Fossil DNA Best Preserved in the Ground

Jan. 11, 2007 — The best way to preserve fossil DNA is to follow nature's lead and leave the bones buried in the ground, according to a study that compared "fresh" and "old" fossils.

As the only remains of extinct species, fossils can provide a wealth of information if their DNA can be retrieved.

However, the most easily accessible samples, those stored in museums after standard conservation procedures, including handling, washing, brushing and storing at room temperature, yield little or no DNA, a French experimental study has concluded.

Eva-Maria Geigl of the Institute Jacques Monot in Paris, France, and colleagues investigated the preservation of ancient DNA under three different situations.

First they compared freshly excavated fossils to 247 preserved herbivore bones that were up to 50,000 years old and originated from 60 different archeological sites in Europe, the Near and Middle East and the Arabian peninsula.

"Freshly excavated and non-treated, unwashed bones contained six times more DNA and yielded twice as many authentic DNA sequences as bones treated with standard procedures," the researchers wrote in the current issue of the Proceedings of the National Academy of Sciences.

To narrow the field to more similar situations, Geigl and colleagues analyzed two different fossil bone groups from the same Neolithic site in Turkey. In one group, the bones had been brushed with water, dried and stored in collections; in the other, the fossils were recently excavated and kept under conditions similar to natural burial.

"The difference in the success rate was striking: it was possible to amplify DNA from five out of eight 'fresh' fossil bones as opposed to none out of eleven 'old' fossil bones," the researchers said.

Geigl obtained final and conclusive evidence from the analysis of 3,200-year-old fossil ribs belonging to an aurochs, an extinct cattle species.

The bones were dug up at a site in France at two different times. Those dug in 1947 had been stored in collections of the natural history museum of Le Mans, while bones uncovered in 2004 were stored according to strict protocols at -4 degrees Fahrenheit (–20°C).

None of the washed, museum-stored rib samples collected in 1947 produced DNA, but all of the recently excavated bones yielded authentic aurochs sequences.

"As much amplifiable DNA was lost during the 57 years when the aurochs bones were stored in a collection as during the previous 3,200 years of burial," the researchers concluded.

According Geigl and colleagues, the study calls for a revision of fossil conservation procedures.

"Our results show very clearly how detrimental standard treatments are to the preservation of DNA in fossil bones," Geigl said.

Most scientists who study ancient DNA know that the sooner DNA is extracted from a fresh fossil, the better the chances something useful is retrieved, said Ross MacPhee of the American Museum of Natural History in New York.

But the French research has revealed that the best practice is not widespread among fossil hunters and museum curators.

"Good conservation practices should not to add to the damage by just shoving bones into museum drawers at ambient temperature and humidity," MacPhee told Discovery News. "I am convinced that the very best practice is to place the fossils in the most inert environment...freezing is the easiest environment to use."