Why?
Why should Mars NOT have life on it? In fact, I think that the Viking lander biological experiment detected it. It seems to me that if you conducted the same experiment on Earth you would get the same results. If you give a soil sample some radioactive water and nutrients and then monitor the radioactive CO2 emitted by the microbes, the result would be positive on Earth as it was on Mars. If now you take another sample of soil and heat it until the microbes died off and then gave it water and nutrients, the result would be negative on Earth as it was on Mars.