Table of Contents
In commenting on the remarkably old DNA in the supposedly 17-million-year-old magnolia leaf, Svante Paabo exclaimed, "The clay was wet, however, and one wonders how DNA could have survived the damaging influence of water for so long." 24
DNA, like all other biological macromolecules, is generally quite unstable and spontaneously breaks down - especially when hydrated or "wet". In living cells, DNA is maintained by repair mechanisms, but after death DNA self-destructs at a rather rapid rate. In a published review of the chemical stability of DNA, Tomas Lindahl (1993) noted, 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.
Lindahl went on to argue for the "contamination" of all such specimens by modern DNA suggesting that, "The apparent observation that fully hydrated plant DNA might be retained in high-molecular mass form for 20 million years is incompatible with the known properties of chemical structure of DNA." 28 In a 1991 issue of Science Jeremy Cherfas expressed his bewilderment noting, "That DNA could survive for such a staggering length of time was totally unexpected - almost unbelievable." 25
Others have noted that, "Certain physical limits seem inescapable. In approximately 50,000 years, water alone strips bases from the DNA and leads to the breakage of strands into pieces so small that no information can be retrieved from them. Oxygen also contributes to the destruction of DNA. Even in ideal conditions in the absence of water and oxygen and at low temperature background radiation must finally erase all genetic information."27Many other authors have suggested a "maximal DNA survival of 50 thousand (Kyr) to 1 million (Myr) years."33
"Dinosaurs hold an enduring fascination. We reported the detection of a protein in a dinosaur bone, published at around the same time as the release of Steven Spielberg's blockbuster, Jurassic Park, [so it] was bound to receive the full media treatment. Our report claimed to have detected osteocalcin immunologically and also to have found an unusual amino acid g-carboxyglutamic acid (Gla) in a dinosaur bone from immature (unheated) sediments. Osteocalcin is peculiarly suited to such spectacular survival, it is very abundant in bone, binds strongly to it and has the distinction of being the only ancient protein ever to have been sequenced." 7
"The lab filled with murmurs of amazement, for I had focused on something inside the vessels that none of us had ever noticed before: tiny round objects, translucent red with a dark center. Then a colleague took one look at them and shouted, 'You've got red blood cells. You've got red blood cells!'. It was exactly like looking at a slice of modern bone. But, of course, I couldn't believe it. I said to the lab technician: 'The bones, after all, are 65 million years old. How could blood cells survive that long?'" 13,14
"The production of antibodies specific for hemoglobin in two rats injected with the trabecular extract is striking evidence for the presence of hemoglobin-derived peptides in the bone extract. . . That the antisera did not react with snake hemoglobin shows that the reactivity is specific and not artifact. . . When considered as a whole, the results support the hypothesis that heme prosthetic groups and hemoglobin fragments were preserved in the tissues of the Late Cretaceous dinosaur skeleton." 16
"But the heme itself is too small to be immunogenic [only about 652 daltons]. We believe that there were possibly 3-4 amino acids from the original protein attached to the heme, and that was what may have spiked the immune response." 17
"I mean can you imagine pulling a bone out the ground after 68 million years and then getting intact protein sequences?" said John Asara of Beth Deaconess Medical Center and Harvard Medical School, lead author of one of the studies. "That's just mind boggling how much preservation there is in these bones."The new finding will be viewed skeptically, admitted one of the researchers involved in the two studies. "It's very, very, very controversial because most people have gone on record saying there's an absolute time limit to anything that's protein or DNA," said Mary Schweitzer, a molecular paleontologist at North Carolina State University.Matthew Carrano, a dinosaur curator at the "Here are the pieces of the protein. If you're going to refute this you have to explain how these pieces got in there," Carrano said in a telephone interview. "It's not another molecule mimicking the protein and giving off a similar signal. This is the actual sequence."32 in Washington, D.C., who was not involved in either study, said the protein findings are robust.
A comparison by Asara's team of the amino-acid sequence from the T. rex collagen to a database of existing sequences from modern species showed it shared a remarkable similarity to that of chickens. Amino acids are the molecular building blocks of proteins; there are 20 of them used by organisms to build proteins, and their precise order is determined by instructions found in DNA."I'm grateful that he was able to get the [amino acid] sequences out. That's the Holy Grail," Schweitzer told LiveScience. . . Until now, family trees have been constructed from the shapes of bones and teeth, a not-always-reliable technique."This finding supports the idea that chickens and T. rex share an evolutionary link and bolsters previous research showing that birds evolved from dinosaurs and that birds are living dinosaurs."Here we have a real molecule from a real dinosaur, and it's much more similar to a bird than it is to anything else," Carrano said.32
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- Svante Paabo, "Ancient DNA", Scientific American, Vol. 269, November 1993, p. 92.
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- Russell H. Vreeland et al., "Isolation of a 250 Million-Year-Old Halotolerant Bacterium from a Primary Salt Crystal," Nature, Vol. 407, 19 October 2000, pp. 897-900.
- Mary H. Schweitzer, Jennifer L. Wittmeyer, John R. Horner, Jan B. Toporski, Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex, Science, March 25, 2005
- Jeanna Bryner, T. Rex Related to Chickens, LiveScience ( Link ) Last Accessed: April 20, 2007; See also: Sharon Begley, T. Rex and His Family, Newsweek, April 23, 2007
- Martin B. Hebsgaard, Matthew J. Phillip and Eske Willerslev, "Geologically ancient DNA: fact or artefact?" TRENDS in Microbiology, Vol.13 No.5, May 2005 ( Link )
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by William A. Hoesch, M.S. *
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