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
samples: 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.
genetic diversity: 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
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.
model: "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 in Astrobiology.