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Scientists recently reported they have found chemical markers of DNA in dinosaur fossils. This claim has stirred up a storm of controversy because dinosaurs are claimed to be millions of years old, much too old for fragile DNA to survive. Not only DNA, but a variety of paleomolecules have been reported in fossils, raising questions about how this could be possible. This short article will give a brief description of some of the issues.
Paleomolecules are organic molecules that have been preserved in their original biochemical state after the organism has died. They include lipids, proteins and DNA (the molecule that carries biological information in the cells). When organisms die, cells decay, due in part to the activity of the enzymes and other chemicals inside the cell. Bacteria also cause decay of the tissue and their molecular constituents. The rate of decay depends on the type of organic molecules and on physical-chemical factors such as humidity, oxygen, temperature, acidity (pH) and the effects of radioactivity. In general, decay is faster when the molecule is exposed to water and oxygen, which are usually abundant in the environment. In optimal conditions, some organic molecules are expected to last for tens to thousands of years in materials like bones, wood and bacteria, but eventually, the organic bonds that hold the atoms together become unstable and the lipids, proteins and DNA break down completely.
Lipids with double (C=C) bonds tend to break down rapidly and become rancid. Proteins tend to be destroyed quickly by microbial attack (bacteria and fungi), hydrolysis (splitting caused by water), low pH (high acidity), and denaturing at high temperatures. DNA is a notoriously unstable molecule under normal exposure to water and air. Therefore, preservation of organic molecules in a multi-million year bone, shell or piece of wood is not expected.
However, in recent years, several reports have been made of preservation of biomolecules in fossils. Unsaturated fats have been found intact in ancient bones and other fossils allegedly hundreds of millions of years old in the uniformitarian geological timeframe. Many reports have been published of ancient proteins of up to a supposed 160 million years in age. These include collagen (the most common), osteocalcin in vertebrate samples, and albumin, among others. The ancient molecules retrieved from fossils have resulted in controversy over how long a biomolecule can last in nature.
Claims of ancient DNA are especially interesting, because if they are valid they might open up the possibility of comparing DNA sequences of various extinct types of organisms. However, the conventional view of the fragility of the DNA molecule has led many scientists to greet all claims of fossil DNA with strong skepticism. A recent report has stirred up more controversy by reporting detection of chemical markers of DNA in calcified cartilage from the bones of a Cretaceous dinosaur named Hypacrosaurus.
Hypacrosaurus is a member of the duckbill group, which could walk on two or four legs and ate vegetation. It was a large dinosaur, almost as big as the famous T. rex. It grew to about 30 feet (9 meters) in length, with an estimated weight similar to that of an Asian elephant (4 metric tons, 8,000 – 9,000 pounds). A concentration of individuals, including adults, juveniles and dozens of nestlings were discovered in the Two Medicine Formation of Montana in the 1980s. The bones for this study came from the skulls of two of the nestlings.
As the skull grows, cartilage is replaced by bone tissue. Growth occurs at the junction of the cartilage and the bone tissue, where cells are actively dividing, and bony tissue is replacing the cartilage. This area of bone growth was the location where DNA chemical markers were found in the study reported here. Microscopic study had shown what appeared to be cellular structure, with dark centers suggesting the possible presence of a nucleus. Follow-up studies confirmed the presence of the protein collagen, and chromosome-like structures with chemical markers of DNA. It is not clear what the state of preservation is for the DNA, but the material responds to DNA-specific chemical tests.
DNA has been reported in many fossil bones and plants but the scientific community has greeted most such reports with skepticim. Contamination with modern DNA is a major problem in these studies. However, denial of the validity of fossil DNA is becoming more difficult as researchers have responded to the challenges of their conclusions by refining their methods and establishing extraordinarily strict experimental protocols.
It seems clear from studies of modern DNA that it should not last long enough to remain in dinosaur fossils. DNA, like all biomolecules, is subject to breakage by the action of water and warm temperatures. Radioactive particles might also damage the chemical bonds. It seems difficult to imagine a bone embedded in soil or sedimentary rocks for tens of millions of years without its DNA being exposed to both water and high temperatures. From these considerations, it is no surprise that many scientists refused at first to believe the reports of biomolecules in dinosaurs, all of which are claimed to be older than 65 million years.
There is still some opposition to some of these claims, but most researchers have come to accept that paleomolecules are indeed being found in fossils from all parts of the geologic column, sparking a search for the mechanisms of preservation. Yet the very fact of preservation of highly degradable proteins and DNA in fossils allegedly ten to hundreds of millions of years of age stretches the imagination beyond what many scientists are able to imagine or explain. Survival of these biomolecules for a few thousand years seem much less of a problem than survival for scores of millions of years.
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