puts on astrobiologist hat
Alexandra was still knee deep in methanotrophs and methanogens and the intricacies of interplanetary similarities in biochemical markers. It was enough to make John’s head hurt.
The fact that there are methanogens
and methanotrophs suggests that these bugs are indigenous to Mars, since they'd form a complete ecology--this sort of synthrophism with methanogens producing methane from inorganic carbon and methanotrophs consuming it is very common on Earth. Bacterial contamination by human missions would be very unlikely, imo, to lead to a complete ecology like this.
Doesn't mean the microbes aren't ultimately terrestrial in origin, just that if they are, they've probably been on Mars for a
long time, long enough to evolve into different metabolic guilds.
What about using the DNA to see how separated it is from known Earth life time wise to try to see if it's actually from Mars or not?
This is exactly how'd they do it--specifically, they'd try to sequence its 16RS ribosomal RNA (since that portion of the genome is
highly conserved across terrestrial life) and construct an evolutionary phylogenetic tree from that.
If they aren't able to sequence the microbes' 16RS at all, then that means the microbes really are extraterrestrial in nature, since their DNA is so different that the sequencing primers used for terrestrial biology can't bind to it.