Just a brief note on the notion that any life from Mars might be unable to directly interact with earth's life due to biochemical constraints, etc.
Many compounds of life have what are known as chiral centers. These typically result from a carbon atom, within a molecule, which has tetrahedral bonds. Such a configuration provides mirror-image molecules (non-superimposable), with both forms having the same physical properties! These are known as stereoisomers. They are distinguished in solution by how they rotate polarized light, designating a D-form and a L-form.
Amino acids are a prime example. If you make them from scratch (non-biologically), one usually ends up with a 50:50 mixture of the D-form and the L-form. Life on earth however "chose" one form - the L-amino acids - for proteins, during its earliest formative period. This is rather curious since one would expect that both forms were present in equal amounts during abiogenesis as the process must have used amino acids from an abiotic source. (Some D-forms are used for peptides and other small compounds.) Whatever the species, for humans, and all life on earth, all proteins are composed of L-amino acids, and D-amino acids can actually be toxic!
So it is a distinct possibility (50:50 for a good guess!) that any life from Mars (or anywhere else aliens might come from) could have evolved using the D-stereoisomers of amino acids (and that is just for their proteins!). Since it is the mirror image of the L-form, this could really happen! As noted before, they have the same physical properties.
The same is true for most chiral biochemicals. And chiral centers are everywhere in earth's biochemical "repertoire". Some are L-forms, and some are D-forms. An alien would probably have some serious problems "fooling around" with life on earth based simply on variations in stereochemistry!
So if an alien from Mars (or anywhere else) that evolved using D-amino acids came here, it would have a rough time just making a living (forget about fooling around!) : Most of what it might eat would be deadly, since they are of opposite stereochemistry. It is a little comforting to know there is some probability that an alien would not eat you, at least not right away! If they got here at all, they would surely know of the possible stereochemical toxins awaiting them. One suspects they would have test animals to feed to find out if we were safe to consume!
The variables here are so enormous as to boggle the mind. If you could make a copy of yourself with the opposite stereoisomers that you use now, and looking at the finished copy like a mirror, one would see the same face looking back at you, with no indication whatever that it is composed of completely different biochemistry! We could call such things "mirror life forms."
This is only one aspect of how life could vary from one origin to the next. There are others to be pondered, but this is one of the most obvious, with an unquestionably big difference.
And that really was a brief note on this subject!