Ancient Penguin, Bear DNA Reveal Pace of Evolution and Extinctions
By Bijal P. Trivedi
National Geographic Today
|March 22, 2002|
Ancient frozen bones excavated from Antarctica, northeast Siberia, the
Yukon, and Alaska have yielded strikingly well preserved DNA that
scientists are using to calculate how quickly life evolves and how it
may have responded to climate change during the last ice age.
recent report comparing DNA from ancient and modern penguins suggests
that evolution may proceed much faster than previously calculated for
penguins, and possibly other species. Another analysis of DNA from brown
bearssome up to 60,000 year-oldrevealed mysterious
extinctions and sudden appearances of new populations.
Both reports are published in the March 22 issue of the journal Science.
Researchers led by David Lambert of Massey University in Palmerston North, New Zealand, collected DNA samples from 7,000-year-old nesting grounds of the Adélie penguin in Antarctica. Underneath the existing colonies lie layers of ancient frozen, fossilized penguin bones with DNA held suspended in time in the cold, dry conditions.
Comparing DNA from 96 penguin bonessome nearly 6,500 years oldwith DNA from blood samples taken from 380 living Adélie penguins, Lambert and his colleagues calculated their evolution rate at between two and seven times faster than previous estimates.
"What is most surprising about Lambert's study," said evolutionary biologist Robert Wayne of the University of CaliforniaLos Angeles, "is that the evolutionary rate seems so high." Wayne cautioned that it is too early to say whether this rate applies to other species.
Lambert's team focused on DNA found in a special cell compartment called the mitochondria. Mitochondrial DNA is inherited directly from the mother without interacting with male DNA. "Mitochondrial DNA is particularly useful for constructing family trees and for studying the rate of evolution because mutations are easy to detect," said Lambert.
Lambert excavated bones from different depths, comparing DNA from each millennium with that in modern bones. This continuous DNA record enabled Lambert to directly measure the bird's evolution through time.
What distinguishes Lambert's study is that is "a completely new approach," said Wayne. Previous efforts to calculate the rate of evolution relied on either very few time points or rare fossil records. Some approaches have extrapolated evolutionary rates over millions of years based on genetic differences between just a couple of generations.
Using the faster rate of evolution, Lambert has suggested that two groups of Adélie penguinsthe Antarctica lineage and the Ross Sea lineagethat differ genetically by about eight percent evolved these differences over only 60,000 years. Previous estimates suggested that this change would have taken at least 200,000 years.
Over the last 100 years, evolutionary biologists have tried to create a family tree showing the relationships between all living forms and to put a time scale on this tree, said Lambert. "Our study reveals that life may be evolving much faster than we anticipated," he said.
Lambert plans to use similar methodology to determine the evolutionary rate for the kiwi bird, a lizard called the tuatar, and a species of Antarctic fish.
Mysterious Bear Extinctions
A study led by Alan Cooper of England's University of Oxford used the genetic records of brown bears from BeringiaSiberia and North America and the connected by a land bridge that existed until 11,000 years agoto determine the effects of climate change on different bear populations.
Cooper's team analyzed genetic changes in 36 brown bear bones, some excavated on field expeditions and others gathered from collections in New York's American Museum of Natural History and the Canadian Museum of Nature in Ottawa.
"If we look at animals today, we see many sub-species, but this doesn't tell us how long these various groups have been around or how long previous groups, now extinct, lasted," said evolutionary biologist Adrian Lister of University College in London.
But with the new DNA evidence, "they are looking directly back into the past and can actually see the movements of different populations," Lister explained.
The analysis revealed "quite a bizarre population history" of the brown bear, said molecular paleontologist Ian Barnes of the University of Oxford, who conducted much of the study.
Insight Into Origin of New Species
The ancient distribution of bears was quite different from now. A type of brown bear that was common near Fairbanks, Alaska, became extinct about 35,000 years ago. "There are no modern bears with these genetic characteristics," said Barnes. The team found no record of brown bears in Beringia between 35,000 and 21,000 years ago.
What is most surprising to Barnes is that the population trends don't seem to correlate with climatic conditions. The last ice age lasted from approximately 90,000 to 10,000 years ago, although there was a slight warming between 35,000 and 21,000 years ago.
"If anything," said Barnes, "we would have predicted an extinction between 21,000 and 16,000 years ago," when the size and thickness of the glacier covering most of Canada and some of the northern United States peaked and when food may have been most scarce.
But the extinction of the Fairbanks bear about 35,000 years ago strikes Barnes as odd, as this suggests a warmer period.
Equally surprising, said Barnes, is the reappearance of brown bears about 21,000 years ago, and the appearance of a new, genetically distinct group of bears near Fairbanks that still exist today.
One explanation is that the mysterious disappearance of the brown bear may not have been the result of climate change but of a fearsome competitorthe three-meter-tall, carnivorous short-faced bearwhich seems to have dominated the region during that period. When the short-faced bear became extinct in Alaska and Canada about 21,000 years ago, the brown bears seemed to reappear and fill that niche.
Lister said Barnes' study "helps us understand the origin of new species" by revealing the genetic and geographical history of brown bears over the last 60,000 years.
Both Lambert and Barnes hope to probe the inhospitable icy reaches of the planet for more hidden genetic treasures that could hold the secrets of life past.
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