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Stability of fossil DNA  and geological time

Fossil DNA in amber and implications for geological time 

The release of Steven Spielberg's blockbuster, Jurassic Park in 1993 meant that dinosaur mania swept the country! The cinemas were packed to capacity, shops were filled with dinosaur books and toys, and newspapers and magazines carried dinosaur-related stories and features almost daily throughout that summer.

Spielberg's film, based on a novel by Michael Crichton, takes as its premise the idea that mosquitoes living in the Jurassic period were trapped in hardening tree resin - amber - after biting dinosaurs. In the film, scientists extract the genetic code, DNA, from dinosaur blood present in the fossil mosquitoes, and use it to clone living dinosaurs!

The film is based partly on science fact. A great deal of attention has been devoted to the fact that scientists have, in recent years, successfully extracted portions of DNA from fossil materials. This has provided them with a lot of new and valuable information. But the cloning of living dinosaurs or other extinct creatures, as portrayed in the film, remains strictly science fantasy.

One of the earliest published reports (Goldenberg et al. 1990) concerned DNA extracted from fossil Magnolia leaves found in lake bottom sediments of Miocene age, supposedly 17-20 million years old. However, most of the DNA which has been recovered is from insects and plants preserved in amber, including a termite estimated to be 25-30 million year old (De Salle et al. 1992), a Hymenaea leaf thought to be 25-40 million years old (Poinar et al. 1993) and a weevil estimated to be 120-135 million years old (Cano et al. 1993). The weevil DNA is currently claimed to be 80 million years older than any other fossil DNA ever extracted and sequenced.

Such discoveries have been widely reported by the media. But what has been largely ignored is the difficulty that these finds present for the standard geological time scale.

DNA, like all other biological macromolecules, is very unstable, and spontaneously breaks down. In living cells, DNA is maintained by repair mechanisms, but after death DNA self-destructs at a rather rapid rate. In a recently published review of the chemical stability of DNA, Tomas Lindahl (1993) has said: `deprived of the repair mechanisms provided in living cells, fully hydrated DNA is spontaneously degraded to short fragments over a time period of several thousand years at moderate temperatures'(p.713).

In a similar vein, Sykes (1991) has commented that in vitro estimates of the rate of spontaneous hydrolysis imply that no DNA would remain intact much beyond 10,000 years. In his review paper, Lindahl goes on to argue that `it seems feasible that useful DNA sequences tens of thousands of years old could be recovered, particularly if the fossil has been retained at low temperature' (p.713), giving as an example DNA from mammoth tissue thought to be 40,000 years old. So, in summary, our knowledge of DNA stability makes it seem highly improbable that this molecule could be preserved for more than a few tens of thousands of years at most.

Yet the fossils from which DNA has been recovered are reckoned, in some cases, to be tens of millions of years old! There is obviously a problem here. It is for this reason that some of the reported finds - especially those which do not involve preservation in amber - are now being viewed skeptically by some scientists. It has been argued that some of the detected residues were the result of contamination by modern DNA, so more recent workers have been conscientious in arguing that they have eliminated this possibility. It is hoped that future studies will be carried out to see whether or not the earlier results can be reproduced.

Nevertheless, the data as presently known do not sit comfortably with the accepted millions of years time scale. Creationist young-earth models, which date the fossils to the time of the Biblical Flood and its immediate aftermath, accommodate the fossil DNA evidence much more easily. Until it is explained how DNA can be preserved for millions of years, or until the earlier published reports are conclusively refuted, creationists will be justified in viewing these finds as further evidence that the fossils are young.

Literature sources:
Cano, R.J. et al. 1993. Nature363: 536-8.
De Salle, R. et al. 1992. Science257: 1933-6.
Golenberg, E.M. et al. 1990. Nature344: 656-8.
Lindahl, T. 1993. Nature362: 709-15.
Poinar, H.N. et al. 1993. Nature363: 677.
Sykes, B. 1991. Nature352: 381-2.

Paul Garner (1993) 

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