As beams of sunlight trickle through the lattice of snow-dusted pine branches, a sleeping reindeer stirs. The buck rises slowly from its bed of pine needles, taking a moment to shake the snow from its tall antlers. It gazes out onto the sea of conifers at the foot of a magnificent glacier, blinks slowly, and begins trudging through the forest in search of some tasty shrubs. A flock of geese honk loudly as they fly in formation overhead. The buck moves beyond the tree line and into a sprawling expanse of tundra, ice, and snow as far as the eye can see. With no shrubs in sight, the buck turns to move back into greener pastures. Suddenly, it stops dead in its tracks. Towering above the reindeer stands a mastodon. The mastodon gazes down lazily at the buck, as if barely registering its presence. The buck, seeing the giant’s lack of concern, hurries back into the forest, feeling fortunate despite the lack of shrubs. Just another morning in Greenland, 2.1 million years ago.
Although we don’t have a way of knowing exactly what happened 2 million years ago, recent discoveries based on some of the oldest DNA evidence suggest it very well could have happened. Last December, Eske Willerslev, a geneticist and paleontologist at the University of Cambridge, and his colleagues were able to take samples of ancient permafrost from the Kap København Formation in Northern Greenland. Using a technique that Willerslev developed, the team extracted DNA that was present just in the environment, so-called environmental DNA or eDNA. eDNA comes from organisms that expired or otherwise shed their DNA around their homes. Fossils are rare finds in Greenland’s frozen desert, but DNA is everywhere. However, researchers have only recently developed the technique to free it intact from its icy prison. DNA tends to bind to silica surfaces in the presence of certain salts, so minerals within soil can sometimes have the right conditions for DNA preservation. The Kap København Formation, with its silica-rich sediment and freezing temperatures, is the perfect place to look for eDNA. Once extracted from the permafrost, the investigators then sequenced the DNA, and reconstructed the genomes of the plants and animals that lived in this ancient ecosystem by comparing them to modern flora and fauna.
The picture they painted was unprecedented. The records show a mixture of both open boreal and Arctic desert flora and fauna. Although you would probably recognize the reindeer, geese, confers, and even the mastodons identified as part of this community, the denizens of these chilly environments were not known to have lived together. If you happened to live in Northern Greenland 2.1 million years ago, bumping into a mastodon while looking for breakfast was plausible indeed.
Within the DNA record, the researchers also found the presence of marine animals, including horseshoe crabs and green algae. This discovery is particularly interesting, because these species could not survive in the present-day versions of these frozen ecosystems. Their presence in the DNA record suggests that the climate 2.1 million years ago was warmer than today by around 11–19 °C. These conditions imply a climate that resembles those forecasted under future warming trends. Understanding the implications of the evidence can give us an idea of what to expect for our ever warming planet, and pushes the boundaries of what paleobiology and DNA technology are capable of.
Although eDNA can tell us what organisms were present in a particular moment in time, it can’t tell us information about their anatomy and life history like fossils can. Additionally, all the vertebrates identified by DNA were herbivores, which may be a function of relative biomass rather than a true lack of carnivores. That bias may also be why the amount of plant evidence dominated that of animal evidence. The authors seem certain that the DNA they found originated from that area, and do not address the possibility that the DNA could have washed into the formation from another area before preservation. The ability of eDNA extraction to tell us the exact species that a given sample belonged to is also limited, as it relies on databases that can only assign membership of DNA material to their broader genetic family.
Despite these limitations, the team’s findings demonstrate the ability to track the ecology and evolution of entire communities in deep time. None of the investigators expected to find mastodons in Greenland, and the fact that they found evidence using new eDNA techniques points to an advantage over using fossil evidence alone. Little is known about the period of Earth’s history around 2.6 million years ago, but this new method has the potential to fill in the gaps of that knowledge on an ecosystem-wide scale.
As a bioinformatician, the fact that DNA could remain stable enough, under the right conditions, to sequence after 2 million years heightens my reverence of the molecule. These findings deepen my appreciation for DNA’s capability and transience, at once the complex essence of life and simply an orderly arrangement of chemicals. The simple nucleotide code of DNA serves as the blueprints for organisms in the present, and now, a story of life in the unrecorded past.
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