DNA gleaned from ancient coral unlocks clues about warming

Samples of Scleractinian coral were among the skeletons researchers found on the New England Seamounts. The samples showed how the creatures were affected by climate changes. (Tim Shank/Woods Hole Oceanographic Institution)

The skeletons in the Earth's closet reveal not only a dark past. They also cast a light on its future.

That is what Tim Shank discovered when he sent an underwater robot to sweep up a basket full of broccoli-like fossils from volcanoes under the sea. Shank, a researcher at Woods Hole Oceanographic Institution, found skeletons of 35,000- to 40,000-year-old corals littered on the New England Seamounts in the North Atlantic. He took them back to his laboratory, extracted what he could of their remaining DNA fragments, and started to piece together their past.

Their story - and similar ones gleaned from the DNA of ancient spiders entombed in ice cores, and the bones of dodos and woolly mammoths - tells us how ancient creatures survived or disappeared as a result of dramatic climate changes. That, in turn, provides a preview of how today's flora and fauna might react to global warming.

These kind of "ancient DNA" studies have become possible in the past few years, and from them researchers have been able to piece together not just when and where long-gone plants and animals lived, but how species adapted, migrated, or perished during periods of melting glaciers and warming temperatures. When scientists recently announced that they had decoded most of the woolly mammoth's genome, it was another sign of dramatic progress in sequencing the DNA of extinct and dead organisms.

"Ten or 12 years ago, maybe even five years ago, this would have been science fiction," said Ross MacPhee, curator of vertebrate zoology at New York's American Museum of Natural History. "This is a totally brave new world that's been actualized by modern genomic sequences."

It was three years ago that Shank began his coral project. Since then, he and his team have found that the cold-water corals, many as vibrant as their tropical counterparts, did not do as well when ocean temperatures rose and currents changed during warmer periods in the planet's past. "In some cases, they went deeper; in others they had massive die-offs," he said.

The deep-sea corals represent the fate of a wealth of marine life. Commercial fish species cluster around them and shift their populations as the corals do. As the Atlantic Ocean heats up as a result of the current global warming trend, Shank worries that fish populations will decline or migrate. Some species, he said, may even go extinct.

The stories revealed by ancient DNA are not all dismal. Genetic study of polar bear fossils predicts a better fate for the endangered species.

Before 2007, it was thought that polar bears evolved from the brown bear fairly recently in the planet's history. But three years ago, a DNA study of polar bear fossils showed that the polar bear originated nearly 130,000 years ago. That means the species survived the last interglacial period of global warming.

The discovery gives MacPhee hope about the polar bear's fate in the coming decades. "It's possible they will survive, even though they will suffer," he said.

Other studies have revealed that climate played a bigger role in past extinctions than once thought. Ancient DNA showed that bison populations, for example, lost nearly all their genetic diversity about 37,000 years ago - well before humans were hunting their tracks.

"It tells us that the environment has a significant impact," said Beth Shapiro, one of the authors of the bison study and a leading researcher in the field. "Humans may not have started the extinction, but they may have ultimately caused it."

The first recognized ancient DNA study was done in the early 1980s, when scientists pulled a short segment of DNA from the preserved skin and teeth of a quagga, an extinct zebra from southern Africa. "It showed that it was possible to extract DNA from dead stuff, which people were excited about," said Shapiro, also an evolutionary biology professor at Pennsylvania State University. Only in the past three years or so, however, have scientists been able to study how entire populations of extinct and dead animals changed genetically in the past.

"All of this is really in its early stages," Shapiro said, noting that technical and statistical methods had to catch up to the fervor to decode the DNA of extinct animals.

Ancient DNA is, for obvious reasons, not easy to come by, and researchers are under increasing pressure to prove the authenticity of their specimens. One reason is that it is painstaking work to keep ancient DNA samples free of contamination by human hair and skin cells while studying it.

Another is that some scientists have not shown proper caution. In the mid-1990s, after the release of the dystopic novel and movie "Jurassic Park" in which fictional scientists used DNA preserved in amber-encased mosquitoes to create dinosaurs, some researchers said they isolated dinosaur DNA. The assertion was discredited when the DNA was discovered to be human.

Researchers have yet to find well-preserved DNA fragments for specimen older than a few hundred thousand years. And even if they could find a few DNA fragments for a Tyrannosaurus Rex, it is highly unlikely that they could decode the entire genome or replicate genes that govern the growth process from egg to adult.

Even without such a showstopper, ancient DNA study is illuminating our understanding of the past - and holds tremendous promise for helping us understand the impact of global warming.

"We can turn the page back in history," said Shank, at Woods Hole. "But the real goal is to say: 'What's going to happen the next time?' "

Venkataraman can be reached at bvenkataraman@globe.com.