My view is that we can reason about organisms living in space because we are one of them. We have a sample size of one, but assuming that properties of different space-faring organisms are distributed normally, it stands to reason that the first sample collected is near to the mean of the underlying distribution.

In other words, imagine a normal distribution with very, very low variance (say, 0.0001). The mean is unknown to you. If you take one sample, your sample will very likely be close to the mean.