Grunsfeld also made another observation about the effect of human exploration on Mars. The robot rovers sent there over the years are believed to have carried Earth microbes despite efforts to sterilize them. "We know there is life on Mars, because we put it there," Grunsfeld said.
Microbial species thrive in every corner of Earth's biosphere. Their rapid growth rates and promiscuous gene swapping provide ample grist for the evolutionary mill over relatively short time spans. In ocean surface waters, for example, microbial doubling times of about 1 day result in an estimated production of around 1030 new cells per year. (This number exceeds that of all individual grains of sand on Earth by about 10 orders of magnitude.) These cells are not static entities, but rather the products and perpetuators of dynamic evolutionary processes. But exactly how fast, to what degree, and by what mechanisms, do free-living microbes change and evolve over time in natural settings?