Life: past, present and future

Molecular methods of taxonomy and phylogeny have changed the way in which l ife on earth is viewed; they have allowed us to transition from a eukaryote -centric (five-kingdoms) view of the planet to one that is peculiarly proka rote-centric, containing three kingdoms, two of which are prokaryotic unice lls. These prokaryotes are distinguished from their eukaryotic counterparts by their toughness, tenacity and metabolic diversity Realization of these features has, in many ways, changed the way we feel about life on earth, ab out the nature of life past and about the possibility of finding life elsew here. In essence, the limits of life on this planet have expanded to such a degree that our thoughts of both past and future life have been altered. T he abilities of prokaryotes to withstand many extreme conditions has led to the term extremophiles, used to describe the organisms that thrive under c onditions thought just a few years ago, to be inconsistent with life. Perha ps the most extensive adaptation to extreme conditions, however, is represe nted by the ability of many bacteria to survive nutrient conditions not com patible with eukaryotic life. Prokaryotes have evolved to use nearly every redox couple that is in abundance on earth, filling the metabolic niches le ft behind by the oxygen-using, carbon-eating eukaryotes. This metabolic pla sticity leads to a common feature in physically stratified environments of layered microbial communities, chemical indicators of the metabolic diversi ty of the prokaryotes. Such 'metabolic extremophily' forms a backdrop by wh ich we can view the energy flow of life on this planet, think about what th e evolutionary past of the planet might have been, and plan ways to look fo r life elsewhere, using the knowledge of energy flow on earth.