Panspermia, not necissarily from Mars but from the same comet/asteriod bombardment that hit both planets.
Meteorite Yields Evidence of Primitive Life on Early Mars
http://www2.jpl.nasa.gov/snc/nasa1.html
In November 2009, a team of scientists at Johnson Space Center, including McKay, argued that since their original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant fossil-like structures in other Martian meteorites.
Deoxyribonucleic acid is a molecule that carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses.
DNA is a compound of elements that are common in the Solar system.
Right now there is probably DNA on all the planets, moons, comets and asteroids in this system.
Just as Human DNA and Elephant DNA differ, DNA on other celestial bodies differ. It is a fingerprint of life but is not life itself.
There are other factors involved.
One very important factor is liquid water (a soluble). Water exists on Mars but it is not in liquid state. Water also exists on Venus but it is not in liquid state.
The Sun is in main sequence. It has been in main sequence for about 1 or 2 billion years. It will remain in main sequence for another 2 to 3 billion years.
In Mars' distant past, it may have been orbiting within the Goldilocks Zone allowing liquid water. It likely had a magnetosphere because it had a molten iron core. At that time, it is a safe assumption to think that life did happen on that planet.
It is also a fairly safe assumption that there is life in the upper atmosphere of Venus (where water droplets can form).
Considering that the only true example of civilization is the one created by humans on this planet that took nearly 4.5 billion years to come about, chances of any civilization on Mars or Venus is unlikely. There just wasn't enough time for it to happen. The life prime time was too short.
