A fundamental question is the nature of the apparent biophysical/biochemical constraint on the upper temperature limit for growth of procaryotes and the kingdoms of eucaryotes. Heat shock proteins (HSPs) are likely candidates to investigate because oftheir multifold roles in cells (molecular chaperones, involvement in mitochondrial membrane synthesis, etc.). Nisbet (1995) suggested that HSPs evolved early as a protection against the very high temperatures around hydrothermal vents. I would add that thermophiles probably also needed protection from local low albedo-induced high temperatures on land. The thermostability of HSPs, along with that of organelles and their membranes (mitochondria, nuclei, etc.) and various enzymatic systems (such as involved in blastula formation and the synthesis of collagen, a key structural protein of Metazoa) should be investigated. Are there any living ther-mophilic amitochondrial eucaryotes that never had mitochondria in their ancestors, with upper temperature limits above the mitochondrial eucary-otes, that might be models of an Archean presence?
Attempts to synthesize and find in nature living models of possible Pre-cambrian land symbioses such as actinolichens (actinobacteria and algae or cyanobacteria) and primitive lichens (e.g., with chytrid mycobionts) should be pursued. Is the anti-lichen model viable for Precambrian land biota?
Finally, an experimental approach to the stabilization and dissolution of a mineral soil by thermophiles should be pursued, which would be relevant to the scenario of early biotic colonization of the land.
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