Question

1. What lines of reasoning support the hypothesis that the first self-replicating systems were based on RNA molecules?
2. Why is the origin of cyanobacteria a critical step in evolution?
3. What evidence supports the idea that mitochondria and chloroplasts were once free-living members of the domain bacteria?

4. What evidence supports the classification of life into three domains?

5. Compare and contrast the physical and chemical conditions on Earth at the time life first arose with conditions today.
   a) Discuss two reasons why animals could not have existed on early Earth.
   b) In what ways has microbial metabolism altered Earth’s biosphere?
   c) How might life on Earth be different if oxygenic photosynthesis had not evolved?

Answer 1

1. The first existing nucleotide sequence that could have existed is RNA. This is based on the evidence that uracil was formed first before thymine. This RNA molecule can directly translate into proteins and also reverse transcribe into DNA. Hence the RNA acted as the intermediate for DNA and protein development.

2. The cyanobacteria is the first microbe in the phylogenetic tree which evolved the oxygen if photosynthesis. This helped in the conversion of CO2 rich environment to a balanced O2-CO2 environment in which subsequent life forms could survive.

3. The evidence that supports mitochondria and chloroplasts were from microbes is that they both have primitive DNA same as the prokaryote and their ribosomal machinery has the same subunits as the prokaryotes.

4. The evidence that supports the classification of life into three domains is based on the 16S rRNA that is found in all organisms.

5. At the time life first arose the environment was a reducing environment and the physical conditions were all extreme conditions such as high temperatures. Due to this the primitive organisms had anoxygenic photosynthesis and were anaerobic in nature.

A. The two reasons that life couldn't have existed is that it was a reducing environment and there were extreme conditions everywhere.

B. Microbial metabolism altered the environment by changing it to an oxidising environment from a reducing environment. It helped in the evolution of higher animals.

C. If oxygenic photosynthesis didn't evolved then the environment wouldn't have changed to an oxidising environment and the reducing environment would've always persisted and the enzymes which evolved would have been different which had to work at higher temperatures.

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