A great deal of humility is necessary to address these questions because of how little is known. There is no theory of evolutionary processes to determine the range of life-forms that might evolve in the universe and in particular on Mars. The Viking landers did not find any biological activity at the landing sites. If the Viking results can be extrapolated to the entire planet, the surface of Mars would appear to be inhospitable to life. However, the Viking landing sites were not chosen to optimize the search for life, and the biological experiments focused on a limited variety of metabolic types and sampled only a few centimeters below the surface at two locations.
Depth is significant in the search for life.
He suggests that perhaps microbial life exists in all locations where microbes can survive. This possibility should engender humility about answering the question of extant life on Mars. The question of whether there is danger of contaminating Earth has been a subject of concern. In 1977, Thomas H. Jukes concluded that there are no biological risks. It is not known whether the informational macromolecule DNA is universally dominant.