EVOLUTIONARY PATTERNS OF NICHE SPECIALIZATION IN THE TREE OF LIFE; IMPLICATIONS FOR EARLY LIFE ON EARTH AND THE SEARCH FOR LIFE ELSEWHERE

The deepest branches in the bacterial domain are found in near-boiling slightly alkaline terrestrial silica-depositing springs, e.g. in Yellowstone National Park. This suggests that the ancestor to the bacterial domain existed in similar high temperature ecosystems. Parsimony reconstructions of ancestral character states show that the ancestor to the Crenarchaeota lived in slightly acidic terrestrial geothermal habitats (likely performing sulfur reduction) and the ancestor to the Euryarchaeota in marine geothermal habitats like deep-sea hydrothermal vents (likely performing methanogenesis). This suggests that very early niche specialization into the three major types of geothermal environments may explain why there are three main lineages of prokaryotes (Bacteria, Cren- and Eury-archaeota). Mapping of character states onto the Cren- and Euryarchaeal trees shows that their habitat specializations lasted over a billion years.

From these ancestors with specialized niches, came further divergences. For example, several archaeal groups diverged into hyperacidic and mesophilic habitats. Similarly, major divergences into mesophilic habitats occurred in the bacterial domain. Phylogenomic dating places these latter divergences before 2.45 Ga, perhaps near the end of the Archean era that witnessed the appearance of microbial mats and stromatolites. From these mesophilic descendents, came further niche specialization into hypersaline and hyperalkaline habitats.

From this evolutionary pattern, an analogous pattern might be expected in the search for life elsewhere. If life spontaneously arose elsewhere, one might predict early niche specialization, followed later by divergences and radiations of new groups, then by further niche specializations. If this life was transferred by Panspermia, one might expect the interplanetary transfer to have occurred early in the solar system, perhaps during late-heavy bombardment or the early Archean era. If so, then the type of organisms transferred must have been hyperthermophiles that populated the deepest branches on the tree of life (not mesophiles). This inoculum then would have led to divergences and radiations, followed by further niche specialization such as colonization of hyperacidic, hypersaline, mesophilic, or hyperalkaline environments.