Scientists have thawed samples of bacteria that were frozen in ice for up to 8,000,000 years in order to figure out whether these bacteria would still be viable and whether their DNA is intact. It turns out they are viable, but the longer they were in ice the more their DNA was fragmented. This has implications as to whether life traveled to Earth from a comet or was evolved on site:

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 from the samples showed that it had become increasingly fragmented as time went on.

By analysing samples of ice varying from 100,000 years to eight million years, Bidle 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 eight 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 in Huntsville, Alabama. 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 genes became available as ice sheets melted," they write in the Proceedings of the National Academy of Sciences.

It doesn't seem to preclude the transmission of life from other planets in our solar system.

I was also interested in the increase in genetic diversity after the ice ages. I don't know if that is just restricted to micro-organisms because if it isn't wouldn't you expect it to be the result of more ecological niches opening up.