He explains that by any measure, the most common organisms have always been, and still are, the bacteria. The complexity distribution is bounded at one side (a living organism cannot be much simpler than bacteria), so an unbiased random walk by evolution, sometimes going in the complexity direction and sometimes going towards simplicity (without having an intrinsic preference to either), will create a distribution with a small, but longer and longer tail at the high complexity end.
The author claims that, by any standard, bacteria are the most prevalent life forms, both in the past and present. The range of biological complexity is limited on one side (living organisms cannot be simpler than bacteria), so an unbiased evolutionary process will produce a distribution with a small but increasingly longer tail at the high complexity end.