Based on the FUNCTIONAL characteristics of the Cyanobacteria and Proteobacteria phyla, which phylum do you think is more deeply rooted in the tree of life (i.e. which phylum branches off closer to the last universal common ancestor)?

Question

Use at least two pieces of evidence to support your answer. How could you test your hypothesis?
What will be an ideal response?

Answer 1

Answer: Proteobacteria are more likely to be deeply rooted in the tree of life compared to Cyanobacteria for two reasons. First, Proteobacteria are very functionally diverse and include anoxygenic phototrophs, sulfur-oxidizers, sulfur-reducers, and a huge array of other anaerobic and aerobic physiologies. In comparison, Cyanobacteria are exclusively oxygenic phototrophs which could indicate that this phylum is younger and has had less time to evolve diverse physiologies. Second, conditions on early Earth were anaerobic and highly reducing. The physiology of many Proteobacteria indicates that ancestors of this group evolved during this time and have retained the ability to utilize a diverse array of reduced compounds for energy. In addition, many anoxygenic phototrophs are Proteobacteria. Anoxygenic photosynthesis evolved prior to oxygenic photosynthesis, which also supports the idea that Proteobacteria may be more deeply rooted than Cyanobacteria. To test this hypothesis, comprehensive phylogenetic trees including many bacterial phyla could be constructed based on SSU rRNA sequences and sequences of other housekeeping genes to determine which phylum is more deeply rooted based on molecular sequences that are not involved in energy generation.