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Once we have granted that any physical theory is essentially only a model for the world of experience, we must renounce all hope of finding anything like the correct theory... simply because the totality of experience is never accessible to us.
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Is there some time scale and levels that you can provide? My guess is that at the time scales involved with the evolution of the Earth, that these factors wouldn't be that large.
Plus; what about the levels as related to the Sun's emissions, and how much intersteller stuff actually makes it past the Heliopause.

I don't know the answers myself, but these are things that I would question.

And, I think that the Drake equation covers your concerns because it represents a snapshot in time, and the number of planets cabable of sustaining life or fraction that evolves life can include the fraction of time that a planet is in a "safety" or "habitable" period.

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[quote=NEOWatcher;1131202]Is there some time scale and levels that you can provide? QUOTE]

I've read that distant galaxies have "active" cores with relation to energy they emit. These galaxies are at an earlier stage of development (relativistically speaking). This I've interpreted to mean that our galaxy once also had an "active" core as well.

http://www.tufts.edu/as/wright_cente.../fr_1_gal.html

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Once we have granted that any physical theory is essentially only a model for the world of experience, we must renounce all hope of finding anything like the correct theory... simply because the totality of experience is never accessible to us.
Hugh Everett

The truest acts of scientific brilliance come from those who push the envelope of conventional wisdom

The answer is pretty much no. Life on earth exists without a apparent problem in places where the background radiation is hundreds to a thousand times higher than average. Since cosmic interstellar radiation is only a small portion of the total radiation life recieves on earth, radiation from space would have to be whoppingly huge compared to now to have an effect. Then there's the fact that life might be quite happy develping under a rock, or under an ocean or beneath an icecap where it would be well protected. We also know life can develop resistance to radiation.

Pass the pipe. There is no evidence earth has ever suffered from blasts of radiation emanating from outside the solar system. We have craters, iridium layers, and crackpots, but no credible evidence supporting your extraordinary claim.

I don't think an actual claim was made.
Augustus Vox asked a question.

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There is some evidence that the Earth has suffered from an event outside the solar system, actually;
http://www.lbl.gov/Science-Articles/...tinction.html#
but is that evidence sufficiently compelling? I'm not entirely convinced.

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