explain?

Answer 17

• mitochondria
• chloroplasts
• flagella & cilia
• mitosis

Question 18

How was the symbiosis theory been used to explain the origin of mitochondria?

Answer 18

• have their own DNA & RNA
• mitochondria have similar DNA to facultative aerobe-proteobacterium (if you sequence the DNA) - suggest that aerobic organism been engulfed by anaerobic organism, which allows them to live symbiotically, until they live as 1
• the radical change of atmospheric composition killed a lot of anaerobic organisms

Question 19

How was the symbiosis theory been used to explain the origin of chloroplasts?

Answer 19

• got their own DNA
• sequencing of chloroplasts leads to great similarities to Cyanobacteria

Question 20

How was the symbiosis theory been used to explain the origin of flagella & cilia?

Answer 20

• some evidence to suggest it was formed as a result of symbiotic relationship - taken up to help move
• lot of evidence to suggest that flagella & cilia are similar to spirochaete bacteria

Question 21

How was the symbiosis theory been used to explain the origin of mitosis?

Answer 21

• centriole spindles look similar to tubules in cilia & flagella
• some suggestion that the process of mitosis originates from spirochaete bacterium

Question 22

What was suggested that a eukaryote was a combination of?

Answer 22

bacteria (e.g. plant chloroplasts & mitochondria), which were used to help deal with oxygenation of the atmosphere