[A. Pole]

"If there were warm, wet places, then life could have developed," Murray said.

It did not need to develop there. Mars orbit is more or less parallel to the Earth and constant so called solar wind is pushing the miriads of dust and spores from the Earth atmosphere in the outside direction. If there are suitable spots the Earthly life had to made some beachhead on the Mars.

[muawiyah]

And, of course, vice versa whenever Mars is hit by an object big enough to knock off chunks that are then thrown toward the Sun and across Earth's orbit.

[RossA]

or vice versa