A series of publications claim that ancient DNA from plants, animals, and microbes — even viable bacterial cells — can survive in amber, halite, soft tissue, and sediments for up to several hundred million years (Goldenberg et al. 1990; Soltis et al. 1992; Cano et al. 1992a, b, 1993; DeSalle et al. 1992, 1993; Poinar et al. 1993; DeSalle 1994; Kennedy et al. 1994; Woodward et al. 1994; Cano and Borucki 1995; Morita 2000; Vreeland et al. 2000; Lambert et al. 2001; Vreeland and Rosenzweig 2002; Fish et al. 2002; Kim et al. 2004). These publications suggest that nucleic acids can persist over geological timescales (i.e., DNA sequences > 1 million years old). Departing from the theoretical evidence, these claims bear a heavy burden of proof. Another interesting study showed that Antarctic ice samples up to 8 million years old not only contain amplifiable DNA but also living bacteria (Bidle et al. 2007). This is a very interesting result as the 8 million-year-old sample is the oldest ice sample ever studied, but also because both the bacteria DNA and the viable cells isolated from the ice are much older that expected. The result is also far reaching compared to the record of long-term DNA survival from Greenland. In a recent study, 450,000- and 800,000-year-old DNA have been extracted from the silty ice of the Dye 3 Ice Core, but not from the much older ice in the GRIP (Greenland Ice Core Project) core (Willerslev et al. 2007).
Recent studies of frozen sediments performed under very strict conditions show that DNA from extinct animals and plants can reproducibly be recovered by independent laboratories from samples dated 300,000-400,000 years old, but not from sediments dated to be 1.5-2 million years old (Willerslev et al. 2003). Another study showed that bacteria DNA can be amplified from 400,000 to 600,000 years old permafrost samples from Siberia, but not from 8.1 million-year-old samples from Antarctica (Willerslev et al. 2004a). These results from permafrost show that DNA from bacteria, extinct animals and plants can reproducibly be recovered from very old samples up to 600,000 years old. Even though these findings could potentially result from leaching of free DNA, they are within what many groups currently accept as maximum ages for DNA survival (Hofreiter et al. 2001a; Smith et al. 2001; Willerslev et al. 2004a, b; Paabo et al. 2004; Willerslev and Cooper 2005).
The long-term survival of bacteria sealed in permafrozen sediments for up to 1 million years have also recently been investigated (Johnson et al. 2007). The study showed evidence of bacteria surviving in samples up to 500,000 years old which make this the oldest independently authenticated DNA to date obtained from viable cells. It is further shown that this long-term survival is closely tied to cellular metabolic activity and DNA repair.
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