Origin of Prokaryotes and Eukaryotes (Wellman)

Question 1

Evidence for prokaryotic life

Answer 1

• Fossil Stromatolites
• Fossil microorganisms
• Carbonaceous matter that can be chemically identified as the product of ancient life

Question 2

Fossil Stromatolites

Answer 2

(Evidence for PRO life)
Stromatolites are formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria.
-Formed as sediment accumulates on top of cyanobacteria, causing it to move back to the surface, leaving defined layers of sediment. This causes the stromatolites to build up.
-First found in Australia’s Shark Bay, where salinity levels are very high. This excludes the life that feed on algae. This allows the cyanobacteria to photosynthesis and grow. A colony of anaerobic bacteria underneath where there’s no oxygen.

Question 3

Fossil Microorganisms

Answer 3

(Evidence for PRO life)
Silicified in cherts
Microorganisms can be observed in chert, which is silica-rich sedimentary rock that preserved fossils very well.

Resistant cell walls called acritarcs preserved in siltstones
Microorganisms can be seen under microscopes when organic residues are removed from the siltsones.

Question 4

Carbonaceous matter that can be identified chemically as the product of ancient life

Answer 4

(Evidence for PRO life)
Rubisco discriminates with carbon 12 over carbon 13. This means carbon 12 is much more abundant in the atmosphere. We can see how life has evolved by measuring the abundance of each isotope during different periods.

Question 5

Prokaryotes dominated the Earth?

Answer 5

Prokaryotes remain relatively unchanged to the present day. They dominated the planet for some 2 billion years.
By 3,500 Ma we have advanced prokaryotic life in the oceans (offering some protection from UV rays). This consisted of complex and diverse communities of stromatolite forming communities on the floors of shallow seas and free living prokaryotes floating around in the oceans.

Both communities include cyanobacteria that produce oxygen as a by-product of their metabolism.

Question 6

Early Oxygen Sinks

Answer 6

• Volcanic gases that readily combine with oxygen.
• Dissolved iron that scavenged oxygen to form BIFs
• Microorganisms carrying out aerobic respiration

Question 7

Volcanic gases that readily combine with oxygen

Answer 7

(Evidence for early oxygen sinks)

N–NO and NO2, H–H2O, CH4–CO2 and H2S–SO4

Question 8

Dissolved iron that scavenged oxygen to form BIFs

Answer 8

(Evidence for early oxygen sinks)
-BIF = Banded Iron Formations (pre-Cambrian rocks)
The early oceans were full of dissolved iron, which had nothing to react with.
When O2 was made, iron oxide was formed which sinks to the bottom of the oceans, banded iron formations are built up.

Question 9

Microorganisms carrying out aerobic respiration

Answer 9

(Evidence for early oxygen sinks)
O2 + organic matter –> H2O + CO2 + cellular energy
The earliest organisms were no doubt facultative, respiring aerobically when O2 was available, but switching back to anaerobic fermentation when it was in short supply.

Question 10

Evidence for oxygenated Earth

Answer 10

• BIFs
• Pyritic conglomerates
• Red beds

Question 11

Pyritic conglomerates

Answer 11

(Evidence for oxygenated Earth)
(Between 3000 and 2400 million years ago)
Iron sulfide (pyrite) -oxidises-> iron sulfate
There could not have been oxygen in the atmosphere when pyritic conglomerates formed because iron sulfide readily oxidises to iron sulfate in oxygen.

Question 12

Red beds

Answer 12

(From 2400 millions ago, and still being formed today)
Red beds, made of iron sulfate, are only formed when oxygen is in the atmosphere, so when these formed, the Earth must have become oxygenated.

Question 13

Consideration of the origin of eukaryotes

Answer 13

When were conditions suitable for the origin of eukaryotes?
Eukaryotes (and mitochondria) are aerobic and need oxygen in order to survive.

-Fossil evidence
By 2 billion years ago, some acritarchs are large enough to suggest that they were Eukaryotes. We can assume that they are eukaryotes instead of prokaryotes because they are much larger. They were found with complex structures in cherts.

Question 14

Theories of how eukaryotes evolved

Answer 14

Theory 1 - Symbiosis:
Organelles were originally bacteria, and 2 species of bacteria were working together symbiotically, until eventually one species ingested the other, forming a nucleus.

Theory 2 - Elaboration of cell membrane:
The cell (which fed by ingesting particles) already had a linear chromosome (as the result of a mutation), and the cell membrane folded inwards forming the nucleus.

Question 15

Symbiosis can be used to explain..

Answer 15

The origin of eukaryotic cells and their organelles:

• Mitochondria and chloroplasts
• Flagella and cilia
• Mitosis

Question 16

Mitochondria and chloroplasts (Symbiosis)

Answer 16

• Mitochondria/Chloroplasts are bacterial size and contain their own DNA. When this was sequenced it appeared very similar to prokaryotic DNA. There was also exchange of genetic material between mitochondria and the nucleus.
• When mitosis occurs, mitochondria and chloroplasts divide and reproduce as well.
• Semi-autonomous.
• It is suggested that chloroplasts originated from cyanobacteria that could reproduce.

Question 17

Flagella and cilia (Symbiosis)

Answer 17

• Suggested to have originated from spirochaete bacteria, which is worm-like in water.
• During mitosis, flagella has to replicate too
• Arguments say that DNA is detectable, but this is still widely speculated.

Question 18

Mitosis (symbiosis)

Answer 18

-Centrioles form at the edge of the cell when dividing. The tubule structure is similar to flagella and cilia.