Theme 5C: The history of life

Question 1

What was the Earth’s early atmosphere like?

Answer 1

• Originally very little oxygen
• Oxygen did not exist in a molecular state

Question 2

What type of energy input was used to transform some of the compounds in the earth to form the first organic compounds?

Answer 2

• Lightning!
• This was replicated in the Urey-Miller experiment

Question 3

When was there a rapid expansion of life?

Answer 3

• During the Phanerozoic eon.

Question 4

Where were some of the first prokaryotes found?

Answer 4

• Found in stromatolites due to the presence of cyanobacteria
• Stromatolites appear in fossils dating back to 2.7 billion years ago, decline in abundance by 500 million years ago

Question 5

How did cyanobacteria contribute to the change in the Earth’s atmosphere?

Answer 5

• Cyanobacteria obtain their energy through photosynthesis, so gaseous oxygen is produced as a by-product
• Converted the Earth’s atmosphere into an oxidizing one
• Rusting of the Earth
• Led to the near-extinction of oxygen-intolerant organisms

Question 6

What was the great oxygenation event?

Answer 6

• When free oxygen accumulated in the atmosphere about 2.5 billion years ago, with an increase about 850 million years ago

Question 7

What is some of the major evidence regarding the great oxygenation event?

Answer 7

• Banded iron formation in different rocks
• Major changes in the number of rock types formed after this event

Question 8

What could explain the long gap before oxygen was able to develop in the atmosphere?

Answer 8

• There was a long period of anoxygenic photosynthesis
• The oxygen that was being produced by the cyanobacteria was most likely being absorbed by the ocean, changing the chemistry of the ocean

Question 9

What’s the endosymbiotic theory?

Answer 9

• The origin of key eukaryotic organelles result of a symbiosis between separate single-celled organisms

Question 10

What’re some of the major evidence supporting the endosymbiotic theory?

Answer 10

• Organelles are bound by membranes
• Organelles have their own DNA
• Mitochondrial DNA sequences similar to bacteria/chloroplast DNA sequences similar to those of some cyanobacteria

Question 11

What’s the Paleozoic era?

Answer 11

• One of the major episodes in the history of life
• When the Cambrian explosion occurred, invasion of land, the appearance of gymnosperms, major groups of tetrapods

Question 12

What was the Cambrian explosion?

Answer 12

• Rapid appearance of many groups of organisms about 530 million years ago.
• Preceded by the appearance of small shell parts (calcium carbonate was more abundant)
• Unusually high number of sites with soft-body preservation
• Includes evidence of arthropods, echinoderms, and a large number of extinct forms
• Features of many modern groups started to apear such as mouths, heads, eyes, and legs

Question 13

Why was there such an increase in oxygen in the atmosphere after the Cambrian explosion?

Answer 13

• Eukaryotic algae developed

Question 14

Why was there such a high rate of mutations in organisms found during the Cambrian explosion?

Answer 14

• The organisms do not have the same proofreading capabilities that we have now, so many mutations were able to occur, creating a lot of variation among the species.

Question 15

What is mass extinction?

Answer 15

• When the rate of extinction greatly exceeds the rate of speciation (i.e., more than 75% of known species in a geographically short interval)

Question 16

Why are mass extinctions significant to evolution?

Answer 16

• Niches are cleared and ecological opportunities are made available to the organisms that remain
• Trims the phylogenetic tree (‘dead clades walking’)