However, if a life form in the deep soil of Mars (for example) was seeded from Earth or vice-versa it is perhaps even more exciting given it would be proof of panspermia
Panspermia is a complex subject. Life may be transferred between planets within a solar system quite readily; we have numerous meteorites from Mars and Venus on Earth, and presumably Mars and Venus both have meteorites which originated on Earth. So it seems possible that life may have transferred spontaneously between any or all of these three planets, and potentially between these planets and various moons, asteroids and comets within our system and vice versa. Indeed, I'd be very disappointed if this has not happened at some point in the long history of our star.
But the chances of transferring life between different solar systems is many orders of magnitude more difficult. Not only are stars very far apart and offer a very small target indeed for potentially life-bearing rocks, but the journey times between stars at typical meteoric speeds are very long indeed. This means that any lifeforms that might find themselves on an interstellar meteor would need to persist for many millions of years before approaching another solar system, making survival very unlikely. This sort of interstellar panspermia might happen on extremely rare occasions, but would probably not result in a galaxy filled with inter-related lifeforms.
One factor could increase the odds of interstellar panspermia; nearly all stars are created inside relatively compact molecular clouds which condense into stellar clusters. These stellar clusters disperse after a period of a few tens or hundreds of millions of years; but while the stars are still close together, life-bearing objects could transfer between their planets more readily. The problem with this is that most planets in young stellar clusters are hot and inhospitable., so this might reduce the odds considerably
Finally there is the possibility that advanced alien civilisations may have moved species around from star to star, which could rapidly increase the habitability of a galaxy by artificial means. I should point out that there is no evidence of this happening on Earth, since we can trace our molecular genetics back for around four billion years, but it may have happened elsewhere in the galaxy on a regular basis.