But the point I was trying to make, about alien life in general, is that even if it is based on the same general chemical architecture as ours, and has evolved in an environment very similar to ours, it will still be very different from us. It may have something different than DNA to carry its genetic history, it's amino acids and proteins would be subtly different, different enzymes would drive its metabolic processes, and so on.

All earth life seems to have derived from a common ancestor, we can eat just about anything that is alive, unless it has engineered some specific metabolic poison into its tissues as a defense. We are quite similar, chemically, to every other organism on this planet. I don't think life on another world, even if it followed the same general plan, would be chemically compatible with us. It is even conceivable that some of its chemical components would be identical in composition, but different in symmetry (right- vs left- handed sugars etc.: same chemical formula, but we can break one down but not the other.).

An alien mosquito would bite us, suck our blood, and promptly die. An alien shark would take a bite out of us and spit it out. An alien snake might find its venom had no effect on us. We would probably get a hideous allergic or immune reaction to their bizarre saliva proteins, though. Alien microbes could not survive in our tissues, alien viruses could not hihjack our cells' nuclear facilities, and vice-versa. Even big predators and parasites would not be able to digest us, and we wouldn't be able to eat the local flora and fauna, either. Our crops would clash with the bacteria in alien soils, and find no helpful symbionts. In short, alien life would be undigestible, if not downright poisonous. And of course, if life is capable of existing on different chemical and physiological/metabolic patterns than what we are familiar with, then it could be extremely toxic.

I don't think we'll be able to eat alien plants or game, and we will probably not have to worry about alien diseases or parasites, either. Parasites (as in the film "Alien"), are highly unlikely. Most parasites have spent millions of years evolving counters to their host's defenses. It's highly unlikely an alien tapeworm will be able to move in and set up shop in us sight unseen, at first meeting.

Because evolution seems to come up independently with many of the same solutions to the same problems, the overall shapes and anatomies of alien life will probably not be too bizarre, I'm sure there will be alien mosquitoes, an ecological niche filled by many taxa in our own world, but they may may not resemble our flying insects very much at the organ and tissue level when we dissect them.

Martian life, unless it was related to us by some panspermia event, would not likely pose a threat to us or vice-versa. The only reason we should sterilize our spacecraft is to make sure subsequent probes don't confuse native with imported life-forms; either on earth or Mars.