seemingly utilizing arsenic rather than phosphorus (emphasis on seemingly).
As with the other questions there are some qualifications for Number 4:
How does life originate?
Strictly speaking the slightly modified version of this question that is really under the microscope is: How did life originate on Earth? However astrobiology has to set its sights high because the origin of life here may or may not represent the same type of origin as life out there. It's also true that one of the biggest scientific motivations for astrobiology is that in order to understand our own origins we desperately need a proper context. One planet with life, even if hugely complex and with a 3-4 billion year history, is still just one sample of a phenomenon. Seeing how this phenomenon operates elsewhere in the universe would help answer all those nagging issues of probability, bio-chemical uniqueness, convergent evolution, and so on.
All signs point towards the existence of microbial life on Earth somewhere in the time-slot of 3.5 to 3.8 billion years ago - mainly from the fingerprints of ancient stromatolite rock structures, products of bacteria and archaea colonies. We really don't know how far back it all goes, owing to the dearth of unaltered continental crust from these times. We especially don't really know what was going on during and before the Late Heavy Bombardment - a period of massive pounding by asteroidal and cometary material about 4.1 to 3.8 billion years ago. It probably also doesn't make sense to be seeking a single 'moment' at which life sprung up and started tweeting. As a phenomenon of self-organizing, information carrying, reproducing, eating, pooping stuff, life in truth represents many interleaved networks and systems.
The bottom line is that right now we have no single clean answer to this question. Many, many hypotheses exist. These range from having life delivered pre-formed from other planets or star systems (panspermia) - which rather avoids the question - to an array of so-called 'pre-biotic' chemical scenarios that lead to the basic biotic molecules that we see today, or at least variants thereof. These include origins in atmospheric chemistry, deep sea vents, clay lattices, and polycyclic aromatic hydrocarbons to name but a few. Then there are ideas that seek to tackle a possible transition from molecules to real, recognizable bio-structures - like cell membranes and catalyzing molecules. Coming from the other direction people have studied the potential ancestral forms of modern DNA. Since RNA molecules can serve as both information storage and enzymes (a role generally taken by proteins in modern biochemistry) then perhaps there was an 'RNA-world' before there was a DNA-world, a simpler, much more fluid and information-loose biology that eventually hit upon the more robust 'hard-drive' data storage offered by the DNA molecular structure.
In short - it's a tough problem. No one suggestion has yet had the ring of completeness, and it may be that there simply is no single mechanism, but rather a parallel series of critical steps and components that have to have a few tens of millions of years to cook the right pie. The environment of the kitchen probably plays a big role. Small rocky planets, with a nice rich chemical wash, and both stellar and geophysical energy input may well provide one of the most conducive environments in the universe...as far as we can tell.
An intriguing avenue of investigation may be coming around the horizon with the advent of artificial biological organisms - this type of reverse-engineering might just provide some unexpected clues. Similarly, as we better understand microbial and viral ecology there may be pointers to fundamental rules that haven't quite come into focus yet. In truth, it sure would be handy to have another biosphere to compare against...