How is it that we (Earthlings only familiar with carbon-based life forms) can decide what other life forms would look like, or be made of?
The simple answer is that we don't decide, we speculate. A lot of science starts as speculative hypotheses. These hypothesis set a direction for research. Sometimes they turn out to be wrong (phlogiston and the lumeniferous ether are examples) and sometimes they turn out to be right. But even the wrong ones can provide a useful guide that eventually gets us moving in the right direction.
I the case of life, what is observed is the importance of proteins and their wide variety. It seems that amino acids are a bit like lego blocks that can be combined in a huge variety of different ways. Since this seems important to biological life, it is reasonable to look to other possible components with similar combining possibilities. This might turn out to be a mistaken idea, but at least it sets an initial direction for investigators to follow.
That's an example I considered. However, it isn't able to persist for long, except in unusual conditions. The problem is, that it is not able to modify its own behavior to increase persistence. For example, it can't reduce its rate of fuel consumption, so as to lengthen the time that the process exists. It's an example of selfishness (I take the term form Dawkins) carried to a self-destructive extreme.
Is a fire alive?
It is interesting that people do actually use that terminology. They will talk of live coals in a fire.
I doubt that biological life just popped into existence. Before biological life there had to be some earlier proto-life systems. Maybe we could put fire as one entry in that category. A proto-life system need not have been based on DNA. Some proto-life systems could have been more capable than fire, at modifying their own behavior. And those self-modifying systems could have evolved into biological life, discovering DNA along the way.
I could be approaching this from the wrong perspective or asking the wrong question ...
The received view is that we require an explanation of how the subjective arises from the objective. But that's backwards. The whole principle of empiricism is that knowledge arises from experience. Thus our objective knowledge arises out of the subjective. How this works has never been adequately explained, although George Berkeley made an attempt (admittedly a misguided one).
Eventually, science should close the circle. That is, we should have an account of how the objective arises from the subjective, but we should also have some understanding of how the subjective arises from what we view of the physical world. If we cannot close the circle, then we should suspect something is amiss in our understanding.
Still, the starting point has to be in how the objective arises from the subjective.
So if electrons say have some properties like mass, charge, spin that weren't all discovered at the same time perhaps they have have some other property that is the basis of this interior awareness, same goes for other particles, atoms, molecules, and forces like electromagnetism.
I don't see the need for special properties of objects (property dualism).
I see an organism as a gatherer of information about the world. The AI paradigm tends to assume a computation system that is a passive receiver of information, and the AI folk want to explain everything in terms of how that information is processed. But that seems mistaken to me. I see an organism as actively involved in collecting information, and even involved in deciding what to consider as information (defining information).
Electrons and the like are mere carriers of information. Information, itself, is an abstraction, and thus has no mass and no energy. Representing that information requires carriers, and we would expect the carriers to have mass and energy. But it is the information itself that is all important, not the carriers.
According to the received view, the processing system receives the information. But that information has no effect whatsoever. The processing is assumed to not even deal with the information. Rather, it is assumed that the processing deals with the carriers of the information as a proxy for handling the information.
My alternative non-traditional view is that the information we gather is the essence of our experience. The qualia simply are the information - not the representation, but the information itself. Our visual experience is of a 3-dimensional world, simply because the information is of a 3-dimensional world.
I am not intending to evade the problem in the above. As I indicated, science does need to close the circle. I make the empiricist assumption that a new person entering the world has no innate knowledge of the world. I expect that an infant has little or no knowledge that it lives in a 3-D world, and therefore is not initially collecting 3D information. The infant has to learn what kind of world it is in, and what kind of information is available, before it can start collecting that information. That program of learning about the world is where I believe investigation needs to be directed.
By the way, we are drifting off topic. You might want to start a separate thread if you want to pursue this further. (I might be amenable to an appropriate GD thread).