Thermophilic ancestry of Bacteria

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The fact that the deepest branches in the bacterial tree are thermophilic, and often extremely thermophilic, implies that the ancestors of Bacteria were also thermophilic, and that mesophily arose later in the tree, approximately at the main evolutionary radiation where most of the non-thermophilic phyla originate. The possibility of a thermophilic ancestry for Bacteria is strengthened by the fact that the deeply-branching and primitive Archaea are likewise thermophilic, and so probably both the Bacteria and Archaea share a thermophilic ancestry.

But not all bacterial thermophiles are primitive and related only to other thermophiles; many are close relatives of otherwise mesophilic species. This means that there are 2 types of fundamentally-different thermophiles:

Ancestral thermophiles (e.g. Thermotoga and Aquifex) have only thermophilic ancestors. These organisms have been thermophiles as far back as we can observe phylogenetically, & so seem never to have adapted to mesophily - they evolved from scratch in thermophilic environments and thermophily is built into them from the basics up.

Adapted thermophiles (e.g. thermophilic Bacillus and Clostridium species) are related to & evolved from mesophiles, (re)adapting to thermophily more recently.

This is a critical difference for those interested in examining how life is possible at high temperatures.