Which only calls forth the question: Where did those microorganisms originate, and who or what "seeded" the Earth with them? Life has to start somewhere. If it began on its own "out there," why couldn't the same thing have happened here?
Life may well have began on its own here. Seeding isn't a necessity, but is a possibility.
It may be extremely rare for life to form on it's own.
Life has an inherent will to survive and is likely to want to expand beyond it's home star system given the chance. As explained above, the most likely way that would happen in my opinion is by spreading microorganisms to all four corners of the galaxy.
Consider how many seed worlds could be created by just one life bearing planet.
The average number could be less than 1 (project is a probable failure) or it could be hundreds (in which case most life bearing worlds in the galaxy would be seed worlds).
The equation we have to look at is this:
a = Probability that a life bearing planet evolves creatures with the will and the means to create seed comets.
b = Number of new life bearing worlds created from one planet's seeding project.
The product N = a*b is the number of daughter worlds that each life bearing world gives birth to (on average).
If N > 1, the growth is exponential, and the galaxy will tend to a state where it's teeming with life, and where almost all life bearing worlds are seed worlds.
If N < 1, the seeding projects are unsustainable, and most life bearing worlds have created life on their own.