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:

• Life may have arrived from space – Natural History Museum
• Could life have survived a fall to Earth? — ScienceDaily
• Are We All Martians? Tool Could Help Detect Earth-Like Life on Mars – Space.com
• Mars Meteorite Structures: Optimism for Alien Life? – Discovery News