Ancient DNA: How Old is Old?

Hidden deep beneath the planet Earth's frozen glaciers, lie worlds hidden from our gaze. The ancient remains of flora and fauna and the DNA locked inside can reveal much about our planet's history and also its future. To get to these worlds involves the drilling of massive ice cores. The ancient DNA from Greenland was found embedded in mud at the bottom of a 2km ice core. The oldest samples are estimated to be around 800,000 years old and they eclipse the previous record holder - 400,000 year old ancient DNA samples, that included the genetic material of woolly mammoths.

The DNA structure of the Greenland samples was in such a good shape that scientists from Copenhagen University, led by Eske Willerslev, were able to extract genetic sequences of ancients plants and insects.

The ancient DNA was dated by using several different methods.

Mitochondrial DNA and amino acids change in a regular way over time. They are molecular clocks that give scientists a strong indication of the age of the biological samples. By using both these methods scientists dated insect DNA as being 450,000 years old.

Two other methods of dating were used. One examined the light given off by the minerals inside dust particles that were found on the ice cores and frozen into place; and the other was radioactive dating of the ice.

The ancient DNA sequences that the scientists were able to study were found to be related to beetles, flies, butterflies, moths and spiders. The ancient plant DNA revealed tree species such as alder, spruce and yew and several herbaceous plants.

To protect the integrity of the ancient DNA samples and prevent contamination, the scientists covered the surface of the frozen cores with plasmid DNA. Polymerase Chain Reaction (PCR) products of this plasmid DNA were only obtained from scrapings off the surface outside the core and not inside, showing that contamination had not taken place.

The interpretation given to the findings of the world's oldest DNA samples was that Greenland had been a green and forested area within the last million years; providing a different picture to the barren wasteland that was previously envisioned.

Ancient Biomolecules from Deep Ice Cores Reveal a Forested Southern Greenland by Eske Willerslev et al. Science, 6 July 2007: Vol. 317.no.5834, pp.111-114

http://www.sciencemag.org/cgi/content/full/317/5834/111#TBL1