According to a new calculation by Alexei Sharov and Richard Gordon, life must have pre-existed the planet Earth.
You can find the study here. You can find some of my own earlier uncredentialed speculations about related matters here.
Sharov and Gordon are both properly credentialed. Each is a Ph.D. with a research role with prestigious (though non-academic) institutions. Nonetheless, there is a good deal of speculation in their work, as one might imagine given the subject.
The gist is that they infer that genetic complexity increased at an exponential pace: it doubles every 376 million years. If that is right, then life originated roughly 9.7 billion years ago (give or take 2.5 billion). And if that is right then it could not have originated on earth. Radiometric dating puts the age of the earth at roughly 4.5 billion years.
So on their guesstimations, life must have been already 5 billion years old before the earth was formed.
Their calculations also indicate that it would have taken life 5 billion years to reach the degree of genetic complexity present in a bacterium. Five billion years, counting forward from the hypothetical date of origin 9.7 billion years ago, allows for the possibility that bacteria drifted onto the new planet Earth and began their history of development/adaptations here.
I applaud the direction of their thinking, as it confirms my prejudices, which is always the best test of science.