We've all heard the story about what we should expect after a nuclear holocaust; everything on Earth obliterated except for cockroaches and Twinkies. What life that may have survived the detonation of nuclear bombs would surely perish due to the ill effects induced by overexposure to radiation. We used to think that the negative effects were so profound and effective that we utilized the process of irradiation to sterilize equipment. The recent discovery of a radiation resistant extremophile, Deinococcus radiodurans, drastically changed our understanding of exposure to radiation as a limiting factor on life.
Radiation is a term we use colloquially. When we talk about the effects of radiation on life, we are talking about radiation in the form of particles (like protons or alpha particles) or as electromagnetic~radiation (i.e. gamma rays, UV light, radio, etc.). Radiation strong enough to be designated as "extreme" doesn't happen often from natural sources on Earth. When exposed to high levels of radiation, the consequences on life can range from damage to nucleic acids (like DNA) to disruption of photosynthesis.
Some extremophiles, like D. radiodurans, can withstand doses of radiation up to 100 times the lethal dose for humans. These organisms do not combat the ill effects of radiation directly. Instead, they have developed mechanisms such as DNA repair that can work to undo the results of overexposure. These mechanisms are highly effective. For instance, D. radiodurans has been found living in the water core of nuclear reactors. Because they have adaped so well, such organisms do not "endure" or "survive" these environment, they thrive in them.
The discovery of radiation resistant organisms has expanded our thoughts about the possible origins of life. Some scientists have contended that life could have originated on another planetary body, like Mars for instance. Impact with Mars could have ejected life-containing debris that would eventually make its way to Earth, subsequently seeding our planet with life. Skeptics argue that life in transport would be highly exposed to cosmic~rays and radiation from the Sun and therefore would not be able to survive the long trip from one planet to another. The discovery of highly efficient DNA repair mechanisms within cells that could combat the exposure to radiation involved in such an event have lent support to the idea that life did not originate on Earth, but rather Earth was seeded with life from elsewhere.
DNA repair mechanisms like those found in D. radiodurans could have been a vital step in evolution of life on Earth. Conditions on early Earth prior to the development of a protective ozone layer would have been hostile to life. Damage due to UV radiation would have been of utmost concern to the first life forms. Understanding the underpinnings of DNA repair in today's living specimens may eventually shed some light on the origin and evolution of life on Earth and elsewhere.