[antonvs]

> You'd have to add up the marginal probabilities of all the other scenarios, to compare with just the Earth scenario.

There aren't even close to billions of other planets that could have been candidates for sending a life-bearing asteroid to Earth. Some of the restrictions are:

1. A planet capable of having developed life.

2. A planet close enough for an asteroid from it (I have trouble typing that because it's such an unlikely scenario) to have reached Earth around the time that life on Earth began. That was ~4 billion years ago, which seems like a lot, but consider that if the recently spotted asteroid 1I/2017 U1 had come direct to us from Proxima Centauri, the closest star, it would have taken at least 48,000 years to get here. That puts a limit on candidate sources of less than 100 times the distance to Proxima, and limits the number of star systems to ~2 million.

Even if you multiply the probabilities in question by a factor of 1e6 to 1e7, it's not going to change the conclusion.

[JoeAltmaier]

Add in : asteroids in close orbits around suns developing life elsewhere (or here). Precursor chemicals to life surviving the asteroid trip even if whole organisms do not (accelerating life development on Earth by millions/a billion years).

We started with, it took an Earth to develop life, but if it came from elsewhere we'd have to expand our ideas to include all other possible sources of life, not just Earthlike. Because our premise changed.