Search tips
Search criteria 


Logo of transbThe Royal Society PublishingPhilosophical Transactions BAboutBrowse By SubjectAlertsFree Trial
Philos Trans R Soc Lond B Biol Sci. 1999 December 29; 354(1392): 1923–1939.
PMCID: PMC1692713

Life: past, present and future.


Molecular methods of taxonomy and phylogeny have changed the way in which life 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 prokarote-centric, containing three kingdoms, two of which are prokaryotic unicells. 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, about the nature of life past and about the possibility of finding life elsewhere. 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. The abilities of prokaryotes to withstand many extreme conditions has led to the term extremophiles, used to describe the organisms that thrive under conditions thought just a few years ago, to be inconsistent with life. Perhaps the most extensive adaptation to extreme conditions, however, is represented by the ability of many bacteria to survive nutrient conditions not compatible with eukaryotic life. Prokaryotes have evolved to use nearly every redox couple that is in abundance on earth, filling the metabolic niches left behind by the oxygen-using, carbon-eating eukaryotes. This metabolic plasticity leads to a common feature in physically stratified environments of layered microbial communities, chemical indicators of the metabolic diversity of the prokaryotes. Such 'metabolic extremophily' forms a backdrop by which we can view the energy flow of life on this planet, think about what the evolutionary past of the planet might have been, and plan ways to look for life elsewhere, using the knowledge of energy flow on earth.

Full Text

The Full Text of this article is available as a PDF (890K).

Selected References

These references are in PubMed. This may not be the complete list of references from this article.
  • Canfield DE, Teske A. Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature. 1996 Jul 11;382(6587):127–132. [PubMed]
  • Des Marais DJ, Strauss H, Summons RE, Hayes JM. Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment. Nature. 1992 Oct 15;359(6396):605–609. [PubMed]
  • Ehrenreich A, Widdel F. Anaerobic oxidation of ferrous iron by purple bacteria, a new type of phototrophic metabolism. Appl Environ Microbiol. 1994 Dec;60(12):4517–4526. [PMC free article] [PubMed]
  • Friedmann EI. Endolithic microorganisms in the antarctic cold desert. Science. 1982 Feb 26;215(4536):1045–1053. [PubMed]
  • Holland HD, Rye R. Evidence in pre-2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota: comment and reply. Geology. 1997 Sep;25(9):857–858. [PubMed]
  • Jørgensen BB, Revsbech NP, Blackburn TH, Cohen Y. Diurnal cycle of oxygen and sulfide microgradients and microbial photosynthesis in a cyanobacterial mat sediment. Appl Environ Microbiol. 1979 Jul;38(1):46–58. [PMC free article] [PubMed]
  • Knoll AH. The early evolution of eukaryotes: a geological perspective. Science. 1992 May 1;256(5057):622–627. [PubMed]
  • MacGregor BJ, Moser DP, Alm EW, Nealson KH, Stahl DA. Crenarchaeota in Lake Michigan sediment. Appl Environ Microbiol. 1997 Mar;63(3):1178–1181. [PMC free article] [PubMed]
  • Mojzsis SJ, Arrhenius G, McKeegan KD, Harrison TM, Nutman AP, Friend CR. Evidence for life on Earth before 3,800 million years ago. Nature. 1996 Nov 7;384(6604):55–59. [PubMed]
  • Nealson KH. Sediment bacteria: who's there, what are they doing, and what's new? Annu Rev Earth Planet Sci. 1997;25:403–434. [PubMed]
  • Nealson KH. The limits of life on Earth and searching for life on Mars. J Geophys Res. 1997 Oct 25;102(E10):23–23,686. [PubMed]
  • Nealson KH. Post-Viking microbiology: new approaches, new data, new insights. Orig Life Evol Biosph. 1999 Jan;29(1):73–93. [PubMed]
  • Nealson KH, Saffarini D. Iron and manganese in anaerobic respiration: environmental significance, physiology, and regulation. Annu Rev Microbiol. 1994;48:311–343. [PubMed]
  • Olsen GJ, Woese CR, Overbeek R. The winds of (evolutionary) change: breathing new life into microbiology. J Bacteriol. 1994 Jan;176(1):1–6. [PMC free article] [PubMed]
  • Rye R, Holland HD. Paleosols and the evolution of atmospheric oxygen: a critical review. Am J Sci. 1998 Oct;298(8):621–672. [PubMed]
  • Shi T, Reeves RH, Gilichinsky DA, Friedmann EI. Characterization of viable bacteria from Siberian permafrost by 16S rDNA sequencing. Microb Ecol. 1997 May-Jun;33(3):169–179. [PubMed]
  • Woese CR. Bacterial evolution. Microbiol Rev. 1987 Jun;51(2):221–271. [PMC free article] [PubMed]
  • Woese CR. There must be a prokaryote somewhere: microbiology's search for itself. Microbiol Rev. 1994 Mar;58(1):1–9. [PMC free article] [PubMed]

Articles from Philosophical Transactions of the Royal Society B: Biological Sciences are provided here courtesy of The Royal Society