Question 7:
From the evolutionary standpoint, extreme thermophiles are more closely connected with which one of the following:
Extreme thermophiles, like those in the domains Archaea and Bacteria, evolved adaptations for high-temperature environments, sharing deep evolutionary roots with methanogens. The correct answer is (B) Methanogens, as both belong to ancient archaeal lineages thriving in extreme conditions.
Correct Answer
(B) Methanogens
Extreme thermophiles, such as hyperthermophilic Archaea (e.g., Pyrococcus, Thermococcus), are most closely related to methanogens evolutionarily. Both groups dominate in the Archaea domain, with methanogens like Methanocaldococcus also being extreme thermophiles adapted to high-heat, anaerobic hydrothermal vents. Phylogenetic studies show Archaea as ancient lineages near the base of the tree of life, suggesting a shared hyperthermophilic last universal common ancestor (LUCA).
Option Breakdown
(A) Cyanobacteria
Cyanobacteria are oxygenic photosynthetic Bacteria, not extreme thermophiles. While some tolerate moderate heat, they lack the hyperthermophilic traits (growth >80°C) and Archaea-specific membrane lipids of true extremophiles. Their evolutionary path diverged early from Archaea.
(B) Methanogens (Correct)
Methanogens are Archaea producing methane under anaerobic conditions, many as extreme thermophiles (e.g., Methanothermobacter). Genomics reveal extensive gene transfer between thermophilic Archaea groups, confirming close phylogenetic ties and shared adaptations like reverse gyrase for DNA stability in heat.
(C) Fungi
Fungi are Eukarya, limited to ~60°C max due to unstable organelles and proteins. They evolved later from a bacterial-archaeal host merger, far removed from prokaryotic thermophiles. No close evolutionary link exists.
(D) Microsporidia
Microsporidia are parasitic Eukarya (fungi relatives), mesophilic intracellular pathogens with reduced genomes. They diverged within eukaryotes, unrelated to thermophilic prokaryotes.
