At first blush, the headlines sound like something out of science fiction: "Scientists recreate ancient ecosystem by studying dirt!" But it's real. The dirt in question, 41 sediment cores taken by Danish paleontologists in 2-million-year-old northern Greenland permafrost in 2005, turned out to be a treasure chest of DNA. They didn't know that back then — the technology wasn't up to the task of teasing out that information — but on an inspired hunch, they stored the cores in a basement freezer at the University of Copenhagen while the extraction methods and the sequencing machines improved incrementally. Every couple of years, they tried, unsuccessfully, to extract DNA from the Greenland samples.
Finally, following a major upgrade in their equipment in 2017, they triumphed. After sifting and sampling some 16 billion "reads" (short fragments of DNA), they were able to identify traces of hundreds of animals and plants, many of which are now extinct. The trick was to match the reads — most as short as 50 base pairs — against entire known genomes, which typically run to 1 billion or so base pairs. Amazingly, that's enough to pinpoint which plant or animal the read came from.
Until now, the oldest known DNA (present in every cell of a living organism) was from a 1.2-million-year-old molar from a mastodon. That was exceptional. Enzymes usually break down DNA within a few hundred thousand years but, in the case of the Greenland cores, DNA — from sloughed off skin, poop or rotting leaves, for instance — had bound electrostatically to minerals in the soil. (DNA has a weak electric charge, as do grains of clay and quartz in the sediment.) The tell-tale fragments of DNA were thus protected over the millennia from decomposition. Ironically, this same electrostatic attraction made the researchers' task more challenging, requiring innovative techniques to unbind the precious reads from the surrounding sediment.
Using the new technology, Danish scientists have been able to identify more than 100 kinds of plants (many of which had only been found previously as fossils), proving that today's polar desert was once a forested estuary dominated by birch and poplar trees together with sedges, horsetails, willows and spruce. Several of the still extant plants no longer grow in Greenland, indicating that the climate back then was warmer, although still subject to the same 24/7 darkness for half the year as now. Not only plants: The researchers have now detected DNA from several dozen species of living or extinct animals, including hares, reindeer, horseshoe crabs, rodents related to modern lemmings, precursors of today's geese and, incredibly, mastodons, extinct relatives of elephants.
It's hard to underestimate the epic nature of these findings on the field of paleontology. We now know it's possible, given the right circumstances — in this case, suitable minerals for the DNA to bind to, plus a cold climate — to develop a picture of an entire ancient ecosystem. The very limit for DNA survival is probably about 4 million years, a quantum jump from the few hundred thousand years to which researchers have previously been limited. In the future, university courses for budding paleontologists will no doubt include the arcana of microbiology, including DNA recovery and sequencing, as this new science transforms our knowledge of the past.
Barry Evans (he/him, firstname.lastname@example.org) was bedazzled by artist Beth Zaiken's rendering of mastodons in Greenland. This story is the result.