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,
- How does the star system (that hosts the planet) survive or protect its planets of those dangerous radiation sources?
- 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?
- 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:
- RNA life at first,
- Cellular life,
- There would be a Great Oxygenation Period to fill the atmosphere with oxygen,
- 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.