frozen in the oldest ice on Earth have been thawed out
and brought back to life in the laboratory, providing
new insights into how long living creatures can be frozen.
However the poor health of the thawed-out microbes has
led their discoveries to cast doubt on a notion long
cherished by some - that life on Earth arrived here
on comets from outside our solar system.
DNA in comets prone to damage by cosmic rays
|• Ice, rock
could protect microbes from radiation
After thawing ice from the mullins and Beacon valleys
in Antarctica, Kay Bidle, a microbiologist at Rutgers
University, New Jersey, and colleagues discovered life
in water from even the oldest samples - estimated to
be around 8 million years old.
"By examining microbes in such ancient ice, our study
significantly extends our understanding over which microbes
retain viability," he notes. However, while some bacteria
taken from 100,000-year-old ice reproduced quite readily,
cells from the oldest ice multiplied only very slowly
and their DNA was badly damaged. Studies of isolated
DNA form the samples showed that it has become increasingly
fragmented as time went on.
By analyzing samples of ice varying form 100,000 years
to 8 million years, Bidles and colleagues calculated
a 'DNA half-life;' the length of DNA fragments in the
ice halves every 1.1 million years.
This fragmentation is consistent with the idea that
the DNA is being damaged by cosmic rays. Bidle and his
colleagues believe this half-life makes it highly unlikely
that life on Earth was carried here on comets from outside
our solar system.
"If you take the speed of a comet and take the distance
it would need to travel it would take longer than 8
million years to do that. In a comet the DNA would be
completely deteriorated," says Bidle However not all
experts are convinced this work proves life was not
carried to Earth by comets from outside our solar system.
"I have to say I don't understand how that conclusion
is drawn from the observation related to the Beacon
Valley ice." Says Richard Hoover, an astrobiologist
at NASA's Marshall Space Flight Center.
Ice and rock could protect microbes deep inside comets
from radiation, he says. Bidle and his team also point
out implications of their work on Earth in terms of
increasing the amount of genetic diversity available
at the end of ice ages. Ice sheets in the process of
melting could provide a Kind of 'gene popsicle' for
other microorganisms that could acquire the newly defrosted
DNA and incorporate it into their genomes.
"Given the widespread influence of lateral gene
transfer within microbial populations, one can envision
periods in Earth's history when large numbers of ancient
when large numbers of ancient genes became available
as ice sheets melted," they write in the proceedings
of the National Academy of Sciences.
In a parallel development,
a team of scientists has succeeded in cultivating microbes
from 8-million year-old samples of Antarctica permafrost
from the same Beacon Valley location as Bidle's ice.
David Gilishinsky, a geocryologist at the Russian Academy
of Sciences, Moscow, and his colleagues found diverse
populations of the oldest microorganisms discovered
in permafrost to date.
And while Bidle thinks he has found life cannot have
come to Earth from beyond our solar system, Gilichinsky
and his colleagues offer some succour for those enthusiastic
about extraterrestrial life closer to home.
"Such studies could help define the limit of dormancy
of frozen life on Earth and in ancient permafrost on
other planets and, potentially, provide a model for
Martian ecosystems," they write in their paper