Without a fossil record or detectable events of when a group of specific microorganisms appeared for the first time, we have little knowledge concerning the timeline or age of microbial species, or how to calibrate their evolutionary divergence (Vreeland and Rosenzweig 2002). Therefore, the age of supposed ancient organisms, including permafrost isolates (Willerslev et al. 2004b), is assumed from the age of their surrounding environment (Drancourt and Raoult 2005). A molecular clock has been postulated estimating that there is a characteristic rate of evolution in small subunit rRNA genes (Ochmann et al. 1999; Vreeland and Rosenzweig 2002). However, there is doubt regarding the validity of assuming a universal molecular clock of sequence evolution, as rates differ between bacterial taxa, and it may be unrealistic in regard to native species of environments such as permafrost that are subjected to low nutrient levels, extremely low temperatures, and long microbial doubling times (Vreeland and Rosenzweig 2002). In addition, recent studies demonstrating microbial activity in permafrost samples at ambient subzero temperatures (Steven et al. 2006; see Chap. 9) further complicate the determination of the age of microorganisms isolated from permafrost. These findings suggest that at least a subpopulation of the permafrost microbial community may constitute an active modern microbial ecosystem rather than "ancient" frozen microbial survivors.
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