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Following this question, I had additional question of habitability. Assuming that life could exist on planet formed roughly twelve billion years ago, allowing complex life to exist as of seven billion years ago, what would be the most plausible explanation of how they survived from:

Given the much, much higher gas densities of the era, I'd be far, far more worried about getting blasted apart by the numerous supernovas popping all over the place, and black hole radiation blasts from the super-massive central black hole and actively feeding nearby black holes. Each of these might be sufficient on its own to sterilize areas ranging from dozens of light years for a supernova to perhaps a whole galaxy for the central super-massive. Plus rare events like a triple neutron star collision, etc. - Serban Tanasa's answer

To keep the question in narrow term,

  1. How does the star system (that hosts the planet) survive or protect its planets of those dangerous radiation sources?
  2. If the above condition is impossible to meet, how could a planet protects its life from such radiation? 2.1 Is it possible for life to evolve underground during the early era, and later (at seven billion years ago) populate the surface? 2.2 Is life in an ocean environment similarly affected by the radiation? Or could they survive?
  3. And finally, if all of the above requirements were impossible, could life itself have evolved in a way that that could resist radiation?

Answers do not have to cover all of the questions above, you should just move to next questions if previous question(s) were deemed impossible.

I'm expecting that the evolution of life on that planet follows similar path as of earthen life, like:

  1. RNA life at first,
  2. Cellular life,
  3. There would be a Great Oxygenation Period to fill the atmosphere with oxygen,
  4. And complex life evolved a a later time (similar to Earth's Cambrian period)

Of course it doesn't have to be on that sequence, and the answer could be more flexible.

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Couple ways it could work:

Magnetic fields
The planet could have a really strong magnetic field. Earths magnetic field protects us from a nuclear furnace that's relatively close by.
Likewise, the sun has a magnetic field which would offer some protection.

Water
Some theories say that early earth was protected from radiation by a shield of water. Being deep underwater for the first billion years or so would also help. Maybe forming around nutrient rich underwater volcanos.

Nebula/Molecular clouds
Gasses could absorb some radiation. A stellar nursery is a molecular cloud where stars are forming, possibly with a planet.

Boring neighborhood
By shear chance the star forms in the most boring part of the galaxy, where nothing much is happening. Considering how many things have to not go wrong for evolution be possible, it seems like a miracle that our world isn't a lifeless rock.

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  • $\begingroup$ I think "boring neighborhood" is a key point. There are lots of boring neighborhoods, even between galaxies. Stars in any of those neighborhoods could easily have planets that wouldn't suffer from the general business of the universe. $\endgroup$
    – user3294068
    Mar 26, 2015 at 21:32
  • $\begingroup$ That's exactly what I thought previously, given sufficiently deep waters, strong magnetic force, and considerably boring neighborhoods. But is it likely to have M-class star at that age on that "boring neighborhoods"? $\endgroup$
    – Hendrik Lie
    Mar 27, 2015 at 3:16
  • $\begingroup$ @HendrikLie Nothing I know says that it couldn't happen, in fact the presence of red dwarf stars is one of the ways that they determine the age of a star cluster. Red dwarfs (G class) would still be burning while yellow dwarfs (M class, like our sun) would have already expanded to a red giant. I'm assuming you chose G class because they have a much longer lifespan than M class, and would still be around after 12 billion years. Someone that knows the astronomy will probably tell me I don't know very much about stars, and they wouldn't be wrong. $\endgroup$
    – AndyD273
    Mar 27, 2015 at 14:27
  • $\begingroup$ @AndyD273 actually, our star is G-class, and M-class is red dwarf :) $\endgroup$
    – Hendrik Lie
    Mar 28, 2015 at 3:16
  • $\begingroup$ Okay, I picked it as the best answer. $\endgroup$
    – Hendrik Lie
    Mar 28, 2015 at 15:32
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My initial answer suggests nothing more than a statistical likelihood. In a young universe supernovas were a lot more frequent. That makes it more likely that any particular world would be close enough to a supernova to risk being sterilized.

It does not mean that all worlds in all galaxies were all constantly sterilized. Perhaps a few (or a lot) escaped unscathed.

Earth life evolved in the oceans, as far as we can tell, the ground we call home was colonized far later. Water does provide significant shielding against radiation. However, it does not provide complete protection. Gamma radiation from supernovas has been proposed as the cause of the end Ordovician extinction, which resulted in the death of nearly 60% of the oceanic life on Earth.

Moreover, once established, life can adapt. There are microorganisms on Earth with amazing resistance to radiation, such as Deinococcus radiodurans, which can survive not only radiation but also cold, dehydration, vacuum, and acid.

As a final real-world consideration, one interpretation of the Fermi Paradox is that conditions were adverse to intelligent life until the recent past, so that we are alone as sentient beings in the galaxy. It is not the sole interpretation, of course.

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  • $\begingroup$ Hmm, perhaps they would experience more frequent mass extinction event than us $\endgroup$
    – Hendrik Lie
    Mar 27, 2015 at 3:18
  • $\begingroup$ Or perhaps intelligent life emerged sufficiently long enough in the past that they perished in the moment of our arrival (in magnitude of billion years) $\endgroup$
    – Hendrik Lie
    Mar 27, 2015 at 3:26
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There are stars in the intergalactic space, if sparse, and we know of no reason for them not to have planets. Of course that means extreme isolation from other systems, no dreams of "flights to Proxima Centauri". Very little hope for interstellar escape when their star goes red giant. But they would be safe from all dangerous neighbors like supernovae and black holes.

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  • $\begingroup$ How if the host star were M-class star? Wouldn't they survive old enough to sustain sufficiently advanced civilization? $\endgroup$
    – Hendrik Lie
    Mar 27, 2015 at 3:19
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    $\begingroup$ @HendrikLie: I have no clue. I mean: we still have about a billion years until the Sun goes too hot for us, but we can't reasonably foresee two decades of civilizational advancements. So how are we to foresee if they develop, say, aftificial suns, planetary propulsion, leave their star and speed towards the nearest galaxy taking the rest of the planetary system with them, for resources... $\endgroup$
    – SF.
    Mar 27, 2015 at 8:35
  • $\begingroup$ How if the star were located on the outer tip of the galaxy? Wouldn't they became reasonably close to other stars but would also be quite safe from supernovas from the dense region of galaxy? $\endgroup$
    – Hendrik Lie
    Mar 28, 2015 at 3:44
  • $\begingroup$ @HendrikLie: I'm afraid not. OTOH the star may be travelling through the space towards the galaxy and enter its outskirts about the time when distant space travel becomes a necessity for the populace (and the universe stabilized enough not to mean an immediate threat). $\endgroup$
    – SF.
    Mar 29, 2015 at 7:23

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