Life’s spread into the solar system may have gotten inadvertent transport via earth’s spacecraft:
- Extremophiles, Surveyor III, and the Possibility of Life Beyond Earth? – Roger Launius’s Blog
- Hardy little space travelers could colonize Mars, space station research shows that — ScienceDaily
Life probably spread already from earth via debris thrown into space from giant meteorite impacts in the distant past: How Life-Bearing Rocks from the Chicxulub Asteroid Impact must have Spread through the Solar System: Earth rocks capable of carrying and protecting life have probably to travelled to Europa, Titan and beyond, say astrobiologists – The Physics arXiv Blog
Also, if life originated on Mars, it would have colonized earth in a similar manner: